5-3 Volcanoes and earthquakes reveal energy from a molten interior driving Earth’s surface to shift positions

Looking out the window, you might wonder if Earth looks more or less the same today as it always has. Trees and buildings come and go, but what about major landforms such as mountains and oceans? And if something as massive as a mountain could change, what force of nature is strong enough to accomplish such a task? Earth is full of energy. Some of that energy is left over from the force of impacts in the early formation of our planet, a time when the planet was completely molten. But the majority of Earth’s internal energy comes from radioactive decay of elements such as uranium, thorium, and potassium deep inside Earth. This energy drives geologic activity that profoundly affects Earth’s surface. How do we actually know what is at the center of Earth? Surprisingly, our search starts with the rocks just under our feet.

An Iron-Rich Planet

The specific kinds of rocks found on and near Earth’s surface provide important clues to our planet’s interior. The densities of typical surface rocks are around 3000 kg/m³, but the average density of Earth as a whole is 5515 kg/m³. (As you saw in Chapter 4, average density is total mass divided by total volume.) The interior of Earth must therefore be composed of substances much denser than those found near the surface. But what is this substance? Why is Earth’s interior denser than its crust? And is Earth’s interior solid like rock or molten like magma?

Iron (chemical symbol Fe) is a good candidate for the substance that makes up most of Earth’s interior for two reasons. First, iron atoms are quite massive—a typical iron atom has 56 times the mass of a hydrogen atom—and second, iron is relatively abundant. Figure 5-9 shows that it is the seventh most abundant element in our part of the Milky Way Galaxy. Other elements such as lead and uranium have more massive atoms, but these elements are quite rare—they would be ranked by their atomic numbers at 82 and 92, respectively, if Figure 5-9 was extended to the right. Hence, the solar nebula could not have had enough of these massive atoms to create Earth’s dense interior. Furthermore, iron is common in meteoroids that strike Earth, which suggests that it was abundant in the planetesimals from which Earth formed.

Figure 5-9: Abundances of the Lighter Elements This graph shows the abundances in our part of the Galaxy of the 30 lightest elements (listed in order of increasing atomic number) compared to a value of 10¹² for hydrogen. The inset lists the 10 most abundant of these elements, which are also indicated in the graph. Notice that the vertical scale is not linear; each division on the scale corresponds to a tenfold increase in abundance. All elements heavier than zinc (Zn) have abundances of fewer than 1000 atoms per 10¹² atoms of hydrogen.

Earth was almost certainly molten soon after its formation, about 4.56 × 10⁹ years ago. Energy released by the violent impacts of numerous meteoroids and asteroids and by the decay of radioactive material likely melted the solid parts collected from the earlier planetesimals. Gravity caused the abundant, dense iron to sink toward Earth’s center, forcing less dense material upward to the surface. Figure 5-10a shows this process of differentiation. This is the same process that happens when Italian salad dressing separates into various layers when left undisturbed for a time. In much the same way, we end up with a planet with the layered structure shown in Figure 5-10b—a central core composed almost entirely of iron, surrounded by a mantle of dense, iron-enriched rocks made from silicon and oxygen. The mantle, in turn, is surrounded by an extremely thin crust of relatively light silicon-rich minerals. We live on the surface of this crust. See Cosmic Connections: Comparing Earth’s Atmosphere and Interior for more information.

Figure 5-10: Chemical Differentiation and Earth’s Internal Structure (a) The newly formed Earth was molten throughout its volume. Dense materials such as iron sank toward the center, while low-density materials rose toward the surface. (b) The present-day Earth is no longer completely molten inside. A dense, solid iron core is surrounded by a less dense liquid core and an even less dense mantle. The crust, which includes the continents and ocean floors, is the least dense of all; it floats atop the mantle like the skin that forms on the surface of a cooling cup of cocoa.

Earthquake Waves as Earth Probes

Go to Video 5-2

How might we confirm that Earth does in fact have a layered structure? The challenge in directly testing this notion is that Earth’s interior is inaccessible. The deepest wells go down only a few kilometers, barely penetrating the surface of our planet. Despite these difficulties, geologists have learned basic properties of Earth’s interior by studying earthquakes and the waves they produce.

Over the centuries, stresses build up in Earth’s crust. Occasionally, these stresses are relieved with an energy burst called an earthquake. Most earthquakes occur deep within Earth’s crust. Earthquakes release energy in the form of at least three different kinds of energy waves, which travel around or through Earth in different ways and at different speeds. The first type of wave, which is analogous to water waves on the ocean, causes the rolling motion that people feel near the source of an earthquake. These are called surface waves because they travel only over Earth’s surface and can cause a tremendous amount of destruction. The two other kinds of waves, called P waves (the P is for “primary”) and S waves (the capital S is for “secondary,” not for the s in “surface” waves, which are sometimes confusingly designated with an L), travel through the interior of Earth. P waves are called longitudinal waves because their oscillations are in the same direction the energy wave travels, like a spring that is alternately pushed and pulled. In contrast, S waves are called transverse waves because their vibrations are perpendicular to the direction in which the energy waves move. S waves are analogous to waves produced by a person shaking a rope up and down (Figure 5-11).

Figure 5-11: Seismic Waves Earthquakes produce two kinds of waves that travel through the body of our planet. P waves are longitudinal waves and analogous to those produced by pushing a spring in and out. S waves are transverse waves and analogous to the waves produced by shaking a rope up and down.

What makes earthquake waves useful for learning about Earth’s interior is that they do not travel in straight lines. Instead, the paths that they follow through the body of Earth are bent because of the varying density and composition of Earth’s interior. Just as light waves bend when they pass from air into glass or vice versa, earthquake waves bend as they pass through different parts of Earth’s interior. By studying how the paths of these waves bend, geologists can map out the general interior structure of Earth.

One key observation about earthquake waves and how they bend has to do with the differences between S and P waves. When an earthquake occurs and releases energy, both S and P waves are observed near the center of the earthquake, but geologists on the opposite side of Earth observe only P waves. The absence of S waves was first explained in 1906 by the British geologist Richard Dixon Oldham, who noted that transverse vibrations such as S waves cannot travel far through liquids. Oldham therefore concluded that our planet has a molten core. Furthermore, there is a region in which neither S waves nor P waves from an earthquake can be detected (Figure 5-12). This “shadow zone” results from the specific way in which P waves are refracted at the boundary between the solid mantle and the molten core. By measuring the size of the shadow zone, geologists have concluded that the radius of the molten core is about 2200 mi (3500 km), about 55% of our planet’s overall radius but about double the radius of the Moon (1080 mi = 1738 km). In 1936, the Danish geologist Inge Lehmann explained that the rare P wave that did successfully pass through Earth is deflected into the shadow zone by a small, solid inner core at the center of our planet. The radius of this inner core is about 800 mi (1300 km).

Figure 5-12: Earth’s Internal Structure and the Paths of Seismic Waves Seismic waves follow curved paths because of differences in the density and composition of the material in Earth’s interior. The paths curve gradually where there are gradual changes in density and composition. Sharp bends occur only where there is an abrupt change from one kind of material to another, such as at the boundary between the outer core and the mantle. Only P waves can pass through Earth’s liquid outer core.

Earth’s Major Layers

The earthquake evidence reveals that our planet has a curious internal structure—a liquid outer core sandwiched between a solid inner core and a mostly solid mantle. Table 5-1 summarizes this structure. To understand this arrangement, we must look at how temperature and pressure inside Earth affect the melting point of rock.

Both temperature and pressure increase with increasing depth below Earth’s surface. The temperature of Earth’s interior rises steadily from about 14°C on the surface to nearly 5000°C at our planet’s center (Figure 5-13).

Figure 5-13: Temperature and Melting Point of Rock Inside Earth The temperature (yellow curve) rises steadily from Earth’s surface to its center. The melting point of Earth’s material (red curve) is also shown on this graph. Where the temperature is below the melting point, as in the mantle and inner core, the material is solid; where the temperature is above the melting point, as in the outer core, the material is liquid.

Earth’s outermost layer, the crust, varies quite a bit in thickness, but overall is only about 3 mi to 20 mi (5 km to 35 km) thick. Given the enormous size of Earth, the crust is an extremely thin layer. It is composed of rocks with a melting temperature far higher than the temperatures actually found in the crust. Thus, the crust is solid.

Earth’s mantle, which extends to a depth of about 1800 mi (2900 km), is largely composed of substances enriched by iron and magnesium. On Earth’s surface, specimens of these substances will melt at slightly over 1000°C. However, the temperature at which rock melts depends on the pressure to which it is subjected—the higher the pressure, the higher the melting temperature. As Figure 5-13 shows, the actual temperatures throughout most of the mantle are less than the melting temperature of all the substances of which the mantle is made. Hence, the mantle is primarily solid. However, the upper levels of the mantle—called the asthenosphere (from the Greek asthenia, meaning “weakness”)—are able to flow slowly and are therefore often referred to as being somewhat “plastic.”

At the boundary between the mantle and the outer core, there is an abrupt change in chemical composition, from silicon-rich materials to almost-pure iron with a small mix of nickel. Because this iron has a lower melting point than the material above it, the melting curve in Figure 5-13 has a “jog” as it crosses from the mantle to the outer core and remains. As a result, the temperature at which rocks melt here is somewhat less than the temperature at depths of about 1800 mi to 3200 mi (2900 km to 5100 km), and the outer core is liquid.

At depths greater than about 3000 mi (5100 km), the pressure is more than 10⁶ times ordinary atmospheric pressure, or about 10⁴ tons per square inch. Because the melting temperature of the iron-nickel mixture under this pressure is higher than the actual temperature (see Figure 5-13), Earth’s inner core is solid.

Heat naturally flows from where the temperature is high to where it is low. Figure 5-13 thus explains why there is a heat flow from the center of Earth outward. We will see in the next section how this heat flow acts as the “engine” that powers our planet’s geologic activity.

Plate Tectonics

When looking at the shape of the ocean between the continents of Africa and South America (Figure 5-14), one naturally wonders if the two continents were closer together in the past. What evidence would you need to be convinced that the two were at one time closer together and that the continents are still moving? What would you think if there were similar fossils on both sides of the Atlantic Ocean? What would you think if you could take GPS satellite measurements and see the two moving apart? Indeed, this is exactly what we observe—the Atlantic Ocean is widening.

Figure 5-14: Fitting the Continents Together Africa, Europe, Greenland, North America, and South America fit together remarkably well. The fit is especially convincing if the edges of the continental shelves (shown in yellow) are used, rather than today’s shorelines. This strongly suggests that these continents were in fact joined together at some point in the past.

Close inspection of Earth’s landforms, both above and beneath the oceans, reveal that Earth’s crust is divided into huge plates whose motions produce earthquakes, volcanoes, mountain ranges, and oceanic trenches. This picture of Earth’s crust, now known as plate tectonics (from the Greek tekton, meaning “builder”), has come to be the central unifying theory of geology, much as the theory of evolution has become the centerpiece of modern biology.

Based on the apparent fit of the continents, the German meteorologist Alfred Wegener advocated “continental drift”—the idea that the continents on either side of the Atlantic Ocean have simply drifted apart. In 1915 he published the theory that there had originally been a single gigantic supercontinent, which he called Pangaea (meaning “all lands”). Based on how fast South America and Africa are separating today, this separation probably began roughly 200 million years ago, during what geologists refer to as the early Jurassic period, when dinosaurs dominated the land.

Over time, geologists have refined this theory, arguing that Pangaea must have first split into two smaller supercontinents, which they called Laurasia and Gondwana, separated by what they called the Tethys Ocean. Gondwana later split into Africa and South America, with Laurasia dividing to become North America and Eurasia. According to this theory, the Mediterranean Sea is a surviving remnant of the ancient Tethys Ocean (Figure 5-15).

Figure 5-15: The Breakup of the Supercontinent Pangaea (a)The shapes of the continents led Alfred Wegener to conclude that more than 200 million (2 × 10⁸) years ago, the continents were merged into a single supercontinent, which he called Pangaea. (b) Pangaea first split into two smaller land masses, Laurasia and Gondwana. (c) Over millions of years, the continents moved to their present-day locations. Among the evidence confirming this picture are nearly identical rock formations 200 million years in age that today are thousands of kilometers apart but would have been side by side on Pangaea.

The actual mechanism that was driving the continents’ motions was not evident until the mid-1950s, when geologists discovered that material is being forced upward to the crust from deep within Earth. Bruce C. Heezen of Columbia University and his colleagues began discovering long mountain ranges on the ocean floors, such as the Mid-Atlantic Ridge, which stretches all the way from Iceland to Antarctica (Figure 5-16). During the 1960s, Harry Hess of Princeton University and Robert Dietz of the University of California carefully examined the floor of the Atlantic Ocean. They concluded that rock from Earth’s mantle is being melted and then forced upward along the Mid-Atlantic Ridge, which is in essence a long chain of underwater volcanoes.

Figure 5-16: The Mid-Atlantic Ridge This artist’s rendition shows the Mid-Atlantic Ridge, an immense mountain ridge that rises up from the floor of the North Atlantic Ocean. It is caused by lava seeping up from Earth’s interior along a rift that extends from Iceland to Antarctica.

The upwelling of new material from the mantle to the crust forces the existing crust to separate, causing seafloor spreading. For example, the floor of the Atlantic Ocean to the east of the Mid-Atlantic Ridge is moving east-ward and the floor to the west is moving westward. By explaining what fills in the gap between continents as they move apart, seafloor spreading helps to fill out the theories of continental drift. Because of the seafloor spreading from the Mid- Atlantic Ridge, South America and Africa are moving apart at a speed of roughly 3 cm per year. Working backward, these two continents would have been next to each other some 200 million years ago (see Figure 5-14).

Convection and Plate Motion

What makes plates move? The high temperatures within Earth cause energy in the form of heat to flow outward from Earth’s hot core to its cool crust. Hot material deep in Earth is less dense than cooler material farther away from the core and tends to rise, much the same way heated air in a giant hot air balloon will rise high above Earth’s surface. As hot mantle material rises, it transfers heat to its surroundings. As a result, the rising material cools and becomes denser. It then sinks downward to be heated again, and the process starts over. Earlier in this chapter, we considered convection cycles that occur in a boiling pot of water (Figure 5-6), which also occur in the atmosphere. Here, too, we find that convection occurs when warmer parts of the partially molten mantle, heated from below, slowly rise. The heat that drives convection in Earth’s outer layers actually comes from very far below, at the boundary between the outer and inner cores. As material deep in the liquid core cools and solidifies to join the solid portion of the core, it releases the energy needed to heat the overlying mantle and cause convection.

The upper level of the mantle, the asthenosphere, is hot and soft enough to permit an oozing, plastic flow. Atop the asthenosphere is a rigid layer, called the lithosphere (from the Greek word for “rock”). The lithosphere is divided into plates that ride along the convection currents of the asthenosphere. Earth’s crust is simply the uppermost layer of the lithosphere, with a somewhat different chemical composition than the lithosphere’s lower regions.

Figure 5-17 shows how convection causes plate movement. Molten subsurface rock seeps upward along oceanic rifts, where plates are separating. The Mid-Atlantic Ridge, shown in Figure 5-16, is an oceanic rift and seafloor spreading is occurring there. Where plates collide, cool crustal material from one of the plates sinks back down into the mantle along a subduction zone. One such subduction zone is found along the west coast of South America, where the oceanic Nazca plate is being subducted into the mantle under the continental South American plate at a relatively speedy 10 cm per year. As the material from the subducted plate sinks, it pulls the rest of its plate along with it, thus helping to keep the plates in motion. New material is added to the crust from the mantle at the oceanic rifts and is “recycled” back into the mantle at the subduction zones. In this way the total amount of crust remains essentially the same.

Figure 5-17: The Mechanism of Plate Tectonics Convection currents in the asthenosphere, the soft upper layer of the mantle, are responsible for pushing around rigid, low-density crustal plates. New crust forms in oceanic rifts, where lava oozes upward between separating plates. Mountain ranges and deep oceanic trenches are formed where plates collide.

Geologists today realize that earthquakes, and the vast majority of volcanoes, tend to occur at the boundaries of Earth’s crustal plates, where the plates are colliding, separating, or rubbing against each other. The boundaries of the plates therefore stand out clearly when earthquake locations are plotted on a map as in Figure 5-18.

Figure 5-18: RIVUXG Earth’s Major Tectonic Plates (a) Earth’s surface is divided into plates that move relative to one another. The plate boundaries are where we find earthquakes, volcanoes, and rising mountain ranges. The arrows indicate whether plates are moving apart (←→), together (→←), or sliding past one another ↑↓. (b) Plates Grinding Past One Another. The San Andreas fault, running up the west coast of North America, formed because the Pacific plate is moving northwest along the North American plate. (c) Plates Pulling Apart. The plates that carry Egypt and Saudi Arabia are moving apart, leaving a space that contains the Red Sea. (d) Plates Colliding. The plates that carry India and China are colliding. As a result, the Himalayas are being thrust upward. In this photograph, taken by astronauts in 1968, Mount Everest is one of the snow-covered peaks near the center.

The boundaries between plates are the sites of some of the most impressive geological activity on our planet. Great mountain ranges, such as the Sierras and Cascades along the western coast of North America and the Andes along South America’s west coast, are thrust up by ongoing collisions between continental plates and the plates of the ocean floor. Subduction zones, where old crust is drawn back down into the mantle, are typically the locations of deep oceanic trenches, such as the Peru-Chile Trench off the west coast of South America. Figures 5-18b, c, and d show three well-known geographic features that resulted from tectonic activity at plate boundaries.

The Cycle of Supercontinents

Plate tectonic theory offers insight into geology on the largest of scales, that of an entire supercontinent. In recent years, geologists have uncovered evidence that points to a whole succession of supercontinents that once broke apart and then reassembled. Pangaea is only the most recent supercontinent in this cycle, which repeats about every 500 million (5 × 10⁸) years. As a result, intense episodes of mountain building have occurred at roughly 500-million-year intervals.

Apparently, a supercontinent sows the seeds of its own destruction because it blocks the flow of heat from Earth’s interior. As soon as a supercontinent forms, temperatures beneath it rise, much as they do under a book lying on an electric blanket. As heat accumulates, the lithosphere domes upward and cracks. Molten rock from the overheated asthenosphere wells up to fill the resulting fractures, which continue to widen as pieces of the fragmenting supercontinent move apart.

It can take a very long time for the heat trapped under a supercontinent to escape. Although Pangaea broke apart some 200 million years ago (see Figure 5-15), the mantle under its former location is still hot and still trying to rise upward. As a result, Africa—which lies close to the center of this mass of rising material—sits several tens of meters higher than the other continents.

The changes wrought by plate tectonics are very slow on the scale of a human lifetime, but they are very rapid in comparison with the age of Earth. For example, the period over which Pangaea broke into Laurasia and Gondwana was only about 0.4% of Earth’s age of 4.56 × 10⁹ years. (To put this in perspective, 0.4% of an average human lifetime is about 4 months.) The lesson of plate tectonics is that the seemingly permanent face of Earth is in fact dynamic and ever-changing.

COSMIC CONNECTIONS: Comparing Earth’s Atmosphere and Interior