Broadly, Earth consists of three main layers: the crust, the mantle, and the core (Figure 3.2). In Figure 3.2 the core accounts for almost half of Earth’s radius, but it amounts to only 16.1% of Earth’s volume. Most of Earth’s volume (82.5%) is its mantle, and only a small fraction (1.4%) is its crust. Figure 3.2 shows that the crust, mantle, and core are subdivided, and these divisions are discussed below.
The Earth’s outermost layer, its crust, is rocky and rigid. There are two kinds of crust:
- Continental crust is thicker, and predominantly felsic in composition. We’ll learn more about what felsic means when we discuss igneous rocks, but for the moment we will say that felsic rocks are made of minerals that have somewhat more silica than other minerals, and which are somewhat less dense.
- Ocean crust is thinner, and predominantly mafic in composition. For the moment we will say that mafic rocks have minerals with higher amounts of magnesium and iron, and are denser.
The crust floats on the mantle. Because continental crust has a lower density, it floats higher in the mantle than ocean crust. If tectonic plates happen to bring ocean crust and continental crust into collision, the plate with ocean crust will be forced down into the mantle beneath the plate with continental crust.
The mantle is ultramafic in composition, meaning it has even more iron and magnesium than mafic rocks, and even less silica. Although the mantle has the same chemical composition throughout, it is broken into layers with different mineral compositions and different physical properties. It can have different mineral compositions and still be the same chemically because the increasing pressure deeper in the mantle causes the minerals to reconfigure themselves. The upper mantle is typically composed of peridotite, a rock dominated by the minerals olivine and pyroxene. In the lower mantle, extreme pressures mean that spinels and garnets are present instead.
The lithosphere can’t be classified neatly as either crust or mantle because it consists of both. It is formed from the crust as well as the uppermost layer of the mantle which is stuck to the underside of the crust. The lithosphere is broken into pieces which we know as tectonic plates.
Beneath the lithosphere is the asthenosphere, a layer of mantle that is partially molten. Because the asthenosphere is partially molten it undergoes plastic deformation, meaning that it deforms by flowing. This happens over very long timescales. The asthenosphere is important for plate tectonics because by deforming, it allows the fragments of lithosphere to move around above it and through it.
The D” (pronounced dee double prime) layer is a mysterious layer beginning approximately 200 km above the boundary between the core and mantle. (This boundary is referred to as the CMB, for core-mantle boundary.) We know it exists because of how seismic waves change speed as they move through it, but it isn’t clear why it’s different from the rest of the mantle. One idea is that it is minerals are undergoing another transition in this region because of pressure and temperature conditions, similar to the transition between the upper and lower mantle.
The core is primarily composed of iron, with lesser amounts of nickel. The core is too light to be only iron and nickel, however, and new research has shown that it also contains an amount of sulphur equal to roughly one tenth the mass of the moon!
The core is extremely hot (~3500° to more than 6000°C), but despite the fact that the boundary between the inner and outer core is approximately as hot as the surface of the sun, only the outer core is liquid. The inner core is solid because the pressure is so much greater at that depth.
In the remainder of this chapter, we’ll look first at how we know about Earth’s interior structure, and then at the properties of the different layers and the processes that take place within them.
Heat is continuously flowing outward from Earth’s interior, and the transfer of heat from the core to the mantle causes convection in the mantle (Figure 3.3). This convection is the primary driving force for the movement of tectonic plates. At places where convection currents in the mantle are moving upward, new lithosphere forms (at ocean ridges), and the plates move apart (diverge). Where two plates are converging (and the convective flow is downward), one plate will be subducted (pushed down) into the mantle beneath the other. Many of Earth’s major earthquakes and volcanoes are associated with convergent boundaries.
- Only a small fraction is melted, and this fraction is dispersed through the asthenosphere in tiny droplets. This means the asthenosphere is mostly solid rock. ↵
- WR Peltier (2007). "Mantle dynamics and the D-doubleprime layer implications of the post-perovskite phase". In Kei Hirose, John Brodholt, Thome Lay, David Yuen. Post-Perovskite: The Last Mantle Phase Transition; Volume 174 in AGU Geophysical Monographs. American Geophysical Union. pp. 217–227. ISBN 978-0-87590-439-9. http://bit.ly/1NPCoUz ↵
- Earth's core contains 90 percent of Earth's sulfur, new research shows, Phys.org, 16 June 2015. http://bit.ly/1IhCeU0 ↵