Chapter 30: The Interior of the Earth

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(Note: This reading quiz contains some material from lecture that is not found in the textbook.) In the chapters remaining, we seek to understand the origin of the continental crust and the oceanic crust and the earth's history in terms of processes that are slow rather than in terms of catastrophic processes. But what happens on the surface of the earth is coupled to what is happening on the inside, so we first see what we know about the interior of the earth and how we know it. Evidence for the nature of the interior of the earth comes from: drilling deep holes (but they never get past the crust); "weighing the earth" (Cavendish Experiment), which tells us that the earth (is, is not?) hollow and (is, is not?) homogeneous (same throughout) because the average density of the earth is about 5.5 grams per cubic centimeter and the density of the crust is much less (2.5-3.2 grams per cubic centimeter); the earth's magnetic field, which tells us that the earth's core is likely to be (solid, liquid?) ; meteorites, which tell us that the at least some of the interior of the earth is likely to consist of (aluminum, uranium, iron?) ; and, "listening" to . The best evidence in the above list about the interior of the earth comes from earthquakes. The seismograph is an application primarily of Newton's (First, Second, Third?) Law of Motion; The speed of earthquake waves moving through the earth's interior depends on the density and of the rock. The greater the density, the (faster, slower?) the wave speed; Earthquake waves travelling through the earth sometimes fail to reach detectors in a band around the earth called a " zone." This zone occurs because of (reflection, refraction, diffraction?) of the waves; the shadow zone from s-waves (shear) is more extensive over the earth's surface than for p-waves (compression). This is taken as evidence that the core of the earth is (solid, liquid, gas?) . The shadow zone for p-waves is taken as evidence that there are (sharp, gradual?) boundaries inside the earth that divide one region from another. The earth is a "differentiated" planet. This means that the material of the earth has segregated into layers. But be careful! There are two schemes to label the layers. If you label the layers according to the composition of the rock, then the outermost layer is the and the next is the . If you label the layers according to the rigidity of the rock, then the outermost layer is the (rigid sphere) and the next is the (weak or mushy sphere). The crust is NOT the same thing as the lithosphere! The density of the earth (decreases, increases?) with depth (from the surface). A typical density in the hydrosphere is 1.0 grams per cubic centimeter (water); in the crust is 2.7-3.2 grams per cubic centimeter; in the mantle is 5 grams per cubic centimeter; and, in the core is about 10-12 grams per cubic centimeter. The crust consists primarily of silicates (both granitic rock and basalt are silicates); the mantle consists of dense oxides and silicates high in (metal elements?) and ; the core consists of (metal element?) and nickel. The speed of earthquake waves (increases, decreases?) sharply with depth in the lithosphere, but (increases, decreases?) slightly to mark the presence of the . With further increase in depth through the mantle, the waves again (increase, decrease?) in speed until they reach the boundary of the outer . At this point, the (s, p?) -waves stop altogether because they encounter the (solid, liquid?) core and the speed of the p-waves (increases, decreases?) sharply. A discontinuity in the speed of the p-waves at about 5000 kilometers marks the boundary to the core of the earth.

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