More than 82 percent of Earth’s volume is contained within the mantle, a nearly 2900-kilometer-thick shell extending from the base of the crust (Moho) to the liquid outer core (see Figure 1). Because S waves readily travel through the mantle, we know that it is a solid rocky layer composed of silicate minerals that are rich in iron and magnesium. However, despite its solid nature, rock in the mantle is quite hot and capable of flow, albeit at very slow velocities.
figure 1 show's that Earth’s layered structure: The properties of
Earth’s layers include the physical state of the material
(solid, liquid, or gas) as well as
how strong the
material is—for example, the distinction between the
strong lithosphere and weak asthenosphere. Studies have shown that
Earth’s layers are mainly determined by density, with the
heaviest materials (iron) at the center and the lightest ones (gases
and liquids) on the outside.The upper Mantle:
Earth’s upper mantle extends from the Moho to a depth of about 660 kilometers and can be divided into three shells:
1. The uppermost mantle is called the lithospheric mantle, and it ranges in thickness from only a few kilometers under the mid-oceanic ridges to perhaps as much as 200 kilometers under the stable continental interiors. This layer and the crust make up Earth’s rigid outer shell, called the lithosphere.
2. Beneath the lithospheric mantle is a weak layer called the asthenosphere. The lithospheric mantle and asthenosphere are compositionally similar; however, lithospheric mantle is strong, and the asthenosphere is weak, as a result of Earth’s temperature structure.
3. The lower portion of the upper mantle, at depths between 410 and 660 kilometers, is called the transition zone.
Rocks brought to the surface by volcanism and other geologic processes have provided geologists with valuable information about the composition of the upper mantle, which is composed mainly of the rock peridotite
(Fig 9).
Peridotite is an ultramafic rock that consists of the minerals olivine and pyroxene, minerals that are rich in iron and magnesium. As a result, the mantle is denser than either the continental crust or the oceanic crust that lie above it. At the depth (and pressures) of the transition zone, the mineral olivine, which is stable in the uppermost mantle, is subjected to greater pressure and collapses into denser structures. In the top half of the transition zone, olivine converts to a more compact structure similar to the mineral spinel and pyroxene converts to a garnet-like structure. Water cycles slowly through Earth and is brought into the mantle by subducting oceanic lithosphere and carried back to the surface by rising plumes of mantle rock. Mineral physics experiments have revealed that the transition zone is capable of holding a great deal of water up to 2 percent of its weight. This is considerably more than for the rocks of the uppermost mantle, which can hold only about 0.1 percent of their weight as water. Because the transition zone represents 10 percent of Earth’s volume, it could potentially hold up to five times the volume of Earth’s oceans. How much water is actually contained within the transition zone has not been determined.
The Lower Mantle:
The lower mantle lies between the transition zone (660 kilometers) and the liquid core (2900 kilometers). Beneath the 660-kilometer
discontinuity, both olivine and pyroxene take the structure of the mineral perovskite (Fe, Mg) SiO3 and related minerals. Because the lower mantle is undoubtedly Earth’s largest layer, occupying 56 percent of the volume of the planet, perovskitestructured silicate minerals are the single most abundant material within Earth.
The D" Layer:
In the lowest few hundred kilometers of the mantle is a highly
variable and unusual region called D" (pronounced “dee double-prime”). The D" layer, the boundary layer between the rocky mantle and the liquid iron outer core, is thought to have large variations in composition as well as temperature (Fig10)
Cool areas in the D" layer are thought to be the graveyard of subducted oceanic lithosphere, whereas the hot areas are the birthplace of deep mantle plumes. The very base of D", the part of the mantle in direct contact with the hot liquid iron core, is like
Earth’s surface in that there are “upside-down mountains” of rock that protrude into the core. Furthermore, in some regions of the core–mantle boundary, the base of D" may be hot enough to be partially molten. Evidence for partial melting comes from zones at the very base of the mantle where S-wave velocities decrease by 30 percent, an indication that the material there is quite weak.
Discovering Boundaries: The Core–Mantle Boundary Evidence that Earth has a distinct central core was uncovered in 1906 by British geologist Richard Dixon Oldham. At locations beyond approximately 100 degrees from the epicenter of a largeearthquake, Oldham observed that P and S waves were absent or very weak. In other words, Oldham found evidence for a central core that produced a shadow zone for seismic waves (Fig 11)
As Oldham predicted, Earth’s core exhibits markedly different properties from the mantle above, which causes considerable refraction of P waves—similarly to how light is refracted as it passes from air to water. In addition, because the outer core is liquid iron, it blocks the transmission of S waves. (Recall that S waves do not travel through liquids.)
The locations of the P- and S-wave shadow zones and how their paths are affected by the core are shown in Fig 11. Whereas some P and S waves still arrive in the shadow zone, they differ greatly from those expected in a planet without a core.
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