Читать книгу Earth Materials - John O'Brien - Страница 25

Earth's core consists primarily of iron (~85%), with smaller, but significant amounts of nickel (~5%) and lighter elements (~8–10%) such as oxygen, sulfur and/or hydrogen. A dramatic decrease in P‐wave velocity and the termination of S‐wave propagation occurs at the 2900 km discontinuity which is Gutenberg discontinuity or core–mantle boundary (CMB). Because S‐waves are not transmitted by nonrigid substances such as fluids, the outer core is inferred to be a fluid. Geophysical studies suggest that the Earth's outer core is a highly compressed liquid with a density of ~10–12 g/cm³. Slowly circulating molten, iron‐rich, very viscous liquids in the outer core are believed to be responsible for the production of most of Earth's magnetic field.

The outer/inner core boundary, the Lehman discontinuity at 5150 km, is marked by a rapid increase in P‐wave velocity and the reemergence of low velocity S‐waves. This suggests that the inner core is rigid. The inner core is solid and has a density of ~13 g/cm³. Density and magnetic studies suggest that the Earth's inner core also consists of largely of iron, with nickel and less oxygen, sulfur, and/or hydrogen than the outer core. Seismic studies have shown that the inner core is seismically anisotropic; that is seismic velocity in the inner core is faster in one direction than in others. This has been interpreted to result from the parallel alignment of iron‐rich crystals or from a core consisting of a single crystal with a fast velocity direction. Recent discoveries suggest that the inner core is divided into two layers with the inner layer more rigid than the outer one and with a different orientation of its fast seismic wave direction (Ishii and Dziewonski 2002; Wang et al. 2015).

In this section, we have discussed the major layers of the geosphere, their composition, and their mechanical properties. This model of a layered geosphere provides us with a spatial context in which to visualize where the processes that generate earth materials occur. In the following sections we will examine the ways in which all parts of the geosphere interact to produce global tectonics. The ongoing story of global‐scale tectonics is one of the most fascinating tales of scientific discovery in the last century and new discoveries continue to be made in this one.