What do seismic waves reveal about the layering of Earth’s crust and mantle? Correlations of seismic wave velocities with rock types have made it possible to use seismic waves to explore the composition of Earth’s interior. These explorations have revealed that the continental crust is made mostly of low-density granitic rock, and that the deep seafloor is composed of basalt and gabbro. The crust and outer part of the mantle make up the rigid lithosphere. Beneath the lithosphere lies the asthenosphere, the weak, ductile layer of the mantle on which the lithospheric plates slide. At the top of the asthenosphere, the temperature is high enough to partially melt peridotite, forming an S-wave low-velocity zone. Below 200 to 250 km, S-wave velocities again increase with depth. At two depths in the mantle, 410 km and 660 km below the surface, S-wave velocities show jumps caused by phase changes in mantle minerals. Below 660 km lies the lower mantle, a layer 2000 km thick, in which seismic wave velocities increase gradually.
What do seismic waves tell us about the layering of Earth’s core? Seismic waves reflected from the core-mantle boundary locate this sharp boundary at a depth of 2890 km. The failure of S waves to penetrate below the core-mantle boundary indicates that the outer core is liquid. A jump in P-wave velocity marks the boundary between the liquid outer core and the solid inner core at a depth of 5150 km. Several lines of evidence suggest that the core is composed mostly of iron and nickel, with minor amounts of some lighter element, such as oxygen or sulfur.
How hot does it get in Earth’s interior? Earth’s interior is hot because it still retains much of the heat generated by its violent formation as well as heat currently being generated by the decay of radioactive isotopes. It has cooled over geologic time, primarily by convection in the mantle and core but also by conduction of heat through the lithosphere. A geotherm is a curve that describes how temperature increases with depth. Within most continental crust, it increases at a rate of 208C to 308C per kilometer. Temperatures near the base of the lithosphere reach 13008C to 14008C, which is hot enough to begin to melt mantle peridotite. The temperature in the liquid core is probably greater than 30008C. The temperature at Earth’s center is probably about 50008C.
What has seismic tomography revealed about structures in the mantle? Seismologists can use seismic tomography to create three-dimensional images of Earth’s interior. Regions where seismic wave velocities increase indicate relatively cool, dense rock; regions where they decrease indicate relatively hot, less dense rock. Tomographic images reveal the structures of plate tectonics close to Earth’s surface, from the upwelling of hot mantle material under mid-ocean ridges to the cold lithosphere that extends deep beneath continental cratons. They also reveal many features of mantle convection, such as the sinking of lithospheric slabs into the lower mantle and the rising of plumes from deep within the mantle.
What does Earth’s gravitational field tell us about its interior? Variations in the strength of gravity over Earth’s surface and corresponding distortions in its shape can be measured by satellites. These variations arise primarily from the temperature variations caused by mantle convection, which affect the density of rock (higher temperatures reduce densities). The observed gravitational field is in agreement with the pattern of mantle convection inferred from seismic tomography.
What does Earth’s magnetic field tell us about the liquid outer core? Convective movements in the outer core stir its electrically conducting iron-rich liquid, forming a geodynamo that produces the magnetic field. At Earth’s surface, the magnetic field produced by the geodynamo is primarily a dipole field, but it has a small nondipole component. Maps of the magnetic field derived from compass readings show that the pattern of magnetic field strengths at Earth’s surface has changed over the last several centuries. All of these observations tell us something about the nature of the rapid convective movements that drive the geodynamo.
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What is paleomagnetism and what is its importance? Geologists have discovered that minerals in some rock types align themselves in the direction of Earth’s magnetic field at the time the rocks form. This remanent magnetization can be preserved in rocks for millions of years. Paleomagnetic stratigraphy tells us that Earth’s magnetic field has reversed (flipped back and forth) over geologic time. The chronology of reversals has been worked out, so that the direction of remanent magnetization of a rock formation can be used as an indicator of its age.