4.1: Structure of the Earth

The continents and ocean basins are not permanent. Instead, the location, shape, and size of these features are continuously changing, albeit imperceptibly slowly on the timescale of human experience. The global-scale processes that continuously reshape the face of the planet are, as yet, far from fully understood. However, we do know that these processes, called plate tectonics, originate deep within the Earth.

Layered Structure of the Earth

The Earth consists of a spherical central core surrounded by several concentric layers of different materials (Fig. 4-2). The layers are arranged by density, with the highest-density material at the Earth’s center and the lowest-density material forming the outer layer, the Earth’s crust. This arrangement came about early in the Earth’s history, when the planet was much hotter than it is today and almost entirely fluid. The densest elements sank toward the Earth’s center and lighter elements rose to the surface (CC1). The Earth’s center is still much hotter than its surface. Heat is generated continuously within the Earth, primarily by the decay of radioisotopes (CC7).

Figure 4-2. A cross section of the Earth showing its layers. Note that the thickness of the lithosphere has been greatly exaggerated in this diagram. If it were drawn to the correct scale, the lithosphere would appear as just a thin line at the Earth’s surface. 

The core, which is about 7000 km in diameter, is composed primarily of iron and nickel, and is very dense. It consists of a solid inner core and a liquid outer core. The mantle, which surrounds the core, is composed of material that is about half as dense as the core. Temperature and pressure both increase with depth below Earth’s surface. As a result, the upper mantle, known as the asthenosphere, is thought to consist of material that is close to its melting point and is “plastic,” and capable of flowing very slowly without fracturing. The best example of such materials in common experience may be glacial ice. Glaciers flow slowly in response to gravity. Within a glacier, pressure due to the weight of the overlying ice changes the properties of the ice deeper than about 50 m so that it too becomes plastic and can flow. Much of the deeper mantle is also thought to be capable of flowing very slowly. The ability to flow is a critically important factor in the tectonic processes that shape the Earth’s surface.

The asthenosphere is surrounded by the lithosphere, the outermost layer of the Earth, which varies from just a few kilometers in thickness at the oceanic ridges to about 100 km in the older parts of the ocean basins and from about 130 to 190 km under the continents. The lithosphere consists of the mostly rigid outer shell of the mantle plus the solid crust that lies on the mantle. The lithosphere is less dense than the asthenosphere and essentially floats on top of the plastic asthenosphere. The oceans and atmosphere lie on top of the lithosphere. Pieces of lithosphere are rigid, but they move across the Earth’s surface and in relation to each other as they float on the asthenosphere. They can be many thousands of kilometers wide, but they are generally less than 200 km thick. Because of these “platelike” qualities, they are called lithospheric plates (or “tectonic plates” or just “plates”). Processes that occur where the plates collide, move apart, or “slide” past each other are the principal processes that create the mountains, trenches, and other surface features of the continents and the ocean floor.

Lithosphere, Hydrosphere, and Atmosphere

Relative to the Earth, the lithosphere is thin, rather like the skin on an apple (Fig. 4-2). At the top of this thin layer are the mountains, ocean basins, and other features of the Earth’s surface. From the deepest point in the ocean to the top of the highest mountain is a vertical distance of approximately 20 km. This 20-km range is very small compared to the Earth’s radius, which is more than 6000 km. Consequently, the planet is an almost smooth sphere (from space it looks smoother than the skin of an orange) on which the mountain ranges and ocean depths are barely perceptible.

There are two types of crust—oceanic and continental—both of which have a substantially lower density than the upper mantle material on which they lie. Oceanic crust has a higher density than continental crust (Fig. 4-3). According to the principle of isostasy (CC2), lithospheric plates float on the asthenosphere at levels determined by their density. Consequently, if the continental and oceanic crusts were of equal thickness, the ocean floor would be lower than the surface of the continents but not as deep as they actually are. However, oceanic crust is much thinner (typically 6–7 km thick) than continental crust (typically 35–40 km), causing an additional height difference between the surface of the continents and the ocean floor (Fig. 4-4).

Figure 4-3. Structure of the lithosphere. Continental crust is typically 35 to 40 km thick, whereas oceanic crust is typically only 6 to 7 km thick. Continental crust has a lower density than oceanic crust, but both continental and oceanic crust have a considerably lower density than upper mantle material.

Figure 4-4. Relative heights of parts of the Earth’s surface. The average elevation of the continents is 841 m above sea level, and the average depth of the oceans is 3865 m. Almost 30% of the Earth’s surface is above sea level. Much of the surface of the continental crust is below sea level.

The density difference between continental and oceanic crust is due to differences in their composition. Both are composed primarily of rocks formed from cooling magma and consist mainly of silicon and aluminum oxides. However, continental crust is primarily granite, and oceanic crust is primarily gabbro or basalt, both of which have higher concentrations of heavier elements, such as iron, and thus a higher density than granite.

Surrounding each continent is a continental shelf covered by shallow ocean waters. The continental shelf is an extension of the continent itself, so this portion of the continental crust surface is submerged below sea level. Both the width and the depth of the shelf vary, but it is generally in waters less than 100 to 200 m deep and ranges in width from a few kilometers to several hundred kilometers. The continental shelf slopes gently offshore to the shelf break, where it joins the steeper continental slope (Figs. 4-4, 4-5). The continental slope generally extends to depths of 2 to 3 km. Seaward of the base of the slope, the ocean floor either descends sharply into a deep-ocean trench (Fig. 4-5) or slopes gently seaward on a continental rise that eventually joins the deep-ocean floor (Figs. 4-4, 4-5). Much of the deep-ocean floor is featureless flat abyssal plain, but other areas are characterized by low, rolling abyssal hills.

Figure 4-5. General features of the seafloor topography between the continents and the deep ocean floor.

A layer of sediment lies on top of the oceanic crustal rocks, constituting part of the crust. The thickness of the sediment is highly variable, for reasons discussed in this chapter and Chapter 6.

The hydrosphere consists of all water in the lithosphere that is not combined in rocks and minerals—primarily the oceans and the much smaller volume of freshwater (Table 5-1). The oceans cover all the oceanic crust and large areas of continental crust around the edges of the continents; all of which total more than two-thirds of the Earth’s surface area.

The atmosphere is the envelope of gases surrounding the lithosphere and hydrosphere and is composed primarily of nitrogen (78%) and oxygen (21%). Although these gases have much lower densities than liquids or solids, they are dense enough to be held by the Earth’s gravity. Less dense gases, including hydrogen and helium, are so light that they tend to escape from the Earth’s gravity into space. Although the light gases were present in large quantities when the Earth first formed, only trace concentrations are present in the atmosphere today. The atmosphere is discussed in Chapter 7.

Studying the Earth’s Interior

The processes that occur beneath the lithospheric plates are very difficult to study because they occur deep within the Earth, beneath kilometers of crustal rocks and upper mantle. Studies of the processes below the crust rely primarily on indirect observations, such as examination of volcanic rocks, studies of earthquake-energy transmission through the Earth, and studies of meteorites. Meteorites are examined because they are believed to represent the types of material that make up the Earth’s core and mantle. Until 2023, all samples of rocks thought to be composed of mantle material were likely chemically and mineralogically altered since no sample had ever been obtained from the mantle itself. However, in 2023, the JOIDES Resolution recovered a 1.268 m long drill core of nearly continuous mantle rock from a site on the Mid-Atlantic Ridge, where mantle rocks were exposed. Analysis of this core will continue for at least several years, but it has already revealed new information about mantle processes related to hydrothermal vent activity that occurs along the ridge.

A technique called “seismic tomography” enables scientists to use earthquake waves to study the Earth’s internal structure in more detail than is possible by other means. It has yielded several intriguing findings about the Earth’s interior. For example, features resembling mountains and valleys have been found on the core–mantle transition zone. The mountain-like features extend downward into the molten core and are as tall as those found on the Earth’s surface. It has been hypothesized that some of these features may be sediment-like accumulations of impurities that float upward out of the liquid nickel/iron core.