Читать книгу Earth Materials - John O'Brien - Страница 22

The mantle is thick (~2900 km) relative to the radius of Earth (~6370 km) and constitutes ~83% of Earth's total volume. The mantle is distinguished from the crust by being very rich in MgO (30–40%) and, to a lesser extent, in FeO. It contains an average of approximately 40–45% SiO₂ which means it has an ultrabasic composition (Chapter 7). Some basic rocks such as eclogite occur in smaller proportions. In the upper mantle (depths to 400 km), the Mg‐rich silicate minerals olivine and pyroxene dominate; spinel, plagioclase and garnet are locally common. These minerals combine to produce generally dark colored ultramafic rocks such as peridotite, the dominate group of rocks in the upper mantle. Under the higher pressure conditions deeper in the mantle similar chemical components combine to produce dense minerals with tightly packed crystal structures. These high‐pressure minerals are produced by transformations that are largely indicated by changes high pressure in seismic wave velocity, which reveal that the mantle contains a number of sublayers (Figure 1.2) as discussed below.