Earth's mantle

Aside from making up the vast majority of Earth's volume, the mantle contributed to the formation of Earth's atmosphere and is a driving factor in the movement of tectonic plates, which in turn are responsible for earthquakes, volcanic activity, mountain building, and other processes. Direct exploration of the mantle is difficult, if not impossible, but drilling projects, computer simulations, and other technologies are allowing researchers to continue to learn more.

Formation of the Earth's Mantle

About 4.6 billion years ago, Earth began to form from the debris of a solar nebula. About 10 million years later, Earth's interior began to differentiate into layers, particularly a core, but also a primitive mantle and crust. This event is known as the iron catastrophe because rising temperatures allowed iron to settle out of Earth's molten interior emulsion, sinking toward the center to form the core. Meanwhile, the remaining material closer to the surface began to cool and harden into the primitive mantle and crust.

The mantle and crust that formed at this point did not exist for very long. According to the giant impact hypothesis, a Mars-sized protoplanet crashed into the young Earth about 4.52 billion years ago. The impact was shallow enough that the forming core remained unperturbed, but much of the primitive mantle and crust were ejected into Earth's orbit, along with Earth's early atmosphere. Some of this debris eventually accreted to form Earth's moon.

The collision released vast amounts of heat, so a large portion of Earth's material that remained became molten. During the next 150 million years, the molten material cooled and hardened to form Earth's new rocky mantle and crust. (The differentiation of Earth's layers did not really end here. The mantle and crust continue to differentiate to this day through the movement of tectonic plates).

Structure and Characteristics of the Earth's Mantle

The interior of the earth is generally described by two sets of divisions: chemical and rheological (physical). The mantle is the earth's middle layer according to the main chemical divisions: core, mantle, crust. It is further divided into an upper mantle and a lower mantle. The rheological divisions are made based on physical characteristics, particularly the way matter flows (or how “elastically” it behaves). The mantle spans several of these rheological layers: the very bottom of the lithosphere, the entire asthenosphere, and the entire mesosphere (not to be confused with one of Earth's atmospheric layers, also called the mesosphere).

The mantle begins at a depth of about 35 kilometers (km), or 22 miles (mi) below Earth's surface and stretches down to about 2,890 km (1,790 mi), and its temperature ranges from 500 degrees Celsius (C), or 932 degrees Fahrenheit (F) near Earth's surface to 4,000 degrees C (7,230 degrees F) at its boundary with Earth's outer core. For comparison, one can consider the surface of the sun, where the temperature is an estimated 5,500 degrees C or 9,932 degrees F. By the core-mantle boundary, pressure is approximately 1.4 million standard atmospheres.

The mantle makes up quite a substantial portion of Earth's interior, accounting for about 84 percent of Earth's total volume. While it is referred to as a solid layer, the mantle's high temperature and its composition (mainly silicates and peridotite, an igneous rock containing high levels of magnesium and iron) allow the upper mantle to behave in a plastic manner, flowing very slowly, up to about one centimeter each year. This provides a rocky sea upon which tectonic plates ride and also serves to conduct heat from the inner layers of Earth. Under more pressure and temperature because of its depth, the lower mantle is denser and more rigid.

The mantle's characteristics vary widely throughout the layer; structural subdivisions are important to keep in mind, and many of these subdivisions refer to areas where seismic waves behave differently than they do in the surrounding areas because of physical differences in the matter. Beginning closest to the surface, the subdivisions are as follows: At a depth of about 35 km (22 mi) under continents and just 5 to 10 km (3-6 mi) under oceans, the Mohorovičić discontinuity (usually referred to as the Moho) separates the mantle from the crust, except at mid-ocean ridges. The Moho is actually the border between the lithosphere and the asthenosphere. This discontinuity was discovered by Croatian seismologist Andrija Mohorovičić in the early twentieth century, who observed unexpected behavior of seismic waves in the region.

The upper mantle includes approximately the bottom 65 km (40 mi) of the rigid, rocky lithosphere (which is broken up into tectonic plates) and all of the asthenosphere, a viscous area that spans the region from 100 to 200 km (62-125 mi) below Earth's surface; parts of it may extend to a much greater depth, nearly 700 km (435 mi) below the surface.

Within the upper mantle, at a depth of about 220 km (137 mi), there is another zone where seismic activity changes abruptly: the Lehmann discontinuity. The velocities of two types of seismic waves, P waves and S waves, increase suddenly at this area, as observed by Danish seismologist Inge Lehmann in 1958.

The transition zone serves as the border between the upper and lower mantle and between the asthenosphere and the mesosphere. It lies between 410 km (255 mi) and 660 km (410 mi) below Earth's surface, and it is marked off by seismic discontinuities at 410 km, at 660 km, and at several other depths within this range. The zone is formed by the changing structure of a substance called olivine within one of the mantle's main components, peridotite. Under increased pressure and temperature as depth increases, olivine's crystalline structure is altered significantly enough to affect the seismic wave paths and velocities.

The lower mantle corresponds to the mesosphere; it is denser and less plastic than the upper mantle. The final 200 km (124 mi) of the mantle is known as the D zone, or the Gutenberg discontinuity, a region marked by an abrupt decrease in seismic wave velocity. This region leads into the core-mantle boundary.

Mantle Convection and Its Effect on Tectonic Plates

Heat transfer by convection within the mantle is the main driving factor of tectonic plate movements (and thus seismic and volcanic activity). Tectonic plates are large chunks of the lithosphere, and they ride slowly along the plastic asthenosphere much like items on a conveyor belt; convection provides the energy that fuels this movement.

In general terms, the process of convection requires a fluid medium such as a liquid or a gas, rather than a solid. While the mantle is generally referred to as solid, the upper layer is plastic enough to support diffusion and advection, two necessary processes that contribute to convection. Diffusion refers to the random movements (Brownian motion) of particles within the medium, while advection refers to the larger-scale moving currents of heat or mass within the medium.

Within the mantle, convection manifests as hot material pushing to the surface while colder material travels down to the core. This movement of matter leads to the large-scale slow movement of the upper mantle as a whole, providing the vehicle for tectonic plates to slowly move as well. One can picture the effect on one tectonic plate: As convection drives hot mantle material up, the material adds to the edge of a nearby tectonic plate by accretion and begins to cool by convection and conduction (direct heat transfer). Meanwhile, the cooler edge of the plate is subducting because it is cooler and denser than the edge, where hot material is being added.

Tectonic activity represents the continuing differentiation of Earth's layers in that the parts of the mantle and crust are continually being broken down and replaced with new material through tectonic-induced seismic and volcanic activity and through the formation of mountains and ocean trenches. Tectonic plates interact at three major types of boundaries: convergent boundaries, divergent boundaries, and transform faults.

At convergent boundaries (also known as collision or destructive plate boundaries), plates move toward each other and meet by colliding or by one plate sliding under the other (subduction). These interactions cause friction, high pressure, and melting, leading to volcanic activity, earthquakes, and mountain formation.

At divergent boundaries (also known as extensional or constructive boundaries), plates move away from each other, allowing convecting plumes of molten magma to flow up into the space from the mantle. Between continental plates, this interaction forms rift valleys. Between oceanic plates, this interaction forms volcanic islands and mid-ocean ridges (underwater mountains).

At transform faults (also known as conservative plate boundaries), material is neither created nor destroyed. These faults are generally located in mid-ocean ridges and between continents. They are typically zigzag-shaped and relieve stress caused by interactions at nearby convergent and divergent boundaries.

The Mantle's Effect on the Evolution of Earth's Atmosphere

Earth's first atmosphere was likely blown away with the primitive mantle and crust during the impact described by the giant impact hypothesis. From volcanic evidence, scientists infer that this earliest atmosphere was a poisonous mix containing about 60 percent hydrogen; 20 percent oxygen, which includes water vapor; 10 percent carbon dioxide; 5 percent hydrogen sulfide; and a variety of other gases.

The mantle had a significant effect on the formation of Earth's next atmosphere (still not quite like today's atmosphere). After the giant impact, what was left of Earth's mantle began to convect violently because of the heat transferred by the collision. The mantle needed to cool and partially harden to allow for more differentiation—particularly the formation of the crust—but the overall mantle temperature was much higher than it is now, so a higher percentage of the mantle was molten rather than solid. This caused heat to be pushed upward by a process called outgassing: Steam and gases were released through cracks in the crust and expelled from volcanoes, contributing to the formation of Earth's new atmosphere.

The mantle was just one factor of several, though. The creation of Earth's atmosphere also was shaped by the effects of solar radiation, early life forms, and, to a large extent, impacting comets, meteorites, and protoplanets, which delivered ice and water into the atmosphere and onto Earth's surface. Some early life forms contributed oxygen to the atmosphere through photosynthesis.

About 3.5 billion years ago, Earth's magnetic field formed, protecting the atmosphere from being stripped away again by solar wind. Over time, other factors altered the atmosphere into its current state, a stratified set of gaseous layers that protect Earth from solar radiation and that heat the surface by the greenhouse effect. Earth's atmosphere now is approximately 78 percent nitrogen, 21 percent oxygen, and 1 percent argon, carbon dioxide, and other gases; it also includes water vapor.

Exploration of the Mantle

Direct exploration of the mantle can be extremely difficult because most of it is buried under kilometers of crust, well beyond the reach of even the most modern drilling technology. Exploration is usually done in the sea, as the crust is thinner there, but drilling under the sea has complications of its own.

The first major attempt to directly explore the mantle was called Project Mohole; the goal was to drill through the crust and into the Moho discontinuity to learn more about Earth's composition, age, and interior processes. The project involved drilling through the sea floor. Phase I, the experimental drilling of five holes, was largely successful and suggested that the more ambitious second and third phases could be attempted. However, the project was abandoned in 1966 as costs rose and after the organizing research group, the American Miscellaneous Society, dissolved in 1964. From the holes drilled during Project Mohole's first phase, the deepest reached about 183 meters (200 yards) below the ocean floor.

In 2007, scientists aboard the RRS James Cook had a chance to explore exposed mantle at a spot between Cape Verde and the Caribbean Sea, where a large hole exists in the crust. Later that same year, a ship called Chikyu set out from Japan to begin a project dubbed Chikyu Hakken; the goal was to drill 7 km under the seabed to reach the mantle—a deeper hole than any previously dug under the ocean. The project was initially expected to be completed in 2012, but the vessel sustained damages in March 2011, during the Tohoku earthquake and resulting tsunami. In 2012, the Chikyu began drilling, eventually reaching a depth of 7,740m (25,390 feet), breaking the world record for deep-sea drilling.

An alternative to drilling has been proposed. This alternative is a self-sinking probe filled with radionuclides, whose decay would melt rock around the probe, allowing it to continue sinking. Hypothetically, these probes could reach the Moho underneath oceanic crust in about six months.

Computer simulations provide an easier, yet less direct, approach to exploring the mantle. In 2009, for example, scientists used a supercomputer to model the distribution of various iron isotopes throughout the mantle and the rest of Earth's interior during Earth's differentiation 4.5 billion years ago. Until technology provides an easier way to explore the mantle directly, much can be learned about Earth's interior structure from seismological data.

Principal Terms

asthenosphere: the second layer (from the top) of the earth's interior when Earth is divided rheologically; corresponds with part of the upper mantle

convection: the transfer of heat or matter in a fluid medium

differentiation: the separation of interior layers of a planetary body caused by chemical and physical differences

discontinuity: a zone where seismic wave velocity changes abruptly from the adjacent zone

lithosphere: the uppermost layer of Earth's interior when Earth is divided rheologically; a hard, rigid, rocky layer that includes the crust and the top of the mantle

mesosphere: the third layer (from the top) of the earth's interior when Earth is divided rheologically; a dense, rigid layer that corresponds with most of the mantle

plastic: describes a solid material with some fluid-like properties; deformable

rheology: the study of the flow of matter; focuses on liquids and soft solids that behave like plastic

seismic wave: a moving, energy-transferring disturbance that occurs because of an event, such as an earthquake, which releases low-frequency acoustic energy

subduction: a process that can occur when tectonic plates collide; one plate slides underneath the other plate

tectonic plate: a slowly moving chunk of Earth's uppermost layer, the lithosphere

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