Answer to Question 1
Answer: The two kinds of crust that exist on Earth are oceanic crust and continental crust. Oceanic crust is found along the ocean floor as well as in mountain ranges. It is found in mountain ranges because of places where subduction zone processes pushed pieces of the seafloor onto the edges on continents, where it would later become embedded in mountain ranges. Oceanic and continental crusts vary in composition and thickness. While oceanic crust is typically around 7 kilometers thick, continental crust is typically about 25 kilometers thick. Continental crust is composed of quartz bearing igneous and metamorphic rocks while oceanic crust is composed of mafic igneous rocks. Both types of crust rest on top of the mantle.
Answer to Question 2
Answer: The internal composition of Earth is inferred by relating calculated seismic-wave velocities to properties of rocks and minerals at temperatures and pressures equivalent to Earth's interior. The crust-mantle boundary, called the Moho, displays a sharp increase in seismic velocity where mafic to felsic igneous and metamorphosed igneous rocks of the crust are underlain by mantle peridotite. The low-velocity zone that exists about 100 kilometers below the surface is evidence that part of the upper mantle is less rigid and may be slightly molten. This less rigid zone defines the top of the asthenosphere and is below the more rigid lithosphere consisting of the crust and uppermost mantle. Crystal-structure transformations of peridotite minerals at high pressure and temperature explain the abrupt increases in seismic velocity at about 410 and 660 kilometers below the surface. The interval between 410 and 660 kilometers depth is the transition zone between the upper mantle and the lower mantle.
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Earth’s Spheres
Precession: Earth’s axis rotates, or “wobbles”
Earth’s Lithospheric Plates
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