Earth's Structure

Processes that occur in the interior of the Earth have profound effects upon the surface of the Earth and its human population. The results of such processes include earthquakes, volcanic activity, and the shielding of life-forms from solar radiation.

Overview

A simple demonstration that the Earth’s interior is different from its surface is to compare the Earth’s average density, calculated by dividing its mass by its volume, with the density of typical rocks from the surface. The average density is about 5.5 grams per cubic centimeter, while the density of typical surface rocks is between about 2.7 and 3.0 grams per cubic centimeter. This means that part of the interior must be composed of much denser material than surface rocks.

Evidence that the interior is differentiated into “layers” of various thicknesses, compositions, and mechanical properties comes primarily from analyzing the Seismic waves produced by earthquakes that travel through the Earth. The thinnest layer is the outermost one known as the crust. The crust varies in thickness from about 5 kilometers under parts of the ocean basins up to about 70 kilometers under the highest mountain ranges of the continents. The crust is composed of a number of different rock types, but there are systematic differences between the crust of continents and that of ocean basins; continental crust is generally granitic (similar to granite), while oceanic crust is generally basaltic (similar to basalt). Both granite and basalt are igneous rocks, meaning that they cooled and hardened from hot molten material, and both are composed of silicate minerals.

However, the silicate minerals in basalt (such as pyroxene, olivine, and calcium-rich plagioclase feldspar) are comparatively rich in iron, magnesium, and calcium, giving them a generally dark color and slightly greater density (about 3.0 grams per cubic centimeter), while the silicate minerals in granite (such as quartz, potassium feldspar, and various micas) are poorer in iron, magnesium, and calcium, making them generally lighter in color and slightly lower in density (about 2.7 grams per cubic centimeter). Note that this distinction does not hold completely: Basalt and similar rocks can be found on continents, and sediments weathered from granite and similar rocks can be found in ocean basins.

The base of the crust is marked by a boundary known as the Mohorovičić discontinuity, or Moho. It represents a change in density of the rock above and below it. Rocks just below the Moho are slightly denser, about 3.3 grams per cubic centimeter, than either continental or oceanic crustal rocks. The rocks below the Moho probably are peridotite. Peridotite, composed mostly of olivine and pyroxene, is similar to basalt, but it is richer still in iron and magnesium. Peridotite is thought to represent the general composition of the layer underlying the crust, called the mantle. The mantle comprises the bulk of the Earth, representing about 80 percent by volume.

In the upper mantle, at depths starting about 100 kilometers beneath the surface and extending down to about 410 kilometers, is a zone of less rigid and more plastic, perhaps even partially melted, material called the asthenosphere. The crust and the part of the mantle above the asthenosphere, acting as a rigid unit, are known collectively as the lithosphere. The change to plastic behavior in the Asthenosphere occurs because temperatures there are close to the melting point of peridotite. Although temperature continues to increase below the asthenosphere, the greater pressures at greater depths are high enough to keep the rock from melting.

The asthenosphere is thought to play an important role in movements of the lithosphere above. According to the theory of plate tectonics, the lithosphere is divided into a number of plates about 100 kilometers thick that are in constant motion at speeds of up to several centimeters per year, driven by hot convection currents of material moving slowly in the plastic asthenosphere. The hot material rises along divergent plate boundaries marked at the surface by the volcanic ridge-rift system that extends through the ocean basins around the globe. The slowly moving convection currents in the asthenosphere then move laterally away from the ridge-rifts, carrying the lithospheric plates above away from the ridge-rifts. As it moves laterally, the asthenosphere cools, becoming denser and sinking back downward. The sites where the convection currents sink are places where lithospheric plates with ocean crust on top dive into the mantle in a process called subduction. At these sites, marked at the surface by trenches in the ocean basin floor, crustal rocks may be carried into the upper mantle to depths as great as 700 kilometers. Below this level, the rock may simply be too dense for the lithospheric plates to penetrate.

There are two lower boundaries within the mantle. At 410 and 660 kilometers below the surface, abrupt increases in density occur. Although one might suspect a change in composition to account for the jump in density, laboratory studies of rocks under pressure suggest an alternative explanation. The primary mineral in peridotite is olivine. The pressures at 410 kilometers and again at 660 kilometers collapse the crystalline structure and produce denser minerals with the same iron and magnesium silicate composition. At the pressure existing at 410 kilometers, olivine converts to the denser mineral called spinel, and at the even higher pressure at 660 kilometers, both spinel and pyroxene collapse to yet a denser mineral known as perovskite. Thus the changes occurring in the mantle to produce the asthenosphere and the discontinuities at 410 and 660 kilometers are not changes in composition but instead changes in physical properties caused by temperature and pressure. The density increases from about 3.3 grams per cubic centimeter at the top of the mantle to about 5.6 grams per cubic centimeter at its base.

The next layer beneath the mantle is the outer core. This layer begins at a depth of about 2,900 kilometers beneath the surface and continues to a depth of 5,100 kilometers. There is a large density increase across the core-mantle boundary, from 5.6 grams per cubic centimeter at the base of the mantle to about 10 grams per cubic centimeter at the top of the core. Iron is the only reasonably abundant element that would have the required density at the tremendous pressure of millions of atmospheres at these depths. However, pure iron would give too high a density, so iron mixed with about 15 percent nickel, sulfur, silicon, and possibly oxygen and even hydrogen has been suggested. At the pressures and temperatures that must exist in the outer core, iron alloys would be in a molten state. Complex currents of metallic iron alloy, generated in the fluid outer core by convection and the Earth’s rotation, give rise to the Earth’s main Magnetic field through a geodynamo process.

The core-mantle boundary represents a composition change from the silicate minerals of the lower mantle to the metals of the core. The boundary is a sharp one, but whether it is smooth and spherical in shape or irregular with “hills” or “peaks” on its surface is not known. There is some evidence from seismology that the lower mantle within 100 kilometers of the core boundary is a transition zone with a change of properties. It may consist of a mix of mantle and core material that is less rigid than the mantle rocks above it.

The innermost layer of the Earth’s interior is the inner core. This region has a radius of about 1,300 kilometers where there is a boundary with the outer core. Increasing pressure at these depths requires that the iron of the inner core is solid. It is thought the solid inner core continues to grow in size as iron in the molten outer core crystallizes as Earth slowly cools. Because the solid inner core is separated from the mantle by the molten outer core, it can rotate independently of the planet’s surface. Seismic studies suggest that the inner core rotates slightly faster than the mantle and crust. The inner core also occasionally shifts its orientation, slowing down and changing the direction of its spin over time. In 2023, scientists reported in the journal Nature Geoscience that the Earth’s inner core was slowing down and was likely on the verge of a reversal.

Research has also suggested that the solid inner core may itself be composed of layers. There is another core within the inner core, called the inner-inner core, that is about 1,180 kilometers (or 733 miles) in diameter and, while still composed of iron, that iron may have a different crystalline structure than the rest of the iron inner core. Scientists have also posited that the inner core is steadily expanding, at a rate of one millimeter per year.

Methods of Study

Much of what is known about the structure of the Earth’s interior comes from the analysis of seismic waves generated by earthquakes or by explosives detonated at or just below the surface. After passing through the Earth, the wave vibrations are recorded on seismographs located all around the world, revealing information about the part of the interior they traveled through.

The seismic waves that pass through the interior are called body waves, because they propagate through the body of the Earth and not along the surface. Body waves are of two varieties: primary (or P) waves, and secondary (or S waves). P waves are the same as acoustic or sound waves. They cause the material they traverse to move back and forth in the direction of wave travel, alternately stretching and compressing it. Like ordinary sound waves, P waves can travel through any sort of material—solid, liquid, or gas. S waves are transverse waves, which move material along the wave path from side to side. Consequently they can travel only through rigid, that is, solid, material; S waves cannot travel through liquids or gases.

Both P waves and S waves cross the asthenosphere of the upper mantle, but with reduced speed (that is why the upper mantle is sometimes called the low-velocity zone), suggesting lower rigidity but not a liquid state, since S waves do propagate through it. Therefore, it seems that the asthenosphere is a solid but plastic region, able to ooze and flow very slowly.

At the core-mantle boundary, P waves abruptly slow down by almost a factor of 2 as they enter the outer core, and S waves disappear, indicating the material of the outer core has no rigidity. Since gases cannot exist at the conditions of the outer core, the material of this region must be a liquid.

Other locations in the interior are marked by increased speeds for both P waves and S waves. There is a sharp increase in speed at the Mohorovičić discontinuity at the base of the crust. The P-wave speed jumps from about 6 kilometers per second above the Moho to around 8 kilometers per second below it, while the S-wave speed jumps from about 4 to 5 kilometers per second.

Below the asthenosphere, both P-wave and S-wave speeds gradually increase to a depth of about 410 kilometers, at which point both sharply increase. Laboratory studies indicate that the increase in density when olivine collapses to form spinel accounts for the increased speeds at this depth. At a depth of around 660 kilometers, a second abrupt increase in both wave speeds occurs, here caused by a second collapse to produce the yet denser mineral perovskite. The speeds for this part of the mantle match those obtained in the laboratories from waves passing through perovskite samples placed under the kinds of pressures found at 660 kilometers. A final increase in speed is observed when P waves pass the outer to inner core boundary. This is due to a phase transition from liquid iron in the outer core to solid iron in the inner core. Such a phase transition is supported by the probable reappearance of S waves in the inner core.

Another way in which the existence of structural boundaries within the Earth can be shown from an analysis of seismic waves is to examine the way the waves reflect and/or refract when they encounter the boundaries. What the waves do depends on the angle at which they approach the boundary, as well as on the properties of materials on both sides of the boundary. P waves are reflected off the Moho, the core-mantle boundary, and the inner-outer core boundary, providing clear evidence that there are sharp boundaries between the crust and mantle, the mantle and outer core, and the outer and inner cores. Waves have also been detected reflecting off the 660-kilometer discontinuity.

Refraction or bending of waves yields further evidence. As P waves cross the mantle-core boundary, they are refracted toward the center of the Earth because their speed is less in the molten outer core. This deflection of P waves passing through the outer core leaves a gap stretching around the Earth in the form of a band extending from 100° to 140° from the epicenter of the earthquake. This gap is known as the P-wave shadow zone because no P waves reach the surface in this band.

Advances in computer science have allowed the identification of even subtler details about the Earth’s interior. Computerized tomography is a technique used in medicine, in which X rays from all directions are analyzed in a computer to give a three-dimensional picture of the internal organs of the human body. Seismic tomography is an analogous approach that uses seismic waves that travel from earthquakes to seismographs around the world to map the Earth’s interior. This technique includes both P and S body waves traveling through the interior as well as surface waves traveling along the surface of the Earth. By looking at the travel times of the different waves, scientists are able to compare speeds along different paths. Such an approach has resulted in maps of slow and fast regions of the mantle that probably represent less rigid (warmer) and more rigid (cooler) regions.

Context

The interior of the Earth has profound effects on the surface. The interior acts as a complex heat engine that is the driving force behind plate tectonics, resulting in the formation and evolution of oceanic and continental crust. In the process, earthquakes and volcanic activity occur that create hazards for the human population on the Earth’s surface. Complete acceptance of the plate tectonic theory could not have occurred without the discovery of the asthenosphere, which makes the movement of the lithospheric plates plausible.

It is fortunate that the Earth has a magnetic field. Without it, the age of discovery and exploration would not have been possible, for navigation by magnetic compasses allowed voyages across uncharted oceans. It is now theorized that the Earth’s magnetic field is generated in the molten metallic outer core by the geodynamo process. An important effect of the core-generated magnetic field is the changes it has undergone through time. In particular, at rather irregular intervals of geologic time, the magnetic poles reverse polarity; during such reversals, the magnetic field decreases in strength. Since the magnetic field shields life-forms on the Earth’s surface from charged particles emitted by the Sun, there is some concern that such field weakenings during polar reversals could result in more cancers and genetic mutations. Some scientists suspect that polar reversals might be at least partly responsible for some of the mass extinctions that have occurred in the geologic past, as well as periods of rapid evolution. Thus, the surface of the Earth as well as the life-forms on it depend upon and are strongly affected by processes occurring within the Earth’s interior.

Bibliography

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