Earth Structure | Earth Science

<!DOCTYPE html> <html lang="en" dir="ltr"> <head>  <meta http-equiv="X-UA-Compatible" content="ie=edge"> <meta charset="utf-8"> <base href="https://www.visionlearning.com"> <title>Earth Structure | Earth Science | Visionlearning</title> <link rel="canonical" href="https://www.visionlearning.com/en/library/earth-science/6/earth-structure/69"> <meta name="description" content="The layers of Earth can be described by what they are made of and how they behave. Learn what these layers are and how scientists determined where they are."> <meta name="keywords" content="what is the structure of earth, what is earth made of, how do waves act, what are seismic waves"> <meta name="viewport" content="width=device-width, initial-scale=1.0, shrink-to-fit=no"> <meta name="msvalidate.01" content="D8E20F39AD48052260032E56DE409970"> <script type="application/ld+json"> { "@context": "http://schema.org/", "@type": "Article", "mainEntityOfPage": { "@type": "WebPage", "@id": "https://visionlearning.com/en/library/earth-science/6/earth-structure/69" }, "name": "Earth Structure", "headline": "Earth Structure: A virtual journey to the center of Earth", "author": { "@type": "Person", "name": "Anne E. Egger, Ph.D." }, "datePublished": "2003-07-02 14:04:00", "dateModified": "2017-02-12T08:30:00+05:00", "image": { "@type": "ImageObject", "url": "/img/library/moduleImages/featured_image_69-23061209065514.jpeg", "width": 696, "height": 464 }, "publisher": { "@type": "Organization", "name": "Visionlearning, Inc.", "logo": { "@type": "ImageObject", "url": "http://visionlearning.com/images/logo.png", "width": 278, "height": 60 } }, "description": "Earth's interior structure is composed of layers that vary by composition and behavior. Using principles of physics like gravity and wave motion, this module explains how scientists have determined Earth's deep structure. Different types of seismic waves are discussed. The module details both compositional and mechanical layers of Earth.", "keywords": "what is the structure of earth, what is earth made of, how do waves act, what are seismic waves", "inLanguage": { "@type": "Language", "name": "English", "alternateName": "en" }, "copyrightHolder": { "@type": "Organization", "name": "Visionlearning, Inc." }, "copyrightYear": "2003"} </script> <meta property="og:url" content="https://visionlearning.com/en/library/earth-science/6/earth-structure/69"> <meta property="og:title" content="Earth Structure | Earth Science | Visionlearning" /> <meta property="og:type" content="website"> <meta property="og:site_name" content="Visionlearning"> <meta property="og:description" content="The layers of Earth can be described by what they are made of and how they behave. 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<li><a href="/en/library/earth-science/6/factors-that-control-earths-temperature/234">Factors that Control Earth's Temperature</a></li> <li><a href="/en/library/earth-science/6/circulation-in-the-atmosphere/255">Circulation in the Atmosphere</a></li> </ul> </div> <button class="accordion__button" id="acc-button-hazards" data-accordion="button" aria-controls="acc-panel-hazards" aria-expanded="false"> <span class="accordion__button__label"> Hazards </span> </button> <div class="accordion__panel" id="acc-panel-hazards" data-accordion="panel" aria-labelledby="acc-button-hazards" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/earth-science/6/natural-hazards-and-risk/288">Natural Hazards and Risk</a></li> </ul> </div> <button class="accordion__button" id="acc-button-earth-history" data-accordion="button" aria-controls="acc-panel-earth-history" aria-expanded="false"> <span class="accordion__button__label"> Earth History </span> </button> <div class="accordion__panel" 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aria-controls="acc-panel-ecology" aria-expanded="false"> <span class="accordion__button__label"> Ecology </span> </button> <div class="accordion__panel" id="acc-panel-ecology" data-accordion="panel" aria-labelledby="acc-button-ecology" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/environmental-science/61/biodiversity-i/276">Biodiversity I</a></li> <li><a href="/en/library/environmental-science/61/biodiversity-ii/281">Biodiversity II</a></li> <li><a href="/en/library/environmental-science/61/ecosystem-services/279">Ecosystem Services</a></li> <li><a href="/en/library/environmental-science/61/population-biology/287">Population Biology</a></li> </ul> </div> <button class="accordion__button" id="acc-button-earth-cycles" data-accordion="button" aria-controls="acc-panel-earth-cycles" aria-expanded="false"> <span class="accordion__button__label"> Earth Cycles </span> </button> <div class="accordion__panel" id="acc-panel-earth-cycles" data-accordion="panel" aria-labelledby="acc-button-earth-cycles" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/environmental-science/61/the-nitrogen-cycle/98">The Nitrogen Cycle</a></li> <li><a href="/en/library/environmental-science/61/the-carbon-cycle/95">The Carbon Cycle</a></li> <li><a href="/en/library/environmental-science/61/the-phosphorus-cycle/197">The Phosphorus Cycle</a></li> </ul> </div> <button class="accordion__button" id="acc-button-scientific-research" data-accordion="button" aria-controls="acc-panel-scientific-research" aria-expanded="false"> <span class="accordion__button__label"> Scientific Research </span> </button> <div class="accordion__panel" id="acc-panel-scientific-research" data-accordion="panel" aria-labelledby="acc-button-scientific-research" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/environmental-science/61/collaborative-research-in-the-arctic-towards-understanding-climate-change/183">Collaborative Research in the Arctic Towards Understanding Climate Change</a></li> <li><a href="/en/library/environmental-science/61/atmospheric-chemistry-research-that-changed-global-policy/211">Atmospheric Chemistry Research that Changed Global Policy</a></li> </ul> </div> </div> </div> <button class="accordion__button" id="acc-button-general-science" data-accordion="button" aria-controls="acc-panel-general-science" aria-expanded="false"> <span class="accordion__button__label"> General Science </span> </button> <div class="accordion__panel" id="acc-panel-general-science" data-accordion="panel" aria-labelledby="acc-button-general-science" role="region"> <div class="accordion accordion--secondary"> <button class="accordion__button" id="acc-button-methods" data-accordion="button" aria-controls="acc-panel-methods" aria-expanded="false"> <span class="accordion__button__label"> Methods </span> </button> <div class="accordion__panel" id="acc-panel-methods" data-accordion="panel" aria-labelledby="acc-button-methods" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/general-science/3/the-scientific-method/45">The Scientific Method</a></li> </ul> </div> <button class="accordion__button" id="acc-button-measurement" data-accordion="button" aria-controls="acc-panel-measurement" aria-expanded="false"> <span class="accordion__button__label"> Measurement </span> </button> <div class="accordion__panel" id="acc-panel-measurement" data-accordion="panel" aria-labelledby="acc-button-measurement" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/general-science/3/the-metric-system/47">The Metric System</a></li> </ul> </div> <button class="accordion__button" id="acc-button-physical-properties" data-accordion="button" aria-controls="acc-panel-physical-properties" aria-expanded="false"> <span class="accordion__button__label"> Physical Properties </span> </button> <div class="accordion__panel" id="acc-panel-physical-properties" data-accordion="panel" aria-labelledby="acc-button-physical-properties" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/general-science/3/temperature/48">Temperature</a></li> <li><a href="/en/library/general-science/3/density-and-buoyancy/37">Density and Buoyancy</a></li> </ul> </div> </div> </div> <button class="accordion__button" id="acc-button-math-in-science" data-accordion="button" aria-controls="acc-panel-math-in-science" aria-expanded="false"> <span class="accordion__button__label"> Math in Science </span> </button> <div class="accordion__panel" id="acc-panel-math-in-science" data-accordion="panel" aria-labelledby="acc-button-math-in-science" role="region"> <div class="accordion accordion--secondary"> <button class="accordion__button" id="acc-button-equations" data-accordion="button" aria-controls="acc-panel-equations" aria-expanded="false"> <span class="accordion__button__label"> Equations </span> </button> <div class="accordion__panel" id="acc-panel-equations" data-accordion="panel" aria-labelledby="acc-button-equations" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/math-in-science/62/unit-conversion/144">Unit Conversion</a></li> <li><a href="/en/library/math-in-science/62/linear-equations/194">Linear Equations</a></li> <li><a href="/en/library/math-in-science/62/exponential-equations-i/206">Exponential Equations I</a></li> <li><a href="/en/library/math-in-science/62/exponential-equations-ii/210">Exponential Equations II</a></li> <li><a href="/en/library/math-in-science/62/scientific-notation/250">Scientific Notation</a></li> <li><a href="/en/library/math-in-science/62/measurement/257">Measurement</a></li> </ul> </div> <button class="accordion__button" id="acc-button-statistics" data-accordion="button" aria-controls="acc-panel-statistics" aria-expanded="false"> <span class="accordion__button__label"> Statistics </span> </button> <div class="accordion__panel" id="acc-panel-statistics" data-accordion="panel" aria-labelledby="acc-button-statistics" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/math-in-science/62/introduction-to-descriptive-statistics/218">Introduction to Descriptive Statistics</a></li> <li><a href="/en/library/math-in-science/62/introduction-to-inferential-statistics/224">Introduction to Inferential Statistics</a></li> <li><a href="/en/library/math-in-science/62/statistical-techniques/239">Statistical Techniques</a></li> </ul> </div> <button class="accordion__button" id="acc-button-trigonometric-functions" data-accordion="button" aria-controls="acc-panel-trigonometric-functions" aria-expanded="false"> <span class="accordion__button__label"> Trigonometric Functions </span> </button> <div class="accordion__panel" id="acc-panel-trigonometric-functions" data-accordion="panel" aria-labelledby="acc-button-trigonometric-functions" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/math-in-science/62/wave-mathematics/131">Wave Mathematics</a></li> </ul> </div> </div> </div> <button class="accordion__button" id="acc-button-physics" data-accordion="button" aria-controls="acc-panel-physics" aria-expanded="false"> <span class="accordion__button__label"> Physics </span> </button> <div class="accordion__panel" id="acc-panel-physics" data-accordion="panel" aria-labelledby="acc-button-physics" role="region"> <div class="accordion accordion--secondary"> <button class="accordion__button" id="acc-button-light-and-optics" data-accordion="button" aria-controls="acc-panel-light-and-optics" aria-expanded="false"> <span class="accordion__button__label"> Light and Optics </span> </button> <div class="accordion__panel" id="acc-panel-light-and-optics" data-accordion="panel" aria-labelledby="acc-button-light-and-optics" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/physics/24/the-nature-of-light/132">The Nature of Light</a></li> <li><a href="/en/library/physics/24/electromagnetism-and-light/138">Electromagnetism and Light</a></li> </ul> </div> <button class="accordion__button" id="acc-button-mechanics" data-accordion="button" aria-controls="acc-panel-mechanics" aria-expanded="false"> <span class="accordion__button__label"> Mechanics </span> </button> <div class="accordion__panel" id="acc-panel-mechanics" data-accordion="panel" aria-labelledby="acc-button-mechanics" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/physics/24/defining-energy/199">Defining Energy</a></li> <li><a href="/en/library/physics/24/waves-and-wave-motion/102">Waves and Wave Motion</a></li> <li><a href="/en/library/physics/24/gravity/118">Gravity</a></li> <li><a href="/en/library/physics/24/thermodynamics-i/200">Thermodynamics I</a></li> </ul> </div> </div> </div> <button class="accordion__button" id="acc-button-process-of-science" data-accordion="button" aria-controls="acc-panel-process-of-science" aria-expanded="false"> <span class="accordion__button__label"> Process of Science </span> </button> <div class="accordion__panel" id="acc-panel-process-of-science" data-accordion="panel" aria-labelledby="acc-button-process-of-science" role="region"> <div class="accordion accordion--secondary"> <button class="accordion__button" id="acc-button-introduction" data-accordion="button" aria-controls="acc-panel-introduction" aria-expanded="false"> <span class="accordion__button__label"> Introduction </span> </button> <div class="accordion__panel" id="acc-panel-introduction" data-accordion="panel" aria-labelledby="acc-button-introduction" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/process-of-science/49/the-process-of-science/176">The Process of Science</a></li> </ul> </div> <button class="accordion__button" id="acc-button-the-culture-of-science" data-accordion="button" aria-controls="acc-panel-the-culture-of-science" aria-expanded="false"> <span class="accordion__button__label"> The Culture of Science </span> </button> <div class="accordion__panel" id="acc-panel-the-culture-of-science" data-accordion="panel" aria-labelledby="acc-button-the-culture-of-science" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/process-of-science/49/the-nature-of-scientific-knowledge/185">The Nature of Scientific Knowledge</a></li> <li><a href="/en/library/process-of-science/49/scientists-and-the-scientific-community/172">Scientists and the Scientific Community</a></li> <li><a href="/en/library/process-of-science/49/scientific-ethics/161">Scientific Ethics</a></li> <li><a href="/en/library/process-of-science/49/scientific-institutions-and-societies/162">Scientific Institutions and Societies</a></li> </ul> </div> <button class="accordion__button" id="acc-button-ideas-in-science" data-accordion="button" aria-controls="acc-panel-ideas-in-science" aria-expanded="false"> <span class="accordion__button__label"> Ideas in Science </span> </button> <div class="accordion__panel" id="acc-panel-ideas-in-science" data-accordion="panel" aria-labelledby="acc-button-ideas-in-science" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/process-of-science/49/theories-hypotheses-and-laws/177">Theories, Hypotheses, and Laws</a></li> <li><a href="/en/library/process-of-science/49/scientific-controversy/181">Scientific Controversy</a></li> <li><a href="/en/library/process-of-science/49/creativity-in-science/182">Creativity in Science</a></li> </ul> </div> <button class="accordion__button" id="acc-button-research-methods" data-accordion="button" aria-controls="acc-panel-research-methods" aria-expanded="false"> <span class="accordion__button__label"> Research Methods </span> </button> <div class="accordion__panel" id="acc-panel-research-methods" data-accordion="panel" aria-labelledby="acc-button-research-methods" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/process-of-science/49/the-practice-of-science/148">The Practice of Science</a></li> <li><a href="/en/library/process-of-science/49/experimentation-in-scientific-research/150">Experimentation in Scientific Research</a></li> <li><a href="/en/library/process-of-science/49/description-in-scientific-research/151">Description in Scientific Research</a></li> <li><a href="/en/library/process-of-science/49/comparison-in-scientific-research/152">Comparison in Scientific Research</a></li> <li><a href="/en/library/process-of-science/49/modeling-in-scientific-research/153">Modeling in Scientific Research</a></li> </ul> </div> <button class="accordion__button" id="acc-button-data" data-accordion="button" aria-controls="acc-panel-data" aria-expanded="false"> <span class="accordion__button__label"> Data </span> </button> <div class="accordion__panel" id="acc-panel-data" data-accordion="panel" aria-labelledby="acc-button-data" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/process-of-science/49/data-analysis-and-interpretation/154">Data Analysis and Interpretation</a></li> <li><a href="/en/library/process-of-science/49/uncertainty-error-and-confidence/157">Uncertainty, Error, and Confidence</a></li> <li><a href="/en/library/process-of-science/49/statistics-in-science/155">Statistics in Science</a></li> <li><a href="/en/library/process-of-science/49/using-graphs-and-visual-data-in-science/156">Using Graphs and Visual Data in Science</a></li> </ul> </div> <button class="accordion__button" id="acc-button-scientific-communication" data-accordion="button" aria-controls="acc-panel-scientific-communication" aria-expanded="false"> <span class="accordion__button__label"> Scientific Communication </span> </button> <div class="accordion__panel" id="acc-panel-scientific-communication" data-accordion="panel" aria-labelledby="acc-button-scientific-communication" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/process-of-science/49/understanding-scientific-journals-and-articles/158">Understanding Scientific Journals and Articles</a></li> <li><a href="/en/library/process-of-science/49/utilizing-the-scientific-literature/173">Utilizing the Scientific Literature</a></li> <li><a href="/en/library/process-of-science/49/peer-review-in-scientific-publishing/159">Peer Review in Scientific Publishing</a></li> <li><a href="/en/library/process-of-science/49/the-how-and-why-of-scientific-meetings/186">The How and Why of Scientific Meetings</a></li> </ul> </div> </div> </div> <button class="accordion__button" id="acc-button-scientists-and-research" data-accordion="button" aria-controls="acc-panel-scientists-and-research" aria-expanded="false"> <span class="accordion__button__label"> Scientists and Research </span> </button> <div class="accordion__panel" id="acc-panel-scientists-and-research" data-accordion="panel" aria-labelledby="acc-button-scientists-and-research" role="region"> <div class="accordion accordion--secondary"> <button class="accordion__button" id="acc-button-scientific-research" data-accordion="button" aria-controls="acc-panel-scientific-research" aria-expanded="false"> <span class="accordion__button__label"> Scientific Research </span> </button> <div class="accordion__panel" id="acc-panel-scientific-research" data-accordion="panel" aria-labelledby="acc-button-scientific-research" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/scientists-and-research/58/collaborative-research-in-the-arctic-towards-understanding-climate-change/183">Collaborative Research in the Arctic Towards Understanding Climate Change</a></li> <li><a href="/en/library/scientists-and-research/58/from-stable-chromosomes-to-jumping-genes/184">From Stable Chromosomes to Jumping Genes</a></li> <li><a href="/en/library/scientists-and-research/58/an-elegant-experiment-to-test-the-process-of-dna-replication/187">An Elegant Experiment to Test the Process of DNA Replication</a></li> <li><a href="/en/library/scientists-and-research/58/the-founding-of-neuroscience/233">The Founding of Neuroscience</a></li> <li><a href="/en/library/scientists-and-research/58/tracking-endangered-jaguars-across-the-border/189">Tracking Endangered Jaguars across the Border</a></li> <li><a href="/en/library/scientists-and-research/58/atmospheric-chemistry-research-that-changed-global-policy/211">Atmospheric Chemistry Research that Changed Global Policy</a></li> <li><a href="/en/library/scientists-and-research/58/revolutionizing-medicine-with-monoclonal-antibodies/220">Revolutionizing Medicine with Monoclonal Antibodies</a></li> <li><a href="/en/library/scientists-and-research/58/uncovering-the-mysteries-of-chronic-mountain-sickness/238">Uncovering the Mysteries of Chronic Mountain Sickness</a></li> </ul> </div> <button class="accordion__button" id="acc-button-profiles-in-science" data-accordion="button" aria-controls="acc-panel-profiles-in-science" aria-expanded="false"> <span class="accordion__button__label"> Profiles in Science </span> </button> <div class="accordion__panel" id="acc-panel-profiles-in-science" data-accordion="panel" aria-labelledby="acc-button-profiles-in-science" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/scientists-and-research/58/luis-e.-miramontes/232">Luis E. Miramontes</a></li> <li><a href="/en/library/scientists-and-research/58/bernardo-houssay/237">Bernardo Houssay</a></li> <li><a href="/en/library/scientists-and-research/58/craig-lee/256">Craig Lee</a></li> <li><a href="/en/library/scientists-and-research/58/david-ho/241">David Ho</a></li> <li><a href="/en/library/scientists-and-research/58/louis-tompkins-wright/244">Louis Tompkins Wright</a></li> <li><a href="/en/library/scientists-and-research/58/carlos-j.-finlay/217">Carlos J. Finlay</a></li> <li><a href="/en/library/scientists-and-research/58/cecilia-payne/290">Cecilia Payne</a></li> <li><a href="/en/library/scientists-and-research/58/jazmin-scarlett/291">Jazmin Scarlett</a></li> <li><a href="/en/library/scientists-and-research/58/ramari-stewart/292">Ramari Stewart</a></li> <li><a href="/en/library/scientists-and-research/58/johnson-cerda/300">Johnson Cerda</a></li> <li><a href="/en/library/scientists-and-research/58/ellen-ochoa/201">Ellen Ochoa</a></li> <li><a href="/en/library/scientists-and-research/58/ruth-benerito/205">Ruth Benerito</a></li> <li><a href="/en/library/scientists-and-research/58/franklin-chang-díaz/219">Franklin Chang Díaz</a></li> <li><a href="/en/library/scientists-and-research/58/percy-lavon-julian/221">Percy Lavon Julian</a></li> <li><a href="/en/library/scientists-and-research/58/luis-walter-alvarez/229">Luis Walter Alvarez</a></li> <li><a href="/en/library/scientists-and-research/58/france-anne-dominic-córdova/230">France Anne-Dominic Córdova</a></li> </ul> </div> </div> </div> </div> </div> </li> <li>  <button class="button" data-toggle="dropdown">Earth Science </button> <div class="nav__dropdown box-shadow-1 padding-1"> <div class="accordion accordion--secondary font-size-sm"> <div class="accordion accordion--secondary"> <button class="accordion__button" id="acc-sub-button-plate-tectonics" data-accordion="button" aria-controls="acc-sub-panel-plate-tectonics" aria-expanded="false"> <span class="accordion__button__label"> Plate Tectonics </span> </button> <div class="accordion__panel" id="acc-sub-panel-plate-tectonics" data-accordion="panel" aria-labelledby="acc-sub-button-plate-tectonics" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/earth-science/6/origins-of-plate-tectonic-theory/65">Origins of Plate Tectonic Theory</a></li> <li><a href="/en/library/earth-science/6/plate-boundaries/66">Plate Boundaries</a></li> <li class="current">Earth Structure</li> </ul> </div> <button class="accordion__button" id="acc-sub-button-earth-cycles" data-accordion="button" aria-controls="acc-sub-panel-earth-cycles" aria-expanded="false"> <span class="accordion__button__label"> Earth Cycles </span> </button> <div class="accordion__panel" id="acc-sub-panel-earth-cycles" data-accordion="panel" aria-labelledby="acc-sub-button-earth-cycles" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/earth-science/6/the-rock-cycle/128">The Rock Cycle</a></li> <li><a href="/en/library/earth-science/6/the-hydrologic-cycle/99">The Hydrologic Cycle</a></li> <li><a href="/en/library/earth-science/6/the-carbon-cycle/95">The Carbon Cycle</a></li> <li><a href="/en/library/earth-science/6/the-nitrogen-cycle/98">The Nitrogen Cycle</a></li> <li><a href="/en/library/earth-science/6/the-phosphorus-cycle/197">The Phosphorus Cycle</a></li> </ul> </div> <button class="accordion__button" id="acc-sub-button-rocks-and-minerals" data-accordion="button" aria-controls="acc-sub-panel-rocks-and-minerals" aria-expanded="false"> <span class="accordion__button__label"> Rocks and Minerals </span> </button> <div class="accordion__panel" id="acc-sub-panel-rocks-and-minerals" data-accordion="panel" aria-labelledby="acc-sub-button-rocks-and-minerals" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/earth-science/6/defining-minerals/119">Defining Minerals</a></li> <li><a href="/en/library/earth-science/6/identifying-minerals/130">Identifying Minerals</a></li> <li><a href="/en/library/earth-science/6/the-silicate-minerals/140">The Silicate Minerals</a></li> </ul> </div> <button class="accordion__button" id="acc-sub-button-atmosphere-and-oceans" data-accordion="button" aria-controls="acc-sub-panel-atmosphere-and-oceans" aria-expanded="false"> <span class="accordion__button__label"> Atmosphere and Oceans </span> </button> <div class="accordion__panel" id="acc-sub-panel-atmosphere-and-oceans" data-accordion="panel" aria-labelledby="acc-sub-button-atmosphere-and-oceans" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/earth-science/6/ocean-currents/282">Ocean Currents</a></li> <li><a href="/en/library/earth-science/6/water-in-the-atmosphere/289">Water in the Atmosphere</a></li> <li><a href="/en/library/earth-science/6/weather-fronts-and-forecasts/303">Weather, Fronts, and Forecasts</a></li> <li><a href="/en/library/earth-science/6/history-of-earths-atmosphere-i/202">History of Earth's Atmosphere I</a></li> <li><a href="/en/library/earth-science/6/history-of-earths-atmosphere-ii/203">History of Earth's Atmosphere II</a></li> <li><a href="/en/library/earth-science/6/earths-atmosphere/107">Earth's Atmosphere</a></li> <li><a href="/en/library/earth-science/6/factors-that-control-earths-temperature/234">Factors that Control Earth's Temperature</a></li> <li><a href="/en/library/earth-science/6/circulation-in-the-atmosphere/255">Circulation in the Atmosphere</a></li> </ul> </div> <button class="accordion__button" id="acc-sub-button-hazards" data-accordion="button" aria-controls="acc-sub-panel-hazards" aria-expanded="false"> <span class="accordion__button__label"> Hazards </span> </button> <div class="accordion__panel" id="acc-sub-panel-hazards" data-accordion="panel" aria-labelledby="acc-sub-button-hazards" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/earth-science/6/natural-hazards-and-risk/288">Natural Hazards and Risk</a></li> </ul> </div> <button class="accordion__button" id="acc-sub-button-earth-history" data-accordion="button" aria-controls="acc-sub-panel-earth-history" aria-expanded="false"> <span class="accordion__button__label"> Earth History </span> </button> <div class="accordion__panel" id="acc-sub-panel-earth-history" data-accordion="panel" aria-labelledby="acc-sub-button-earth-history" role="region"> <ul class="nav text-color-link"> <li><a href="/en/library/earth-science/6/extinction/295">Extinction</a></li> <li><a href="/en/library/earth-science/6/paleontology-and-the-fossil-record/301">Paleontology and the Fossil Record</a></li> <li><a href="/en/library/earth-science/6/mass-extinctions/294">Mass Extinctions</a></li> </ul> </div> </div> </div> </div> </li> </ul> </nav>  <div id="theTop"></div> <main id="skip-header-content"> <div class="margin-bottom-5"> <div class="container narrow wide--lg margin-y-4"> <article class="module"> <header class="module__header"> <span class="subcategory"> <strong><em>Plate Tectonics</em></strong> </span> <h1>Earth Structure: <sub><em>A virtual journey to the center of Earth</em></sub></h1> <p class="byline">by Anne E. Egger, Ph.D.</p> </header> <nav class="module__tabs"> <ul class="tabs-nav tabs-nav--pill tabs-nav--horizontal--md library"> <li> <a href="/en/library/earth-science/6/earth-structure/69/reading" class="is-active" aria-current="page" >Reading</a> </li> <li> <a href="/en/library/earth-science/6/earth-structure/69/quiz" >Quiz</a> </li> <li> <a href="/en/library/earth-science/6/earth-structure/69/resources" >Teach with this</a> </li> </ul> </nav> <script type="application/ld+json"> { "@context": "http://schema.org", "@type": "AudioObject", "contentUrl": "https://www.visionlearning.com/img/library/moduleAudio/module_69.mp3", "description": "Recording of Earth Structure : Earth's interior structure is composed of layers that vary by composition and behavior. Using principles of physics like gravity and wave motion, this module explains how scientists have determined Earth's deep structure. Different types of seismic waves are discussed. The module details both compositional and mechanical layers of Earth.", "encodingFormat": "mp3", "name": "module_69.mp3" } </script> <div class="module__audio"> <div class="audio-player border border-radius"> <audio id="audio"> <source src="https://www.visionlearning.com/img/library/moduleAudio/module_69.mp3" type="audio/mpeg"> Your browser does not support the audio element. </audio> <div class="audio-player__title"> <p>Listen to this reading</p> <span class="audio-player__timestamp" id="timestamp"> 00:00 </span> </div> <div class="audio-player__controls" id="controls"> <button class="button button--icon-only" id="play-pause-button"> <span class="icon icon-play" aria-hidden="true"></span> </button> <div class="audio-player__progress" id="progress-bar" tabindex="0" aria-valuemin="0" aria-valuemax="100" aria-valuenow="0" aria-label="Use arrow keys to forward or rewind the audio" role="slider"> <div class="audio-player__progress__fill"> <span class="audio-player__thumb"></span> </div> </div> <div class="audio-player__volume-container"> <button id="mute-button"> <span class="icon icon-volume"></span> </button> <div class="audio-player__volume" tabindex="0" aria-valuemin="0" aria-valuemax="100" aria-valuenow="100" aria-label="Use arrow keys to adjust volume" role="slider"> <div class="audio-player__volume__fill"> <span class="audio-player__thumb"></span> </div> </div> </div> </div> </div> </div> <hr class="module__divider" />  <div class="module__tools"> <aside class="module__tools__container border-radius box-shadow-1"> <div class="tabs tabs--toggle-mobile--lg" role="tablist"> <ul class="tab__buttons"> <li> <button class="button button--icon-over-text" aria-label="In this module" aria-controls="tab-panel-module__tools" aria-selected="true" role="tab"> <span class="button__icon"> <span class="icon icon-list" aria-hidden="true"></span> </span> <span class="button__text">Contents</span> </button> </li> <li> <button class="button button--icon-over-text" aria-controls="tab-panel-toggle-terms" aria-selected="false" role="tab"> <span class="button__icon"> <span class="icon icon-glossary-highlight"></span> </span> <span class="button__text">Glossary Terms</span> </button> </li> <li> <button class="button button--icon-over-text" aria-controls="tab-panel-toggle-ngss" aria-selected="false" role="tab"> <span class="button__icon"> <span class="icon icon-ngss" aria-hidden="true"></span> </span> <span class="button__text">NGSS</span> </button> </li> </ul> <div class="tabs__panel shown" id="tab-panel-module__tools" aria-labelledby="tab-button-module__tools" role="tabpanel"> <div class="table-of-contents"> <p class="table-of-contents__title"> Table of Contents </p> <ul class="table-of-contents__nav"> <li><a href="/en/library/earth-science/6/earth-structure/69#toc_1">Evidence about Earth's interior</a> </li> <li><a href="/en/library/earth-science/6/earth-structure/69#toc_2">Seismic waves</a> </li> <li><a href="/en/library/earth-science/6/earth-structure/69#toc_3">The Moho</a> </li> <li><a href="/en/library/earth-science/6/earth-structure/69#toc_4">Shadow zones</a> </li> <li><a href="/en/library/earth-science/6/earth-structure/69#toc_5">The layers of Earth</a> </li> <li> <ul> <li><a href="/en/library/earth-science/6/earth-structure/69#toc2_1">Compositional layers</a> </li> </ul> </li> <li> <ul> <li><a href="/en/library/earth-science/6/earth-structure/69#toc2_2">Mechanical layers</a> </li> </ul> </li> </ul> </div> </div> <div class="tabs__panel" id="tab-panel-toggle-terms" aria-labelledby="tab-button-toggle-terms" role="tabpanel"> <div class="reading-toggle"> <div class="reading-toggle__switch"> <div class="form-entry__option__switch"> <label> <input type="checkbox" name="termsToggleSwitch" id="terms-toggle-switch" /> <span class="switch__slider"></span> <span class="option__label text-decoration-none font-size-md"> Highlight Glossary Terms </span> </label> </div> </div> <div class="reading-toggle__help"> <p> <em> Activate glossary term highlighting to easily identify key terms within the module. Once highlighted, you can click on these terms to view their definitions. </em> </p> </div> </div> <div class="glossary-container"></div> </div> <div class="tabs__panel" id="tab-panel-toggle-ngss" aria-labelledby="tab-button-toggle-ngss" role="tabpanel"> <div class="reading-toggle"> <div class="reading-toggle__switch"> <div class="form-entry__option__switch"> <label> <input type="checkbox" name="ngssToggleSwitch" id="ngss-toggle-switch" /> <span class="switch__slider"></span> <span class="option__label text-decoration-none font-size-md"> Show NGSS Annotations </span> </label> </div> </div> <div class="reading-toggle__help"> <p> <em> Activate NGSS annotations to easily identify NGSS standards within the module. Once highlighted, you can click on them to view these standards. </em> </p> </div> </div> <div class="ngss-container"></div> </div> </div> </aside> <div class="margin-3"> <script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js?client=ca-pub-9561344156007092" crossorigin="anonymous"></script>  <ins class="adsbygoogle" style="display:block" data-ad-client="ca-pub-9561344156007092" data-ad-slot="7634263342" data-ad-format="auto" data-full-width-responsive="true"></ins> <script> (adsbygoogle = window.adsbygoogle || []).push({}); </script> </div> </div>     <div class="module__main"> <div class="module__main__container"> <div class="accordion">  <button class="accordion__button" id="acc-button-key-concepts" data-accordion="button" aria-controls="acc-panel-key-concepts" aria-expanded="true" tabindex="0"> Did you know? </button> <div class="accordion__panel shown show" id="acc-panel-key-concepts" data-accordion="panel" aria-labelledby="acc-button-key-concepts" role="region"> <div class="accordion__panel__content"> <p>Did you know that although earthquakes can be very destructive, they provide a wealth of information about Earth's interior? Miners, geologists, and others have always wondered what lies below the surface of Earth, but heat and pressure make it impossible to explore deep into its interior. However, seismic waves produced by earthquakes reveal the structure and composition of our planet.</p> </div> </div>  <button class="accordion__button" id="acc-button-table-of-contents" data-accordion="button" aria-controls="acc-panel-table-of-contents" aria-expanded="false" tabindex="0"> Key concepts </button> <div class="accordion__panel" id="acc-panel-table-of-contents" data-accordion="panel" aria-labelledby="acc-button-table-of-contents" role="region" aria-hidden="true"> <div class="accordion__panel__content"> <ul class="bulleted"><li><p>Our knowledge about the structure Earth's interior comes from studying how different types of seismic waves, created by earthquakes, travel through Earth.</p></li> <li><p>Earth is composed of multiple layers, which can be defined either by composition or by mechanical properties.</p></li> <li><p>The crust, mantle, and core are defined by differences in composition.</p></li> <li><p>The lithosphere, asthenosphere, mesosphere, and outer and inner cores are defined by differences in mechanical properties.</p></li></ul> </div> </div>  <button class="accordion__button" id="acc-button-terms-you-should-know" data-accordion="button" aria-controls="acc-panel-terms-you-should-know" aria-expanded="false" tabindex="0"> Terms you should know </button> <div class="accordion__panel" id="acc-panel-terms-you-should-know" data-accordion="panel" aria-labelledby="acc-button-terms-you-should-know" role="region" aria-hidden="true"> <div class="accordion__panel__content"> <dl> <dt><a href="/en/glossary/view/boundary">boundary </a></dt> <dd> a line or limit that divides one area from another </dd> <dt><a href="/en/glossary/view/dense">dense </a></dt> <dd> compact, packed close together, having a high mass in relation to volume </dd> <dt>wave </dt> <dd> a motion of rising and falling in curves; undulation</dd> </dl> </div> </div> </div> <section> <p>The deepest places on Earth are in South Africa, where mining companies have excavated 3.5 km into Earth to extract gold. No one has seen deeper into Earth than the South African miners because the <mark class="term" data-term="heat" data-term-def="A measure of the total internal energy of a substance that can be increased or decreased when objects with different temperatures&hellip;" data-term-url="/en/glossary/view/heat/1506">heat</mark> and pressure felt at these depths prevents humans from going much deeper. Yet Earth's radius is 6,370 km – how do we begin to know what is below the thin skin of the Earth when we cannot see it?</p> <p><section id="toc_1" class=""> <h2>Evidence about Earth's interior</h2></p> <p>Isaac <mark class="term" data-term="Isaac Newton" data-term-url="/en/glossary/view/Isaac+Newton" data-term-def="English alchemist, physicist, astronomer and mathematician born in Woolsthorpe-by-Colsterworth, Lincolnshire (1643-1727). In 1672, Newton offered an experimental proof that light is&hellip;">Newton</mark> was one of the first scientists to theorize about the structure of Earth. <mark id="ngss-5" class="ngss">Based on his studies of the <mark class="term" data-term="force" data-term-def="An influence (a "push or pull") that changes the motion of a moving object (e.g., slows it down, speeds it up,&hellip;" data-term-url="/en/glossary/view/force/883">force</mark> of <mark class="term" data-term="gravity" data-term-def="The natural force that attracts a body toward the center of the Earth, or toward another physical body having mass." data-term-url="/en/glossary/view/gravity/11223">gravity</mark>, <mark class="term" data-term="newton" data-term-def="A metric (or SI) unit measuring force and named for English physicist Isaac Newton. One newton (N) represents the force needed&hellip;" data-term-url="/en/glossary/view/newton/5596">Newton</mark> calculated the <mark class="term" data-term="average" data-term-def="In statistics, average commonly refers to the arithmetic mean, also called simply "mean," which is one measure of the mid-point of&hellip;" data-term-url="/en/glossary/view/average/8542">average</mark> <mark class="term" data-term="density" data-term-def="A measure of the compactness of a substance given by the mass per unit volume (d = m/v). Common units of&hellip;" data-term-url="/en/glossary/view/density/863">density</mark> of Earth and found it to be more than twice the density of the rocks near the <mark class="term" data-term="surface" data-term-def="The outside or external part; the topside face of something." data-term-url="/en/glossary/view/surface/8275">surface</mark>.</mark> From these results, Newton realized that the interior of Earth had to be much denser than the surface rocks. His findings excluded the possibility of a cavernous, fiery underworld inhabited by the dead, but still left many questions unanswered. Where does the denser material begin? How does the composition differ from surface rocks?</p><p>Volcanic vents like <mark class="term" data-term="Shiprock" data-term-def="Although it is commonly called a "volcanic neck," Shiprock (located near the Four Corners of Utah, Colorado, Arizona, and New Mexico)&hellip;" data-term-url="/en/glossary/view/Shiprock/1861">Shiprock</mark> occasionally bring up pieces of Earth from as deep as 150 km, but these rocks are rare, and we have little hope of taking Jules Verne's <em>Journey to the Center of the Earth.</em> Instead, much of our knowledge about the internal structure of the Earth comes from remote <mark class="term" data-term="observation" data-term-def="1. The act of noticing something. 2. A record of that which has been noticed." data-term-url="/en/glossary/view/observation/8255">observations</mark> – specifically, from observations of <mark class="term" data-term="earthquake" data-term-def="1. The sudden motion or slip along a fault. 2. The ground shaking that results from the release of seismic energy&hellip;" data-term-url="/en/glossary/view/earthquake/1664">earthquakes</mark>.</p><p><mark id="ngss-6" class="ngss">Earthquakes can be extremely destructive for humans, but they provide a wealth of information about Earth's interior. This is because every <mark class="term" data-term="earthquake" data-term-def="1. The sudden motion or slip along a fault. 2. The ground shaking that results from the release of seismic energy&hellip;" data-term-url="/en/glossary/view/earthquake/1664">earthquake</mark> sends out an array of <mark class="term" data-term="seismic wave" data-term-def="A form of sound wave that travels away from the source of an earthquake. There are several types of seismic waves,&hellip;" data-term-url="/en/glossary/view/seismic+wave/1674">seismic waves</mark> in all directions, similar to the way that throwing a stone into a pond sends out <mark class="term" data-term="waves" data-term-def="The motion of rising and falling in curves; undulation." data-term-url="/en/glossary/view/waves/8274">waves</mark> through the water.</mark> <mark id="ngss-7" class="ngss">Observing the behavior of these seismic <mark class="term" data-term="waves" data-term-url="/en/glossary/view/waves" data-term-def="The motion of rising and falling in curves; undulation.">waves</mark> as they travel through the Earth gives us insight into the materials the waves move through.</mark></p> <div class="container margin-y-4 text-align-center"> <script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js?client=ca-pub-9561344156007092" crossorigin="anonymous"></script>  <ins class="adsbygoogle" style="display:inline-block;width:300px;height:250px" data-ad-client="ca-pub-9561344156007092" data-ad-slot="9090201191"></ins> <script> (adsbygoogle = window.adsbygoogle || []).push({}); </script> </div> <div class="comprehension-checkpoint margin-y-4"> <h6 class="comprehension-checkpoint__header"> <span> <span class="icon icon-question"></span> </span> Comprehension Checkpoint </h6> <form name="cc6167"> <div class="form-entry"> <div class="form-entry__field"> <span class="form-entry__field__label">Our knowledge about the internal structure of Earth comes mainly from</span> <div class="form-entry__option"> <div class="form-entry__option__radio" data-answer="correct"> <label> <input id="q1-6167-0-option-a" name="quiz-option-6167" type="radio" value="earthquakes." > <span class="option__label"> <span class="screen-reader-only">a.</span> earthquakes. </span> </label> <span class="quiz__response" id="response-6167-0"> <strong>Correct!</strong> </span> </div> <div class="form-entry__option__radio" data-answer="incorrect"> <label> <input id="q1-6167-1-option-b" name="quiz-option-6167" type="radio" value="deep mine shafts." > <span class="option__label"> <span class="screen-reader-only">b.</span> deep mine shafts. </span> </label> <span class="quiz__response" id="response-6167-1"> <strong>Incorrect.</strong> </span> </div> </div> </div> </div> </form> </div> </section> <section id="toc_2"> <h2>Seismic waves</h2><p>An <mark class="term" data-term="earthquake" data-term-def="1. The sudden motion or slip along a fault. 2. The ground shaking that results from the release of seismic energy&hellip;" data-term-url="/en/glossary/view/earthquake/1664">earthquake</mark> occurs when rocks in a fault zone suddenly slip past each other, releasing stress that has built up over time. The slippage releases seismic <mark class="term" data-term="energy" data-term-def="An abstract property defined as the capacity to do work. The basic forms of energy include chemical, electrical, mechanical, nuclear, and&hellip;" data-term-url="/en/glossary/view/energy/1497">energy</mark>, which is dissipated through two kinds of <mark class="term" data-term="waves" data-term-def="The motion of rising and falling in curves; undulation." data-term-url="/en/glossary/view/waves/8274">waves</mark>, <mark class="term" data-term="P-wave" data-term-def="A compression wave produced by an earthquake. The “P” is from the Italian “primero,” indicating that the P-waves were the first&hellip;" data-term-url="/en/glossary/view/P~wave/1672">P-waves</mark> and <mark class="term" data-term="S-wave" data-term-def="A “ripple” wave produced by an earthquake. The “S” is from the Italian “Segundo,” indicating that S-waves arrived at seismic stations&hellip;" data-term-url="/en/glossary/view/S~wave/1676">S-waves</mark>. The distinction between these two <mark class="term" data-term="waves" data-term-url="/en/glossary/view/waves" data-term-def="The motion of rising and falling in curves; undulation.">waves</mark> is easy to picture with a stretched-out Slinky<sup>®</sup>. If you push on one end, a compression wave passes through the Slinky<sup>®</sup> parallel to its length (see P-waves video). If instead you move one end up and down rapidly, a "ripple" wave moves through the Slinky<sup>®</sup> (see S-waves video). The compression waves are P-waves, and the ripple waves are S-waves.</p><div class="figure-two-across"> <div> <div class="figure"><figure> <a href="https://www.visionlearning.com/images/figure-images/69-a.mp4" class="mfp-iframe" title="Illustration of a P-wave/compression wave."> <span class="fa fa-search-plus"></span> <img src="https://www.visionlearning.com//images/figure-images/69-a.jpg" alt="placeholder"> </a> <figcaption>Illustration of a <span class="no-wrap">P-wave/compression</span> wave.</figcaption> </figure></div> </div> <div> <div class="figure"><figure> <a href="https://www.visionlearning.com/images/figure-images/69-b.mp4" class="mfp-iframe" title="Illustration of a S-wave/ripple wave."> <span class="fa fa-search-plus"></span> <img src="https://www.visionlearning.com/images/figure-images/69-b.jpg" alt="placeholder"> </a> <figcaption>Illustration of a <span class="no-wrap">S-wave/ripple</span> wave.</figcaption> </figure></div> </div> </div><p><mark id="ngss-8" class="ngss">Both kinds of <mark class="term" data-term="waves" data-term-def="The motion of rising and falling in curves; undulation." data-term-url="/en/glossary/view/waves/8274">waves</mark> can <mark class="term" data-term="reflect" data-term-def="To change direction in response to hitting a surface; to bounce off in a different direction." data-term-url="/en/glossary/view/reflect/8276">reflect</mark> off of boundaries between different materials: They can also refract, or bend, when they cross a <mark class="term" data-term="boundary" data-term-def="A line or limit that divides one area from another." data-term-url="/en/glossary/view/boundary/8272">boundary</mark> into a different material. But the two types of <mark class="term" data-term="waves" data-term-url="/en/glossary/view/waves" data-term-def="The motion of rising and falling in curves; undulation.">waves</mark> behave differently depending on the composition of the material they are passing through. One of the biggest differences is that <mark class="term" data-term="S-wave" data-term-def="A “ripple” wave produced by an earthquake. The “S” is from the Italian “Segundo,” indicating that S-waves arrived at seismic stations&hellip;" data-term-url="/en/glossary/view/S~wave/1676">S-waves</mark> cannot travel through <mark class="term" data-term="liquid" data-term-def="The state of matter characterized by its condensed nature and ability to flow. Unlike gases, molecules within a liquid often experience&hellip;" data-term-url="/en/glossary/view/liquid/8727">liquids</mark> whereas <mark class="term" data-term="P-wave" data-term-def="A compression wave produced by an earthquake. The “P” is from the Italian “primero,” indicating that the P-waves were the first&hellip;" data-term-url="/en/glossary/view/P~wave/1672">P-waves</mark> can. We feel the arrival of the P- and S-waves at a given location as a ground-shaking <mark class="term" data-term="earthquake" data-term-def="1. The sudden motion or slip along a fault. 2. The ground shaking that results from the release of seismic energy&hellip;" data-term-url="/en/glossary/view/earthquake/1664">earthquake</mark>.</mark></p><p><mark id="ngss-14" class="ngss">If Earth were the same composition all the way through its interior, <mark class="term" data-term="seismic wave" data-term-def="A form of sound wave that travels away from the source of an earthquake. There are several types of seismic waves,&hellip;" data-term-url="/en/glossary/view/seismic+wave/1674">seismic waves</mark> would radiate outward from their source (an earthquake) and behave exactly as other <mark class="term" data-term="waves" data-term-def="The motion of rising and falling in curves; undulation." data-term-url="/en/glossary/view/waves/8274">waves</mark> behave – taking longer to travel further and dying out in <mark class="term" data-term="velocity" data-term-def="The speed and direction in which a given object is traveling, measured in distance per unit time (for example meters per&hellip;" data-term-url="/en/glossary/view/velocity/5594">velocity</mark> and strength with distance, a <mark class="term" data-term="process" data-term-def="Method, procedure; series of actions or steps." data-term-url="/en/glossary/view/process/8256">process</mark> called <mark class="term" data-term="attenuation" data-term-def="The behavior of waves as they radiate out from a source. As distance from the source increases, intensity of the waves&hellip;" data-term-url="/en/glossary/view/attenuation/1660">attenuation</mark>.</mark> (See Figure 1.)</p>  <div class="figure"> <figure> <button class="lightbox-button" data-lightbox-src="/img/library/large_images/image_4858.gif" data-lightbox="image"> <img src="/img/library/modules/mid69/Image/VLObject-4858-100331060340.gif" alt="Figure 1: Seismic waves in an Earth of the same composition." /> </button> <figcaption> <p><strong>Figure 1</strong>: Seismic waves in an Earth of the same composition.</p> </figcaption> </figure> </div> <p>Given <mark class="term" data-term="Isaac Newton" data-term-url="/en/glossary/view/Isaac+Newton" data-term-def="English alchemist, physicist, astronomer and mathematician born in Woolsthorpe-by-Colsterworth, Lincolnshire (1643-1727). In 1672, Newton offered an experimental proof that light is&hellip;">Newton</mark>'s <mark class="term" data-term="observation" data-term-def="1. The act of noticing something. 2. A record of that which has been noticed." data-term-url="/en/glossary/view/observation/8255">observations</mark>, if we assume Earth's <mark class="term" data-term="density" data-term-def="A measure of the compactness of a substance given by the mass per unit volume (d = m/v). Common units of&hellip;" data-term-url="/en/glossary/view/density/863">density</mark> increases evenly with depth because of the overlying pressure, wave <mark class="term" data-term="velocity" data-term-def="The speed and direction in which a given object is traveling, measured in distance per unit time (for example meters per&hellip;" data-term-url="/en/glossary/view/velocity/5594">velocity</mark> will also increase with depth and the <mark class="term" data-term="waves" data-term-def="The motion of rising and falling in curves; undulation." data-term-url="/en/glossary/view/waves/8274">waves</mark> will continuously refract, traveling along curved paths back towards the <mark class="term" data-term="surface" data-term-def="The outside or external part; the topside face of something." data-term-url="/en/glossary/view/surface/8275">surface</mark>. Figure 1 shows the kind of pattern we would expect to see in this case. By the early 1900s, when seismographs were installed worldwide, it quickly became clear that Earth could not possibly be so simple.</p></section> <section id="toc_3"> <h2>The Moho</h2><p>Andrija Mohorovičić was a Croatian scientist who recognized the importance of establishing a <mark class="term" data-term="network" data-term-def="An interconnected system; an interrelated net-like arrangement of parts." data-term-url="/en/glossary/view/network/8740">network</mark> of seismometers. Though his scientific career had begun in <mark class="term" data-term="meteorology" data-term-def="The scientific study of the atmosphere including the processes that cause particular weather conditions." data-term-url="/en/glossary/view/meteorology/11227">meteorology</mark>, he shifted his <mark class="term" data-term="research" data-term-def="A study or an investigation." data-term-url="/en/glossary/view/research/8257">research</mark> pursuits to seismology around 1900, and installed several of the most advanced seismometers around central Europe in 1908. His timing was fortuitous, as a large <mark class="term" data-term="earthquake" data-term-def="1. The sudden motion or slip along a fault. 2. The ground shaking that results from the release of seismic energy&hellip;" data-term-url="/en/glossary/view/earthquake/1664">earthquake</mark> occurred in the Kupa Valley in October 1909, which Mohorovičić felt at his home in Zagreb, Croatia. <mark id="ngss-9" class="ngss">He made careful <mark class="term" data-term="observation" data-term-def="1. The act of noticing something. 2. A record of that which has been noticed." data-term-url="/en/glossary/view/observation/8255">observations</mark> of the arrivals of P- and <mark class="term" data-term="S-wave" data-term-def="A “ripple” wave produced by an earthquake. The “S” is from the Italian “Segundo,” indicating that S-waves arrived at seismic stations&hellip;" data-term-url="/en/glossary/view/S~wave/1676">S-waves</mark> at his newly-installed stations, and noticed that the <mark class="term" data-term="P-wave" data-term-def="A compression wave produced by an earthquake. The “P” is from the Italian “primero,” indicating that the P-waves were the first&hellip;" data-term-url="/en/glossary/view/P~wave/1672">P-waves</mark> that measured more than 200 km away from an earthquake's epicenter arrived with higher <mark class="term" data-term="velocity" data-term-url="/en/glossary/view/velocity" data-term-def="The speed and direction in which a given object is traveling, measured in distance per unit time (for example meters per&hellip;">velocities</mark> than those within a 200 km radius. Although these results ran counter to the concept of <mark class="term" data-term="attenuation" data-term-def="The behavior of waves as they radiate out from a source. As distance from the source increases, intensity of the waves&hellip;" data-term-url="/en/glossary/view/attenuation/1660">attenuation</mark>, they could be explained if the <mark class="term" data-term="waves" data-term-def="The motion of rising and falling in curves; undulation." data-term-url="/en/glossary/view/waves/8274">waves</mark> that arrived with faster velocities traveled through a medium that allowed them to speed up, having encountered a structural <mark class="term" data-term="boundary" data-term-def="A line or limit that divides one area from another." data-term-url="/en/glossary/view/boundary/8272">boundary</mark> at a greater depth.</mark></p><p><mark id="ngss-10" class="ngss">This recognition allowed Mohorovičić to define the first major <mark class="term" data-term="boundary" data-term-def="A line or limit that divides one area from another." data-term-url="/en/glossary/view/boundary/8272">boundary</mark> within Earth’s interior – the boundary between the <mark class="term" data-term="crust" data-term-def="The uppermost 5-70 km of the Earth. There are two types of crust: continental and oceanic. Continental crust ranges from 10-70&hellip;" data-term-url="/en/glossary/view/crust/880">crust</mark>, which forms the <mark class="term" data-term="surface" data-term-def="The outside or external part; the topside face of something." data-term-url="/en/glossary/view/surface/8275">surface</mark> of Earth, and a denser layer below, called the <mark class="term" data-term="mantle" data-term-def="The middle portion of the interior of the Earth, starting below the crust at 5-70 km below the Earth’s surface and&hellip;" data-term-url="/en/glossary/view/mantle/889">mantle</mark> (Mohorovičić, 1910). <mark class="term" data-term="seismic wave" data-term-def="A form of sound wave that travels away from the source of an earthquake. There are several types of seismic waves,&hellip;" data-term-url="/en/glossary/view/seismic+wave/1674">Seismic waves</mark> travel faster in the mantle than they do in the crust because it is composed of denser material. Thus, stations further away from the source of an <mark class="term" data-term="earthquake" data-term-def="1. The sudden motion or slip along a fault. 2. The ground shaking that results from the release of seismic energy&hellip;" data-term-url="/en/glossary/view/earthquake/1664">earthquake</mark> received <mark class="term" data-term="waves" data-term-def="The motion of rising and falling in curves; undulation." data-term-url="/en/glossary/view/waves/8274">waves</mark> that had made part of their journey through the denser rocks of the mantle. The <mark class="term" data-term="waves" data-term-url="/en/glossary/view/waves" data-term-def="The motion of rising and falling in curves; undulation.">waves</mark> that reached the closer stations stayed within the crust the entire time. Although the official name of the crust-mantle boundary is the Mohorovičić discontinuity, in honor of its discoverer, it is usually called the <mark class="term" data-term="Moho" data-term-def="The commonly used abbreviation of the Mohorovicic Discontinuity, the boundary between the crust and the mantle." data-term-url="/en/glossary/view/Moho/1668">Moho</mark> </mark>(see the interactive animation below).</p> <a class="interactive-animation" href="https://www.visionlearning.com/library/animations/Seismic_Wave/Seismic_Wave.html" target="_blank"> <img class="interactive-animation__image" src="/images/anim-snaps/ia-seismic-wave.png" width="200" alt="Moho" /> <p class="interactive-animation__title"> <em>Interactive Animation:</em> <strong class="link-new-window"> <span class="link__text">Moho</span> </strong> </p> </a> </section> <section id="toc_4"> <h2>Shadow zones</h2><p><mark id="ngss-11" class="ngss">Another <mark class="term" data-term="observation" data-term-def="1. The act of noticing something. 2. A record of that which has been noticed." data-term-url="/en/glossary/view/observation/8255">observation</mark> made by <mark class="term" data-term="seismologist" data-term-def="A scientist who studies earthquakes and the behavior of seismic waves in the Earth." data-term-url="/en/glossary/view/seismologist/1675">seismologists</mark> was the fact that <mark class="term" data-term="P-wave" data-term-def="A compression wave produced by an earthquake. The “P” is from the Italian “primero,” indicating that the P-waves were the first&hellip;" data-term-url="/en/glossary/view/P~wave/1672">P-waves</mark> die out about 105 <mark class="term" data-term="degree" data-term-def="[<strong>temperature</strong>] One graduated unit of measure on a Fahrenheit or Celsius temperature scale. On the Kelvin scale, graduations are called Kelvins. <br> <br> [<strong>geometry</strong>]&hellip;" data-term-url="/en/glossary/view/degree/8535">degrees</mark> away from an <mark class="term" data-term="earthquake" data-term-def="1. The sudden motion or slip along a fault. 2. The ground shaking that results from the release of seismic energy&hellip;" data-term-url="/en/glossary/view/earthquake/1664">earthquake</mark>, and then reappear about 140 degrees away, arriving much later than expected. This region that lacks P-waves is called the P-wave shadow zone (Figure 2). <mark class="term" data-term="S-wave" data-term-def="A “ripple” wave produced by an earthquake. The “S” is from the Italian “Segundo,” indicating that S-waves arrived at seismic stations&hellip;" data-term-url="/en/glossary/view/S~wave/1676">S-waves</mark>, on the other hand, die out completely around 105 degrees from the earthquake (Figure 2). Remember that S-waves are unable to travel through <mark class="term" data-term="liquid" data-term-def="The state of matter characterized by its condensed nature and ability to flow. Unlike gases, molecules within a liquid often experience&hellip;" data-term-url="/en/glossary/view/liquid/8727">liquid</mark>. The S-wave shadow zone indicates that there is a liquid layer deep within Earth that stops all S-waves but not the P-waves.</mark></p> <div class="container margin-y-4 text-align-center"> <script async src="https://pagead2.googlesyndication.com/pagead/js/adsbygoogle.js?client=ca-pub-9561344156007092" crossorigin="anonymous"></script>  <ins class="adsbygoogle" style="display:inline-block;width:300px;height:250px" data-ad-client="ca-pub-9561344156007092" data-ad-slot="3321739899"></ins> <script> (adsbygoogle = window.adsbygoogle || []).push({}); </script> </div>  <div class="figure"> <figure> <button class="lightbox-button" data-lightbox-src="/img/library/large_images/image_4863.png" data-lightbox="image"> <img src="/img/library/modules/mid69/Image/VLObject-4863-100331070335.gif" alt="Figure 2:The P-wave and S-wave shadow zones." /> </button> <figcaption> <p><strong>Figure 2:</strong>The P-wave and S-wave shadow zones.</p> </figcaption> </figure> </div> <p><mark id="ngss-12" class="ngss">In 1914, <mark class="term" data-term="Beno Gutenberg" data-term-def="German geophysicist, born in Darmstadt (1889-1960). Gutenberg is best known for precisely determining the depth to the core of the Earth&hellip;" data-term-url="/en/glossary/view/Gutenberg%2C+Beno/4487">Beno Gutenberg</mark>, a German <mark class="term" data-term="seismologist" data-term-def="A scientist who studies earthquakes and the behavior of seismic waves in the Earth." data-term-url="/en/glossary/view/seismologist/1675">seismologist</mark>, used these shadow zones to calculate the size of another layer inside of the Earth, called its <mark class="term" data-term="core" data-term-def="The innermost layer of the Earth, which starts at ~2900 km depth. The core is composed mainly of iron and consists&hellip;" data-term-url="/en/glossary/view/core/1663">core</mark>. He defined a sharp core-mantle <mark class="term" data-term="boundary" data-term-def="A line or limit that divides one area from another." data-term-url="/en/glossary/view/boundary/8272">boundary</mark> at a depth of 2,900 km, where <mark class="term" data-term="P-wave" data-term-def="A compression wave produced by an earthquake. The “P” is from the Italian “primero,” indicating that the P-waves were the first&hellip;" data-term-url="/en/glossary/view/P~wave/1672">P-waves</mark> were refracted and slowed and <mark class="term" data-term="S-wave" data-term-def="A “ripple” wave produced by an earthquake. The “S” is from the Italian “Segundo,” indicating that S-waves arrived at seismic stations&hellip;" data-term-url="/en/glossary/view/S~wave/1676">S-waves</mark> were stopped.</mark></p> <div class="comprehension-checkpoint margin-y-4"> <h6 class="comprehension-checkpoint__header"> <span> <span class="icon icon-question"></span> </span> Comprehension Checkpoint </h6> <form name="cc6175"> <div class="form-entry"> <div class="form-entry__field"> <span class="form-entry__field__label">Scientists figured out that there is a liquid layer deep within Earth by observing</span> <div class="form-entry__option"> <div class="form-entry__option__radio" data-answer="correct"> <label> <input id="q1-6175-0-option-a" name="quiz-option-6175" type="radio" value="when there is an earthquake, S-waves -- which cannot travel through liquid -- die out at 105 degrees." > <span class="option__label"> <span class="screen-reader-only">a.</span> when there is an earthquake, S-waves -- which cannot travel through liquid -- die out at 105 degrees. </span> </label> <span class="quiz__response" id="response-6175-0"> <strong>Correct!</strong> </span> </div> <div class="form-entry__option__radio" data-answer="incorrect"> <label> <input id="q1-6175-1-option-b" name="quiz-option-6175" type="radio" value="that ponds of water have waves after an earthquake." > <span class="option__label"> <span class="screen-reader-only">b.</span> that ponds of water have waves after an earthquake. </span> </label> <span class="quiz__response" id="response-6175-1"> <strong>Incorrect.</strong> </span> </div> </div> </div> </div> </form> </div> </section> <section id="toc_5"> <h2>The layers of Earth</h2><p><mark id="ngss-13" class="ngss">On the basis of these and other <mark class="term" data-term="observation" data-term-def="1. The act of noticing something. 2. A record of that which has been noticed." data-term-url="/en/glossary/view/observation/8255">observations</mark>, geophysicists have created a cross-section of Earth</mark> (Figure 3). The early seismological studies previously discussed led to definitions of compositional boundaries; for example, imagine oil floating on top of water – they are two different materials, so there is a compositional <mark class="term" data-term="boundary" data-term-def="A line or limit that divides one area from another." data-term-url="/en/glossary/view/boundary/8272">boundary</mark> between them.</p><p>Later studies highlighted <mark class="term" data-term="mechanical" data-term-def="Involving physical force or motion." data-term-url="/en/glossary/view/mechanical/8516">mechanical</mark> boundaries, which are defined on the basis of how materials act, not on their composition. Water and oil have the same mechanical properties – they are both <mark class="term" data-term="liquid" data-term-def="The state of matter characterized by its condensed nature and ability to flow. Unlike gases, molecules within a liquid often experience&hellip;" data-term-url="/en/glossary/view/liquid/8727">liquids</mark>. On the other hand, water and ice have the same composition, but water is a <mark class="term" data-term="fluid" data-term-def="Able to flow because the intermolecular forces allow the molecules to move around in relation to one another. Both liquids and&hellip;" data-term-url="/en/glossary/view/fluid/8724">fluid</mark> with far different mechanical properties than <mark class="term" data-term="solid" data-term-def="A collection of atoms or molecules that are held together so that, under constant conditions, they maintain a defined shape and&hellip;" data-term-url="/en/glossary/view/solid/7571">solid</mark> ice.</p>  <div class="figure"> <figure> <button class="lightbox-button" data-lightbox-src="/img/library/large_images/image_4859.gif" data-lightbox="image"> <img src="/img/library/modules/mid69/Image/VLObject-4859-100331070306.png" alt="Figure 3: Compositional and mechanical layers of Earth's structure." /> </button> <figcaption> <p><strong>Figure 3:</strong> Compositional and mechanical layers of Earth's structure.</p> <span class="credit">image ©Dr. Anne E. Egger CC BY-NC-SA 4.0</span> </figcaption> </figure> </div> </section> <section id="toc2_1"><h3>Compositional layers</h3><p><mark id="ngss-15" class="ngss">There are two major types of crust: <mark class="term" data-term="crust" data-term-def="The uppermost 5-70 km of the Earth. There are two types of crust: continental and oceanic. Continental crust ranges from 10-70&hellip;" data-term-url="/en/glossary/view/crust/880">crust</mark> that makes up the ocean floors and crust that makes up the continents. <mark class="term" data-term="oceanic crust" data-term-def="The uppermost layer of the earth beneath the ocean basins. Oceanic crust is constantly forming at mid-ocean ridges and is constantly&hellip;" data-term-url="/en/glossary/view/oceanic+crust/1670">Oceanic crust</mark> is composed entirely of <mark class="term" data-term="basalt" data-term-def="A dark, fine-grained igneous rock formed by cooling of iron- and magnesium-rich lava above the surface of the earth. Basalt is&hellip;" data-term-url="/en/glossary/view/basalt/1661">basalt</mark> extruded at <mark class="term" data-term="mid-ocean ridge" data-term-def="A linear, elevated region of the ocean floor where magma is upwelling and extruding, and new crust is forming. The ridges&hellip;" data-term-url="/en/glossary/view/mid~ocean+ridge/868">mid-ocean ridges</mark>, resulting in a thin (~ 5 km), relatively <mark class="term" data-term="dense" data-term-def="Compact, packed close together; having a high mass in relation to volume." data-term-url="/en/glossary/view/dense/8273">dense</mark> (~3.0 g/cm<sup><font size= "1">3</font></sup>) crust. <mark class="term" data-term="continental crust" data-term-def="The uppermost layer of the Earth that forms the continents. Unlike oceanic crust, continental crust is created and destroyed very slowly,&hellip;" data-term-url="/en/glossary/view/continental+crust/1662">Continental crust</mark>, on the other hand, is made primarily of less dense rock such as <mark class="term" data-term="granite" data-term-def="A light-colored, coarse-grained igneous rock formed by cooling of silica-rich magma below the surface of the Earth. Granite is considered to&hellip;" data-term-url="/en/glossary/view/granite/1665">granite</mark> (~2.7 g/cm<sup><font size= "1">3</font></sup>). It is much thicker than oceanic crust, ranging from 15 to 70 km. At the base of the crust is the <mark class="term" data-term="Moho" data-term-def="The commonly used abbreviation of the Mohorovicic Discontinuity, the boundary between the crust and the mantle." data-term-url="/en/glossary/view/Moho/1668">Moho</mark>, below which is the <mark class="term" data-term="mantle" data-term-def="The middle portion of the interior of the Earth, starting below the crust at 5-70 km below the Earth’s surface and&hellip;" data-term-url="/en/glossary/view/mantle/889">mantle</mark>, which contains rocks made of a denser material called <mark class="term" data-term="peridotite" data-term-def="A rock composed primarily of the mineral olivine. The high percentage of iron and magnesium in olivine creates a very dense&hellip;" data-term-url="/en/glossary/view/peridotite/1671">peridotite</mark> (~3.4 g/cm<sup><font size= "1">3</font></sup>). This compositional change is predicted by the behavior of <mark class="term" data-term="seismic wave" data-term-def="A form of sound wave that travels away from the source of an earthquake. There are several types of seismic waves,&hellip;" data-term-url="/en/glossary/view/seismic+wave/1674">seismic waves</mark> and it is confirmed in the few samples of rocks from the mantle that we do have.</mark></p><p><mark id="ngss-16" class="ngss">At the core-mantle <mark class="term" data-term="boundary" data-term-def="A line or limit that divides one area from another." data-term-url="/en/glossary/view/boundary/8272">boundary</mark>, composition changes again. <mark class="term" data-term="seismic wave" data-term-def="A form of sound wave that travels away from the source of an earthquake. There are several types of seismic waves,&hellip;" data-term-url="/en/glossary/view/seismic+wave/1674">Seismic waves</mark> suggest this material is of a very high <mark class="term" data-term="density" data-term-def="A measure of the compactness of a substance given by the mass per unit volume (d = m/v). Common units of&hellip;" data-term-url="/en/glossary/view/density/863">density</mark> (10-13 g/cm<sup><font size= "-2">3</font></sup>), which can only correspond to a composition of metals rather than rock. The presence of a magnetic field around Earth also indicates a molten metallic <mark class="term" data-term="core" data-term-def="The innermost layer of the Earth, which starts at ~2900 km depth. The core is composed mainly of iron and consists&hellip;" data-term-url="/en/glossary/view/core/1663">core</mark>. Unlike the <mark class="term" data-term="crust" data-term-def="The uppermost 5-70 km of the Earth. There are two types of crust: continental and oceanic. Continental crust ranges from 10-70&hellip;" data-term-url="/en/glossary/view/crust/880">crust</mark> and the <mark class="term" data-term="mantle" data-term-def="The middle portion of the interior of the Earth, starting below the crust at 5-70 km below the Earth’s surface and&hellip;" data-term-url="/en/glossary/view/mantle/889">mantle</mark>, we don't have any samples of the core to look at, and thus there is some <mark class="term" data-term="controversy" data-term-def="An argument, disagreement, or difference of opinion that involves many people. A true scientific controversy involves a sustained debate within the&hellip;" data-term-url="/en/glossary/view/controversy/8241">controversy</mark> about its exact composition. Most scientists, however, believe that iron is the main constituent.</mark></p><p>These compositional layers are shown in Figure 3.</p> <div class="comprehension-checkpoint margin-y-4"> <h6 class="comprehension-checkpoint__header"> <span> <span class="icon icon-question"></span> </span> Comprehension Checkpoint </h6> <form name="cc6178"> <div class="form-entry"> <div class="form-entry__field"> <span class="form-entry__field__label">The crust, mantle, and core are defined as compositional layers of Earth because</span> <div class="form-entry__option"> <div class="form-entry__option__radio" data-answer="correct"> <label> <input id="q1-6178-0-option-a" name="quiz-option-6178" type="radio" value="they are made of different things." > <span class="option__label"> <span class="screen-reader-only">a.</span> they are made of different things. </span> </label> <span class="quiz__response" id="response-6178-0"> <strong>Correct!</strong> </span> </div> <div class="form-entry__option__radio" data-answer="incorrect"> <label> <input id="q1-6178-1-option-b" name="quiz-option-6178" type="radio" value="they behave differently." > <span class="option__label"> <span class="screen-reader-only">b.</span> they behave differently. </span> </label> <span class="quiz__response" id="response-6178-1"> <strong>Incorrect.</strong> </span> </div> </div> </div> </div> </form> </div> </section> <section id="toc2_2"><h3>Mechanical layers</h3><p>The compositional divisions of Earth were understood decades before the development of the <mark class="term" data-term="theory" data-term-def="A scientific theory is an explanation inferred from multiple lines of evidence for some broad aspect of the natural world and&hellip;" data-term-url="/en/glossary/view/theory/4854">theory</mark> of <mark class="term" data-term="plate tectonics" data-term-def="The study of the processes by which the earth's crust has attained its present structure." data-term-url="/en/glossary/view/plate+tectonics/11229">plate tectonics</mark> – the idea that Earth's <mark class="term" data-term="surface" data-term-def="The outside or external part; the topside face of something." data-term-url="/en/glossary/view/surface/8275">surface</mark> consists of large plates that move (see our <a href="/library/module_viewer.php?mid=65">Plate Tectonics I</a> module). <mark id="ngss-17" class="ngss">By the 1970s, however, geologists began to realize that the plates had to be thicker than just the <mark class="term" data-term="crust" data-term-def="The uppermost 5-70 km of the Earth. There are two types of crust: continental and oceanic. Continental crust ranges from 10-70&hellip;" data-term-url="/en/glossary/view/crust/880">crust</mark>, or they would break apart as they moved. In fact, plates consist of the crust acting together with the uppermost part of the <mark class="term" data-term="mantle" data-term-def="The middle portion of the interior of the Earth, starting below the crust at 5-70 km below the Earth’s surface and&hellip;" data-term-url="/en/glossary/view/mantle/889">mantle</mark>; this rigid layer is called the <mark class="term" data-term="lithosphere" data-term-def="The rigid upper layer of Earth consisting of the crust and the upper mantle. Earth’s tectonic plates are composed of pieces&hellip;" data-term-url="/en/glossary/view/lithosphere/1666">lithosphere</mark> and it ranges in thickness from about 10 to 200 km. Rigid lithospheric plates "float" on a layer called the <mark class="term" data-term="asthenosphere" data-term-def="The semi-molten layer of the earth which starts at ~70-200 km depth and ends at 660 km depth. The asthenosphere is&hellip;" data-term-url="/en/glossary/view/asthenosphere/1659">asthenosphere</mark> that flows like a very <mark class="term" data-term="viscosity" data-term-url="/en/glossary/view/viscosity" data-term-def="The measurement of a fluid's resistance to shear or flow. Highly viscous fluids resist motion due to their molecular composition that&hellip;">viscous</mark> <mark class="term" data-term="fluid" data-term-def="Able to flow because the intermolecular forces allow the molecules to move around in relation to one another. Both liquids and&hellip;" data-term-url="/en/glossary/view/fluid/8724">fluid</mark>, like Silly Putty<sup>®</sup>. It is important to note that although the asthenosphere can flow, it is not a <mark class="term" data-term="liquid" data-term-def="The state of matter characterized by its condensed nature and ability to flow. Unlike gases, molecules within a liquid often experience&hellip;" data-term-url="/en/glossary/view/liquid/8727">liquid</mark>, and thus both S- and <mark class="term" data-term="P-wave" data-term-def="A compression wave produced by an earthquake. The “P” is from the Italian “primero,” indicating that the P-waves were the first&hellip;" data-term-url="/en/glossary/view/P~wave/1672">P-waves</mark> can travel through it.</mark> At a depth of around 660 km, the pressure becomes so great that the mantle can no longer flow, and this <mark class="term" data-term="solid" data-term-def="A collection of atoms or molecules that are held together so that, under constant conditions, they maintain a defined shape and&hellip;" data-term-url="/en/glossary/view/solid/7571">solid</mark> part of the mantle is called the <mark class="term" data-term="mesosphere" data-term-def="1. The lowermost portion of the mantle. 2. The layer of the atmosphere above the stratosphere, where temperature decreases with altitude." data-term-url="/en/glossary/view/mesosphere/1667">mesosphere</mark>. The lithospheric mantle, asthenosphere, and mesosphere all share the same composition (that of peridotite), but their <mark class="term" data-term="mechanical" data-term-def="Involving physical force or motion." data-term-url="/en/glossary/view/mechanical/8516">mechanical</mark> properties are significantly different. Geologists often refer to the asthenosphere as the jelly in between two pieces of bread: the lithosphere and mesosphere.</p><p>The <mark class="term" data-term="core" data-term-def="The innermost layer of the Earth, which starts at ~2900 km depth. The core is composed mainly of iron and consists&hellip;" data-term-url="/en/glossary/view/core/1663">core</mark> is also subdivided into an inner and outer core. The outer core is <mark class="term" data-term="liquid" data-term-def="The state of matter characterized by its condensed nature and ability to flow. Unlike gases, molecules within a liquid often experience&hellip;" data-term-url="/en/glossary/view/liquid/8727">liquid</mark> molten metal (and able to stop S-waves), while the inner core is <mark class="term" data-term="solid" data-term-def="A collection of atoms or molecules that are held together so that, under constant conditions, they maintain a defined shape and&hellip;" data-term-url="/en/glossary/view/solid/7571">solid</mark>. (Because the composition of the core is different than that of the <mark class="term" data-term="mantle" data-term-def="The middle portion of the interior of the Earth, starting below the crust at 5-70 km below the Earth’s surface and&hellip;" data-term-url="/en/glossary/view/mantle/889">mantle</mark>, it is possible for the core to remain a liquid at much higher pressures than <mark class="term" data-term="peridotite" data-term-def="A rock composed primarily of the mineral olivine. The high percentage of iron and magnesium in olivine creates a very dense&hellip;" data-term-url="/en/glossary/view/peridotite/1671">peridotite</mark>.) <mark id="ngss-18" class="ngss">The distinction between the inner and outer core was made in 1936 by <mark class="term" data-term="Inge Lehmann" data-term-def="Danish physicist born in Osterbro by the Lakes (1888-1993). In 1925, Lehmann became interested in seismology while working as assistant to&hellip;" data-term-url="/en/glossary/view/Lehmann%2C+Inge/4561">Inge Lehmann</mark>, a Danish <mark class="term" data-term="seismologist" data-term-def="A scientist who studies earthquakes and the behavior of seismic waves in the Earth." data-term-url="/en/glossary/view/seismologist/1675">seismologist</mark>, after improvements in seismographs in the 1920s made it possible to "see" previously undetectable <mark class="term" data-term="seismic wave" data-term-def="A form of sound wave that travels away from the source of an earthquake. There are several types of seismic waves,&hellip;" data-term-url="/en/glossary/view/seismic+wave/1674">seismic waves</mark> within the <mark class="term" data-term="P-wave" data-term-def="A compression wave produced by an earthquake. The “P” is from the Italian “primero,” indicating that the P-waves were the first&hellip;" data-term-url="/en/glossary/view/P~wave/1672">P-wave</mark> shadow zone. These faint <mark class="term" data-term="waves" data-term-def="The motion of rising and falling in curves; undulation." data-term-url="/en/glossary/view/waves/8274">waves</mark> indicated that they had been refracted again within the core when they hit the <mark class="term" data-term="boundary" data-term-def="A line or limit that divides one area from another." data-term-url="/en/glossary/view/boundary/8272">boundary</mark> between the inner and outer core.</mark></p><p>The <mark class="term" data-term="mechanical" data-term-def="Involving physical force or motion." data-term-url="/en/glossary/view/mechanical/8516">mechanical</mark> layers of Earth are also shown in Figure 3, in comparison to the compositional layers.</p><p>Our picture of the interior of Earth becomes clearer as imaging techniques improve. <mark id="ngss-19" class="ngss">Seismic tomography is a relatively new technique that uses <mark class="term" data-term="seismic wave" data-term-def="A form of sound wave that travels away from the source of an earthquake. There are several types of seismic waves,&hellip;" data-term-url="/en/glossary/view/seismic+wave/1674">seismic waves</mark> to measure very slight temperature variations throughout the <mark class="term" data-term="mantle" data-term-def="The middle portion of the interior of the Earth, starting below the crust at 5-70 km below the Earth’s surface and&hellip;" data-term-url="/en/glossary/view/mantle/889">mantle</mark>. Because <mark class="term" data-term="waves" data-term-def="The motion of rising and falling in curves; undulation." data-term-url="/en/glossary/view/waves/8274">waves</mark> move faster through cold material and slower through hot material, the images they receive help scientists "see" the <mark class="term" data-term="process" data-term-def="Method, procedure; series of actions or steps." data-term-url="/en/glossary/view/process/8256">process</mark> of <mark class="term" data-term="convection" data-term-def="The movement or circulation of a fluid due to variations in its density as a result of the transfer of heat&hellip;" data-term-url="/en/glossary/view/convection/877">convection</mark> in the mantle</mark> (see our <a href="/library/module_viewer.php?mid=66">Plates, Plate Boundaries, and Driving Forces</a> module). These and other images offer a virtual journey into the center of Earth.</p></section> <footer class="module__main__footer"> <hr class="border-color-dark"> <p class="citation"> <em> Anne E. Egger, Ph.D. “Earth Structure” Visionlearning Vol. EAS (1), 2003. </em> </p>  <div class="title-list" name="further"> <p class="h6 title-list__title"> Further Reading </p> <ul class="grid grid--column-2--md grid--column-3--md gap-1"> <li> <a class="no-hover-focus height-100" href="/en/library/Earth-Science/6/The-Origins-of-Plate-Tectonic-Theory/65"> <article class="flex-row align-items-center flex-column--md align-items-start--md height-100 theme-light padding-2 gap-2"> <div class="width-30 width-auto--md"> <img class="border-radius box-shadow-1" src="/img/library/moduleImages/featured_image_65-23061209065447.jpg" alt="Origins of Plate Tectonic Theory"> </div> <div class="flex-grow-shrink"> <h2 class="h6 font-weight-normal"> Origins of Plate Tectonic Theory: <em>From early ideas to mapping the ocean floor</em> </h2> </div> </article> </a> </li> <li> <a class="no-hover-focus height-100" href="/en/library/General-Science/3/Density/37"> <article class="flex-row align-items-center flex-column--md align-items-start--md height-100 theme-light padding-2 gap-2"> <div class="width-30 width-auto--md"> <img class="border-radius box-shadow-1" src="/img/library/moduleImages/featured_image_37-23061210060349.jpg" alt="Density and Buoyancy"> </div> <div class="flex-grow-shrink"> <h2 class="h6 font-weight-normal"> Density and Buoyancy: <em>Definitions and units</em> </h2> </div> </article> </a> </li> <li> <a class="no-hover-focus height-100" href="/en/library/Physics/24/Waves-and-Wave-Motion/102"> <article class="flex-row align-items-center flex-column--md align-items-start--md height-100 theme-light padding-2 gap-2"> <div class="width-30 width-auto--md"> <img class="border-radius box-shadow-1" src="/img/library/moduleImages/featured_image_102-23061210060944.jpg" alt="Waves and Wave Motion"> </div> <div class="flex-grow-shrink"> <h2 class="h6 font-weight-normal"> Waves and Wave Motion: <em>Describing waves</em> </h2> </div> </article> </a> </li> </ul> </div> </footer> </div>    </article> </div> </div> </main> <script id="ngssCommentdata" type="application/json"> [{"ngss_tag_id":null,"type":"p","tag":null,"name":null,"description":null,"comment":"Newton used calculations related to his study of the force of gravity to calculate the density of the Earth. SEP.5. Using Mathematical and Computational Thinking","is_public":"1","mod_ngss_comment_id":"5","display_order":"1","dimension":"p","dimension_full":"Science and Engineering Practices"},{"ngss_tag_id":null,"type":"dci","tag":null,"name":null,"description":null,"comment":"Earthquake waves provide evidence about the structure of the Earth's interior. ESS2.A: Earth Materials and Systems.","is_public":"1","mod_ngss_comment_id":"6","display_order":"2","dimension":"dci","dimension_full":"Disciplinary Core Ideas"},{"ngss_tag_id":null,"type":"cc","tag":null,"name":null,"description":null,"comment":"There are many phenomena we cannot observe directly, and therefore other forms of data provide us with \"proxy\" evidence. In this case, seismic waves provide us with indirect evidence about the structure of the Earth's interior. CC.3: Scale Proportion and Quantity. ","is_public":"1","mod_ngss_comment_id":"7","display_order":"3","dimension":"cc","dimension_full":"Crosscutting Concepts"},{"ngss_tag_id":null,"type":"dci","tag":null,"name":null,"description":null,"comment":"Seismic waves have the same properties of compression waves (P-waves), and ripple waves (S-waves). Wave refraction that occurs at different interfaces in the Earth's interior provides evidence for its internal structure. PS4.A: Wave Properties.","is_public":"1","mod_ngss_comment_id":"8","display_order":"4","dimension":"dci","dimension_full":"Disciplinary Core Ideas"},{"ngss_tag_id":null,"type":"p","tag":null,"name":null,"description":null,"comment":"Mohorovicic made observations from data collected from his installed seismic stations. 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