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.Preview Mode
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