Geology - Inside the Earth - Quick Guides | Geology

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Study GuideGeology–Inside the Earth1. The Earth’s CrustThecrustis the thin, outermost layer of the Earth. Even though it feels solid and huge to us, it isactually very thin compared to the rest of the planet. Scientists have learned a lot about the crust bystudying how earthquake waves move through it.1.1 Crust ComposiƟon: What the Crust Is Made OfScientists useP waves(a type of earthquake wave) to study Earth’s interior. These waves travel atdifferent speeds depending on the type of rock they move through.•Inoceanic crust, P waves travel faster—about7 km per second.•Incontinental crust, they travel a bit slower—about6 km per second.These speeds match the types of rocks found in each crust:•Oceanic crustis mostlybasalt and gabbro. These rocks are rich inmagnesium and silica,and this composition is sometimes calledsima.•Continental crustis mainly made of rocks likegranite and gneiss, which contain lots ofsilica and aluminum. This type of composition is calledsialic.So, by measuring wave speeds, scientists can tell what kinds of rocks make up different parts of thecrust.1.2 Crust Thickness and Density: Not All Crust Is the SameThe crust also differs inthicknessanddensity.•Continental crustis quite thick, ranging from30 to 50 km(18–30 miles).•Oceanic crustis much thinner, only about5 to 8 km(3–5 miles).The continental crust is thickest undermountain ranges. Here, it pushes downward into the mantle,forming what is called amountain root, much like the roots of a tree underground.

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Study GuideDensity matters too:•Continental crust has a density of about2.7 g/cm³.•Oceanic crust is denser, about3.0 g/cm³.Because it is less dense, continental crust can“float” higherthan oceanic crust, similar to how apiece of wood floats on water.1.3 The Mohorovicic DisconƟnuity (Moho): A Major BoundaryTheMohorovicic discontinuity, usually called theMoho, is theboundary between the crust andthe mantle. It is the first major internal boundary inside the Earth.The Moho is named afterAndrija Mohorovicic, who discovered it in1909. His work provided the firststrong evidence that Earth haslayersinside it.•The depth of the Moho varies from about5 km to 50 km(3–30 miles) below the surface.•It is shallow beneath oceans and much deeper beneath continents,especially undermountains.2. The MantleThemantlelies beneath the crust and makes up most of Earth’s interior. Scientists study the mantlemainly by observing howseismic (earthquake) wavesmove through it. These waves give usimportant clues about what the mantle is like inside.2.1 Upper and Lower Mantle: Two Main PartsSeismic data show that most of the mantle is made ofsolid rock.In fact,P wavestravel through the mantle at an average speed of about8 km per second. This fastspeed suggests the mantle is made ofultramafic rocks, especiallyperidotite, which are rich in ironand magnesium.Based on how P waves behave, scientists divide the mantle into two main layers:•Upper mantle○Starts just below the crust, at depths of about5 to 50 km(3–30 miles)

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Study Guide○Extends down to about670 km(400 miles)•Lower mantle○Begins at about670 km(400 miles)○Extends all the way down to2,900 km(1,740 miles), where it meets the outer core2.2 The Lithosphere: Earth’s Rigid Outer ShellChanges in P wave speeds reveal another important structureinside Earth.Thelithosphereis made up of:•Thecrust•Theuppermost part of the mantleThis layer forms Earth’sbrittle, rigid outer shell—the part that breaks into tectonic plates.•Under theoceans, the lithosphere is about75 km thick•Undercontinents, it can be as thick as175 km•Its maximum depth is thought to be no more than200 km(120 miles)2.3 The Asthenosphere: A Weaker, SoŌer LayerBelow the lithosphere lies theasthenosphere. At this boundary, P wavesslow down, which tellsscientists the rock here behaves differently.Key features of the asthenosphere:•Also called thelow-velocity zone•About200 km thick•Rocks arehotter,weaker, andpartially melted•Material here is moreplastic, meaning it can slowly flowBecause it is weaker, the asthenosphere is thought to be the layeron which tectonic plates move.It is also considered apossible source of magma.

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Study Guide2.4 Other Important Boundaries in the MantleSeismic data show two more important boundaries within the mantle, at depths of:•400 km(240 miles)•670 km(400 miles)These boundaries are not caused by changes in chemistry, but byhigh pressure and temperature.Under these conditions:•Mineralsrearrange their atomic structure•They becomemore compact and denserThis means the mantle may bechemically similar throughout, but itsmineral compositionchanges with depth.The670 km boundaryis especially important. Scientists believe it marks both:•Aphysical boundary•Achemical boundarybetween the upper and lower mantle2.5 The Lower Mantle: Deep and PowerfulThelower mantleis very thick and extends down to the outer core. It is believed to consist ofultramafic rocksthat are:•Plastic•Possiblypartially melted•Extremely hot and under intense pressure3.IsostaƟc EquilibriumThe Earth’s outer shell is not fixed in place. Instead, it stays balanced through a process calledisostatic equilibrium, often shortened toisostasy. This idea helps explain why mountains are tall,why continents sit higher than oceans, and why the crust can slowly rise or sink over time.

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Study Guide3.1FloaƟng Crust and BalanceLarge plates made ofcrust and upper mantle, together called thelithosphere, rest on top of thesofter, denserasthenosphere. Even though everything is solid rock, this setup works a bit likefloating.•The lithosphere “floats” on the slowly flowing asthenosphere.•How deep a block of crust sinks depends on itsweight.•Heavier blocks sink deeper.•Lighter blocks sit higher.This balance between the crust and the mantle is what we callisostasy.3.2Mountains and Mountain RootsTall landforms likemountain rangeshave a lot of mass. Because of this:•A tall block of crust pushesdeeper into the mantle.•This deep extension is called amountain root.•The higher themountain, the deeper the root below it.

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Study GuideWhen crustal blocks are separated byfaults, each block can settle at a different height, depending onhow heavy it is.3.3A System That Adjusts Over TimeIsostasy is not a one-time process. It constantly adjusts as the surface of the crust changes.For example:•When a mountain rangeerodes, material is removed.•The crust becomes lighter and begins torise upward.•The eroded material is deposited assedimentnearby.•These sediment-covered areas become heavier andsink deeperinto the asthenosphere.Regions that aretectonically stable(with little plate movement) are usually close to perfect isostaticbalance. Scientists can even estimate theviscosity of the mantleby measuring how fast the crustadjusts to these changes.3.4Ice Sheets and Crustal ReboundA clear real-world example of isostasy comes from thePleistocene Epoch, when thick ice sheetscovered large parts of the continents.•The massive weight of the ice pushed the crustdownward.•Whenthe ice melted, the weight was removed.•The crust then began toslowly rise back up.This process is calledcrustal rebound, and it is still happening today in parts ofGreat Lakes.3.5A Possible Link to Mountain BuildingSome geologists suggest thatplate subductionmay create large bodies of magma that stick to thebottom of continents and cool there.•This would make the crustthicker and heavier.•To maintain isostatic balance, the crust would need torise, forming mountains.

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Study GuideHowever, this idea isnot widely accepted, and scientists continue to study how mountains form.4. The Earth’s CoreThecoreis the deepest part of the Earth. Scientists cannot see it directly, so they rely onseismicwaves from earthquakesto learn what it is like. By studying how these waves travel—or fail totravel—through Earth, scientists have uncovered important details about the core’s structure andcomposition.4.1 Shadow Zones: Clues from Seismic WavesWhen an earthquake happens, it sends out different types of seismic waves. Two important ones areP wavesandS waves.P-Wave Shadow Zones•P wavescan travel through solids, liquids, and gases.•When P waves reach thecore, theybend (refract) inward.•This bending creates areas on the opposite side of the Earth where no P waves are detected.These areas are calledP-wave shadow zones. They lie between:•The last P waves that travel straight through the mantle, and•The first P waves that arebent as they pass through the core.By studying these shadow zones, scientists can estimate thelocation, size, and shapeof the core.

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