The Interior of the Earth. The Internal Layers

Living as we do on the surface of the Earth, we cannot directly observe the interior of the planet. However, scientists have been able to learn a great deal about the structure, composition, and behavior of the Earth’s interior using indirect methods.

The interior of the Earth can be described in two different ways depending on the type of indirect information we use and the physical characteristics we are trying to discern. Historically, the interior has been most intensely and accurately studied by investigating the passage of earthquake waves through the body of the Earth. The velocity of earthquake waves depends on the type of rock they travel through and whether the rock is solid, partially molten, or a complete melt.

When these waves encounter boundaries separating layers that are composed of different types of rocks, or that have varying amounts of molten material, their speed and direction of travel will change. They may be reflected, refracted, or transmitted through these boundaries. Detailed modeling of the paths taken by earthquake waves and their expected travel times through the Earth as a function of distance has produced an Earth model consisting of four major layers as illustrated in figure 2.1.

Fig. 2.1. The layered structure of the Earth

At the planet’s center is an inner core; its radius is 1222 km (759 mi). It is solid and nearly five times as dense as common surface rocks such a granite because of the tremendous pressure at that depth. The inner core is magnetized and very

hot (4000-5500°C). The physical properties of the core indicate that its principal component is iron, with lesser amounts of lighter elements that most likely include nickel, sulfur, and oxygen. The inner core is surrounded by a shell, 2258 km (1402 mi) thick, of similar composition and lower temperature (3200°C); this shell is called the outer core. As early as 1926 studies of the tidal deformation of the Earth made it clear that at least a portion of the core must behave like a fluid.

In 1936, seismologist Inge Lehmann used earthquake data to establish the fact that there was a solid inner core surrounded by a “fluid” outer core. Although it behaves like a fluid, the outer core may not be completely molten. It would behave like a fluid even if as much as 30% of it were composed of suspended crystals, most probably of iron oxides and sulfides, that had solidified from the melt. Recent studies have determined that the inner core rotates about 3° per year faster than the mantle. This increase in eastward rotation is thought to be caused by motion in the fluid outer core.

Material in the outer core is believed to flow across the top of the inner core, inward toward the inner core’s rotation axis, and then eastward. These fluid motions in the outer core also continuously regenerated the Earth’s magnetic field. The next layer, the mantle, contains the largest mass of material of any of the layers (about 70% of the Earth’s volume). This layer is 2866 km (1780 mi) thick; it is less dense and cooler (1100°-3200°C) than the core and is composed of magnesium-iron silicates.

A thin layer of the upper mantle 50 to 100 km (30-60 mi) thick behaves rigidly; it is underlain by a region extending to a depth of about 200 to 250 km (125-155 mi) that is easily deformed and flows slowly over the remainder of the mantle beneath it. This arrangement allows the thin, rigid upper layer to move both horizontally and vertically.

A reduction in the speed of seismic waves traveling deep in the mantle near the core-mantle boundary is evidence that there may be a partially molten layer varying in thickness between 5 and 40 km (3-25 mi) at the base of the mantle. The observed reduction in velocity indicates that between 5% and 30% of this layer is molten rock. The Earth’s outermost layer is the cold, rigid, thin surface layer called the crust. There are two kinds of crust. Oceanic crust is relatively dense, with an average thickness of about 7 km (4-3 mi). Continental crust is relatively light and averages about 40 km (25 mi) in thickness. See figure 2.1 and table 2.1 for a comparison of these features and their properties.

Even though studies of earthquake waves have shown that the crust and mantle consist of different types of rocks and have a distinct boundary separating them, similarities in the mechanical behavior of the crust and uppermost part of the mantle allow us to think of them as a single unit separate from other regions of the mantle.

Therefore, the crust and mantle can he subdivided in a different way from that illustrated in figure 2.1; this subdivision is based on how the material in these regions behaves in response to applied stresses over longer time periods than those involved in the passage of seismic waves. These stresses are generated by slow movement in the mantle as high-temperature, low-density material rises and cooler, denser material sinks.

Stresses are also caused by volcanic action and by gains and losses of glacial ice that increase or decrease the weight of material on the surface of the Earth. This way of subdividing the interior of the Earth redefines the regions historically known as the crust and the mantle into three layers. The surface layer is called the lithosphere (see fig. 2.4); it is relatively strong and behaves rigidly. The tectonic plates discussed in section 2.4 are composed of lithosphere.

The lithosphere moves on top of the layer beneath it, the asthenosphere, which is weaker and behaves plastically. Below the asthenosphere the mantle becomes relatively strong once again. This layer is the mesosphere. The lithosphere and asthenosphere are discussed in more detail in table 2.2 and section 2.2.