Earth’s Interior
Earth’s Interior
The radius of Earth—that is, the distance from its center to its surface—is about 3, 950 mi (6, 370 km). Geologists understand the structure and composition of the surface by direct observation and by analysis of rock samples raised by drilling projects; however, the depth of drill holes and, therefore, the depth limit of scientists’ ability to directly observe Earth’s interior is severely limited. Even the deepest drill holes (about 7.5 mi [12 km], created by a Soviet scientific project that involved 24 years of drilling) penetrate less than 0.2% of the distance to Earth’s center. Thus, we know more about the layers near Earth’s surface than about the depths, and can only investigate conditions deeper in the interior through indirect means.
Geologists collect indirect information about the deep interior from several different sources. Some rocks found at the surface, such as kimberlite, originate deep in Earth’s crust and in the mantle. These rocks provide geologists with samples of the composition of Earth’s interior; however, their depth limit is still on the order of a few tens of miles. Another source of information, because of its ability to probe Earth to its very core, is more important: seismic waves. When an earthquake occurs anywhere on the planet, seismic waves—mechanical vibrations transmitted by the solid or liquid rock of Earth’s interior—travel outward from the earthquake center. The speed, motion, and direction of seismic waves changes dramatically at depth different levels within Earth, and these are known as seismic transition
zones. From such data, scientists have concluded that Earth is composed of three basic parts: the crust, the mantle, and the core.
The crust
The outermost layer of Earth is the crust or litho-sphere, a thin shell of rock that covers the globe. There are two types of crust: (1) the continental crust, which consists mostly of light-colored rock of granitic composition and which underlies the continents, and (2) the oceanic crust, which consists mostly of dark-colored rock of basaltic composition and underlies the oceans. The continents have an average elevation of about 2, 000 ft (609 m) above sea level, while the average elevation (depth) of the ocean floor is 10, 000 ft (3, 048 m) below sea level. An important difference between continental and oceanic crust is their difference in density. Continental crust has a lower average density (2.6 g/cm3) than does oceanic crust (3.0 g/cm3). This density difference allows the continents to float permanently on the upper mantle, persisting more or less intact for billions of years. Oceanic crust, in contrast, is barely able to float on the mantle (which has a density of about 3.3 g/cm3).
As oceanic crust ages, it accumulates a heavy under-layer of cooled mantle rock; the resulting two-layer structure eventually sinks of its own weight into the mantle, where it is melted down and recycled. Because of this recycling process, no oceanic crust older than about 200 million years exists on the surface of Earth. About 16% of the mantle consists of recycled oceanic crust; only about 0.3% consists of recycled continental crust.
Another difference between the oceanic crust and continental crust is their difference in thickness. The oceanic crust is 3–6 mi (5–10 km) thick, while the continental crust averages about 20 mi (35 km) in thickness and can reach 40 mi (70 km) in certain sections, particularly those found under recently elevated mountain ranges such as the Himalayas.
The bottom of the crust (both the oceanic and continental varieties) is determined by a distinct seismic transition zone termed the Mohorovicic´ discontinuity. The Mohorovicic´ discontinuity, commonly referred to as “the Moho” or the “M-discontinuity,” is the transition or boundary between the bottom of the crust and the solid, uppermost layer of the mantle (the lithospheric mantle). As the thickness of the crust varies, the depth to the Moho varies, from 3–6 mi (5– 10 km) under the oceans to 20–40 mi (35–70 km) under the continents.
The Moho was first discovered by the Croatian geophysicist Andrija Mohorovičic´ (1857–1936) in 1908. On October 8, 1908, Andrija Mohorovičic´ observed seismic waves from an earthquake in Croatia. He noticed that both the compressional (or primary [P]) waves and the shear (or secondary [S]) waves, at one point in their journey, picked up speed as they traveled farther from the earthquake. This suggested that the waves had been deflected. He noted that this increase in speed seemed to occur at a depth of about 30 mi (50 km). Since seismic waves travel faster through denser material, he reasoned that there must be an abrupt transition at that depth from the material of the crust to denser rocks below. This transition zone was later named for its discoverer. The Moho is a relatively narrow transition zone, estimated to be 0.1–1.9 mi (0.2–3 km) thick. It is defined by the level within the Earth where P wave velocity increases abruptly from an average speed of 4.3 mi/sec (6.9 km/sec) to about 5.0 mi/sec (8.1 km/sec).
The mantle
Underlying the crust is the mantle, which comprises about 82% of Earth’s volume and 65% of its mass. The uppermost section of the mantle, which is solid, is called the lithospheric mantle. This section extends from the Moho down to an average depth of 40 mi (70 km), fluctuating between 30 and 60 mi (50– 100 km). The density of this layer is greater than that of the crust, averaging 3.3 g/cm3. Like the crust, this section is solid, and is cool relative to the material below. The lithospheric mantle, combined with the overlying solid crust, is termed the lithosphere, a word derived from the Greek lithos, meaning rock. At the base of the lithosphere is another seismic transition, the Gutenberg low velocity zone. At this level, the velocity of S waves decreases dramatically, and seismic waves appear to be absorbed more strongly than elsewhere within the earth. Scientists interpret this to mean that the layer below the lithosphere is a “weak” or “soft” zone of partially melted material (1–10% molten material). This zone is termed the asthenosphere, from the Greek asthenes, meaning “weak.”
This transition between the lithosphere and the asthenosphere is named after German geologist Beno Gutenberg (1889–1960), who made several important contributions to our understanding of Earth’s interior. It is at this level that some important Earth dynamics occur, affecting those of us here at Earth’s surface. At the Gutenberg low velocity zone, the lithosphere is carried on top of the weaker, less-rigid asthenosphere, which seems to be in continual circulation. This circulatory motion creates stress in the rigid rock layers above it, and the slabs or plates of the lithosphere are forced to jostle against each other like ice cubes floating in a bowl of swirling water. This motion of the lithospheric plates is known as plate tectonics (from the Greek tektonikos, meaning construction), and is responsible for many surface phenomena, including earthquakes, volcanism, mountain-building, and continental drift.
The asthenosphere extends to a depth of about 155 mi (250 km). Below that depth, seismic wave velocity increases, suggesting an underlying denser, solid phase.
The rest of the mantle, from the base of the asthenosphere at 155 mi (250 km) to the core at 1, 800 mi (2, 900 km), is called the mesosphere (“middle sphere”). Mineralogical and compositional changes are suggested by sharp velocity changes in the meso-sphere. Notably, there is a seismic discontinuity at about 250 mi (410 km) of depth, attributed to a possible mineralogical change (presumably from an abundance of the mineral olivine to the mineral spinel), and another at about 400 mi (660 km), attributed to a possible increase in the ratio of iron to magnesium in mantle rocks. Except for these variations, down to 560 mi (900 km) the mesosphere seems to consist of predominantly solid material that displays a relatively consistent pattern of gradually increasing density and seismic wave velocity with increasing depth and pressure. Below the 560 mi (900 km) depth, the P and S wave velocities continue to increase, but the rate of increase declines with depth.
Although much of the mantle is solid by everyday standards, the entire mantle actually convects or circulates like a pot of boiling water. Images produced by analysis of seismic waves confirm that dense slabs of oceanic crust plunge all the way through the mantle to the outer surface of the core, which shows that the entire mantle is in motion, mixing thoroughly with itself over geological time—millions of years.
The core
At a depth of 1, 800 mi (2, 900 km) there is another abrupt change in the seismic wave patterns, the Gutenberg discontinuity or core-mantle boundary
KEY TERMS
Continental crust— Layer of crust (about 21 mi or 35 km thick) that underlies Earth’s continents; comprised of light-colored, relatively lightweight granitic rock.
Core— The part of Earth below 1, 800 mi (2, 900 km). Comprised of a liquid outer core and a solid inner core.
Gutenberg discontinuity— The seismic transition zone that occurs at 1, 800 mi (2, 900 km) and separates the lower mantle (solid) and the underlying outer core (liquid).Alsoknownasthecore-mantleboundary(CMB).
Gutenberg low velocity zone— The transition zone that occurs at 30–60 mi (50–100 km), between the rigid lithosphere and the underlying “soft” or partially melted asthenosphere.
Lithospheric mantle— The rigid uppermost section of the mantle, less than 60 mi (100 km) thick. This section, combined with the crust, constitutes the litho-sphere, or the solid and rocky outer layer of Earth.
Mantle— The thick middle layer of Earth that extends from the core to the crust, a thickness of almost 1, 800 mi (2, 900 km). The mantle is predominantly solid, although it includes the partially melted asthenosphere.
Mesosphere— The solid section of the mantle directly beneath the asthenosphere. Extends from 150 mi (250 km) down to 1, 800 mi (2, 900 km).
Mohorovičić discontinuity— The seismic transition zone indicated by an increase in primary seismic wave velocity that marks the transition from the crust to the uppermost section of the mantle.
Oceanic crust— Thin (3–6-mi [5–10-km] thick) crust that floors the ocean basins and is composed of basaltic rock: denser than continental crust.
P waves— Primary or compression waves that travel through Earth, generated by seismic activity such as earthquakes; can travel through solids or liquids.
S waves— Secondary or shear waves that travel through Earth, generated by seismic activity such as earthquakes; cannot travel through liquids (e.g., outer core).
Seismic transition zone— A layer in Earth’s interior where seismic waves undergo a change in speed and partial reflection; caused by change in composition, density, or both.
Seismic wave— A disturbance produced by compression or distortion on or within Earth, which propagates through Earth materials; a seismic wave may be produced by natural (e.g., earthquakes) or artificial (e.g., explosions) means.
(CMB). The density change at the CMB is greater than that at the interface of air and rock on Earth’s outer surface. At the CMB, P waves decrease while S waves disappear completely. Because S waves cannot be transmitted through liquids, it is thought that the CMB denotes a phase change from the solid mantle above to a liquid outer core below. This phase change is believed to be accompanied by an abrupt temperature increase of 1, 300°F(704°C). This hot, liquid outer core material is much denser than the cooler, solid mantle, probably due to a greater percentage of iron. It is believed that the outer core consists of a liquid of 80– 92% iron, alloyed with lighter element. The composition of the remaining 8–20% is not well understood, but it must be a compressible element that can mix with liquid iron at these immense pressures. Various candidates proposed for this element include silicon, sulfur, or oxygen.
The actual boundary between the mantle and the outer core is a narrow, uneven zone that contains undulations on the order of 3–6 mi (5–8 km) high. These undulations are affected by heat-driven convection activity within the overlying mantle, which may be the driving force for plate tectonics. The interaction between the solid mantle and the liquid outer core is also important to Earth dynamics for another reason; eddies and currents in the iron-rich, fluid outer core are ultimately responsible for the Earth’s magnetic field.
There is one final, deeper transition, evident from seismic wave data: within Earth’s core, at a depth of about 3, 150 mi (5, 100 km), P waves encounter yet another seismic transition zone. This indicates that the material in the inner core is solid. The immense pressures present at this depth probable cause a phase change, from liquid to solid. Density estimates are consist with the hypothesis that the solid, inner core is nearly pure iron.
The heat that keeps the whole interior of Earth at high temperatures is derived from two sources: heat of formation and radioactive metals. As Earth accreted from the original solar nebula, impacts of new material delivered sufficient energy to melt most or all of the forming planet’s bulk. As most of the new Earth’s iron sank its center through its bulkier, lighter elements (silicon, oxygen, etc.), further energy was released, sufficient to raise the temperature of the core by several thousand degrees Centigrade. Radioactive elements such as uranium and thorium, mostly resident in the mantle, have continued to supply Earth’s interior with heat in the billions of years since its formation; however, Earth’s interior continues to cool, steadily losing its primordial heat to space through the crust. As the core cools, its inner, solid portion grows at the expense of its outer, liquid portion. The current rate of thickening of the inner core is about 0.04 inch (1 mm) per year.
See also Magma.
Resources
BOOKS
Carlson, R.W. The Mantle and Core. San Diego, CA: Elsevier Science, 2005.
PERIODICALS
Deuss, Arwen, et al. “The Nature of the 660-Kilometer Discontinuity in Earth’s Mantle from Global Seismic Observations of PP Precursors.” Science. 311 (2006): 198-201.
Kerr, Richard A. “Mantle Dynamics: Rising Plumes in Earth’s Mantle: Phantom or Real?” Science. 313 (2006): 1726.
Lee, Cin-Ty Aeolus. “Are Earth’s Core and Mantle on Speaking Terms?” Science. 306 (2004): 64-65.
Mary D. Albanese