From Dust to Life by Chambers John; Mitton Jacqueline;

Author:Chambers, John; Mitton, Jacqueline;
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2013-06-14T16:00:00+00:00

EARTH

To understand how Earth became the planet we see today, it helps to understand its structure. Geologists are at a distinct disadvantage compared to astronomers in that they can’t see directly into Earth the same way astronomers can view the heavens. However, geologists have learned a good deal about the planet’s interior through seismometry. When an earthquake occurs, it generates waves that travel through rocks at Earth’s surface and in its interior. These waves travel around the globe, where their arrival is recorded by a network of seismometers. Their speed depends on the density of the rock through which they are traveling, so by measuring how long it takes the waves to reach different places, it is possible to measure the density of rocks at different depths inside Earth.

Earth is divided into a series of distinct layers, each with a different density (Figure 9.3). A thin layer at the top constitutes the familiar crust. Underneath the crust lies a denser, rocky mantle that makes up most of the planet’s bulk. Rocks from the crust and mantle generally have somewhat different compositions as well as different densities—mantle rocks are richer in magnesium, for example, while crustal rocks tend to contain more silicon. Beneath these rocky layers there is a dense core that takes up about half Earth’s diameter and accounts for roughly 30 percent of its mass. Geologists have no direct samples of the core, but its density is so high that it must be 90 percent iron and other metals, and only 10 percent lighter elements.

We can tell that Earth has an iron-rich core in another way too. Certain elements, such as gold, platinum, and iridium, have a strong chemical affinity for iron. These siderophile elements are much less abundant in the rocks that form Earth’s crust and mantle than one might expect. It appears that the siderophile elements joined chemically with much of Earth’s iron and sank with it to the center, leaving behind a crust and mantle depleted in siderophile elements. In fact, laboratory experiments designed to reproduce conditions deep in Earth’s interior show that siderophile elements should be even rarer in the crust and mantle than they actually are. The discrepancy can be explained if Earth acquired a small amount of its bulk after its core had finished forming. This “late veneer,” amounting to perhaps half a percent of Earth’s mass, brought with it most of the siderophile elements that we find in surface rocks today.

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