Chapter 11

aving considered the Earth’s formation in the last chapter, let’s now use the tools of geochemistry we acquired in the first eight chapters to consider how the Earth works. The Earth, unlike many of its neighbors, has evolved over its long history and it remains geologically active. Four and a half billion years later, it is very different place than it was first formed. Certainly one of the main objectives of geochemistry is to understand this activity and unravel this history. The solid Earth consists of 3 distinct layers: crust, mantle, and core. As Table 11.01 shows, these layers become progressively denser with depth, reflecting a stable density stratification of the planet (one shared by the other terrestrial planets). Unfortunately, only the shallowest of these layers, the crust, is accessible to direct study. The crust may well be the most interesting part of the planet, but it represents less than half a percent of its mass. Even most of the crust is out of reach; the deepest boreholes drilled in the continental crust, which is 35 km thick on average, are only 12 km deep; the deepest borehole in oceanic crust, which is on average 7 km thick, is only 1.5 km deep. Thus study of the solid Earth necessarily relies on indirect approaches and on rare samples of deeper material brought to the surface through geologic processes. We’ll begin by considering the geochemistry of the mantle because it represents the largest fraction of the Earth, both by mass (67%) and volume (88%). Although remote, the mantle is important for a number of reasons. For one, the crust has been formed from it. For another, convection within the Earth’s mantle drives most tectonic activity that affects us on the surface, including plate motions, earthquakes, and volcanoes. We’ll next consider the core. As we found in the previous chapter, the core formed quite early – more or less simultaneously with formation of the Earth itself. While we do have a few samples of the Earth’s mantle, we have none at all from the core, so its composition and history must be inferred entirely indirectly. Lastly, we’ll consider the Earth crust – the part of the Earth we inhabit and are most familiar with. The crust, particularly the continental crust, is the most varied, and arguably the most interesting, part of our planet. We’ll find that the continental crust, unlike the core which is as old as the Earth itself, has grown through geologic time through magmatism. The crust, particularly the continental crust, is where the geologic history of the planet is preserved.