Earth's Crust

• CATEGORY: Earth

The crust is the outermost layer of the Earth. The dynamic changes involved in the creation and destruction of crustal rock also fuel volcanoes, cause earthquakes, concentrate mineral deposits, and liberate gases and water that form the atmosphere and ocean.

Overview

The crust of the Earth is the outermost layer of the Earth. It is distinct from the region of rock lying beneath it, called the mantle, in that the rocks that comprise the crust have different compositions and a lower density. Continental crust is composed mostly of granitic rocks with densities around 2.7 grams per cubic centimeter (168.5 pounds per cubic foot), and oceanic crust is composed mostly of basaltic rocks with densities around 3.0 grams per cubic centimeter (187.3 pounds per cubic foot). In contrast, rocks from the upper mantle have densities around 3.3 grams per cubic centimeter (206 pounds per cubic foot), and probably are composed mostly of peridotite.

The rocks of the Earth’s crust are quite varied. They can be classified as belonging to one of three broad groups, depending on how they formed: igneous, sedimentary, and metamorphic. These three groups are parts of what is referred to as the rock cycle, which depicts the way in which rocks from each of these groups can provide the raw material to form rocks in any other group.

Igneous rocks are formed by cooling and crystallization from molten material called lava (if on the surface) or magma (if below the surface). Igneous rocks that cool and harden on the surface are said to be extrusive, and those that cool and harden below the surface are said to be intrusive. Intrusive igneous rocks cool more slowly and thus usually contain larger mineral crystals (large enough to be seen with the unaided eye). Extrusive igneous rocks cool more rapidly and thus either contain smaller, microscopic mineral crystals or are glassy, containing no crystals at all. Some common igneous rocks are granite and gabbro (intrusive) and rhyolite, basalt, and obsidian (extrusive).

Sedimentary rocks are formed from sediment, the remnants of other rocks that were weathered and eroded when exposed at the surface. Weathering processes may be chemical (such as dissolving in water or acid, or being oxidized by oxygen from the atmosphere) or physical (such as being broken apart when water in cracks freezes and expands, or roots grow into cracks). Sediment can be transported by various agents (such as moving water or blowing wind) and is ultimately deposited in layers. The sediment may then be compacted and cemented, forming sedimentary rock. Some common sedimentary rocks include sandstone, shale, and limestone.

Metamorphic rocks are formed from other rocks that have been subjected to pressures and temperatures high enough to change the crystalline structure of the minerals in the rock (but not high enough to melt the rock). Such changes often occur in the deeper parts of the crust, where the temperature and pressure are greater (around 400 to 1,200° Celsius, and 3,000 to 15,000 atmospheres, or 752 to 2,192° Fahrenheit and 303,975,000 to 1,519,875,000 pascals). Some common metamorphic rocks are slate, schist, gneiss, and marble.

The boundary between the crust and the mantle is known as the Mohorovičić Discontinuity or simply the Moho (named for Croatian seismologist Andrija Mohorovičić, who helped to determine the interior structure of the earth). The depth of the Moho, and thus the thickness of the crust, varies widely. The crust is thickest under the continents, averaging about 40 kilometers (24.8 miles) and reaching a maximum of 70 kilometers (43.4 miles) beneath young high mountain chains such as the Himalayas. Under the ocean basins, the crustal thickness averages only about 7 kilometers (4.3 miles).

The crust and uppermost mantle (down to a depth of about 100 kilometers) constitute the rigid lithosphere, which is divided into a number of separate blocks called plates. These plates are slowly moving, driven by slow convective motions in the soft, plastic part of the mantle called the asthenosphere, which is directly beneath the lithosphere.

The crust is constantly being created, deformed, and destroyed by processes at plate boundaries. There are three types of boundaries between plates: divergent, where plates are pulled apart; convergent, where plates collide; and transform or side-slip, where plates slide horizontally past each other. Divergent plate boundaries, where plates move apart, are marked by the ocean ridge-rift system where new ocean basin crust is produced by hot upwelling magma. The Mid-Atlantic Ridge (located in the Atlantic Ocean about midway between North and South America to the west and Europe and Africa to the east) and the Red Sea (between Africa and Arabia) are examples of divergent boundaries. Convergent plate boundaries, where plates come together, can be divided into three subtypes, depending on the nature of the plate (oceanic or continental) on each side of the boundary: oceanic-oceanic, oceanic-continental, and continental-continental. At oceanic-oceanic and oceanic-continental convergent boundaries, subduction occurs. Subduction is a process in which an oceanic plate descends back down into the mantle. Subduction zones are marked by deep-sea trenches that are located where oceanic plates bend down into the mantle, there to be heated and remelted, fueling chains of volcanoes parallel to the trench. The Mariana Trench in the western Pacific and the Peru-Chile Trench along the west coast of South America are examples of oceanic-oceanic and oceanic-continental convergent boundaries, respectively. At continental-continental convergent boundaries, neither plate subducts, but instead crumple and buckle along the collision zone forming high mountain ranges like the Himalayas between India and the rest of Asia. At transform or side-slip plate boundaries, two plates slide horizontally past each other. The San Andreas fault in California is a classic example of a transform plate boundary.

New ocean basin crust forms at the ocean ridge-rift system, which is located along divergent plate boundaries. As the existing lithospheric plates are pulled apart, probably by very slow lateral flow in the underlying asthenosphere, upwelling magma fills the gap. It cools and hardens, producing the igneous rocks basalt and gabbro, which add new bands of oceanic crust to the plates. The newly formed oceanic crust spreads away from the ocean ridge-rift in a process known as seafloor spreading. The age of oceanic crust increases systematically with distance from the ocean ridge-rift system. The oldest known seafloor crust, about 340 million years old, is found in the Mediterranean Sea. Apparently in that sort of time frame or less, oceanic crust reaches a subduction zone and is recycled back into the mantle. In contrast, many continental rocks are several hundred million years old, and the oldest continental rocks found so far, from the Northwest Territories of Canada and western Australia, are about 4 billion years old, almost as old as the 4.5 billion-year age of the Earth.

Continental crust does not subduct. It remains on or near the surface and is recycled there through the processes and stages of the rock cycle. It is initially created along subduction zones where oceanic crust is consumed. As the lithospheric plate descends back down into the mantle, it is heated. Eventually, differential melting occurs; the minerals with lower melting temperatures melt and rise as molten blobs of magma, either cooling and hardening below the surface as igneous intrusions or erupting at the surface as volcanoes. These igneous intrusions and volcanoes are composed of granite, rhyolite, and similar igneous rocks that are less dense than the basalts and gabbros of oceanic crust. They form continental crust, which, because of its lower density, does not subduct but remains at the surface. When two continental plates collide, they crumple and weld themselves together, forming a high mountain range with roots that extend downward, increasing crustal thickness there. Thus, continents grow with time by two processes occurring along their edges: volcanism near subduction zones and accretion. Continents also can be broken apart when rifts develop in them, like the East African Rift. If the rift continues to grow, eventually it becomes a long narrow arm of the ocean, called a linear sea, like the Red Sea between Africa and the Arabian Peninsula.

The crust is thickest under young mountain belts, called orogenic belts, piling upward and sinking downward simultaneously to form a thick wedge of rock. In this sense, it is much like a buoyant iceberg, floating with the majority of its mass below the water. The buoyancy of the less dense crustal rocks floating on the more dense mantle rocks is known as isostasy or flotational equilibrium. Just as the iceberg must reach a flotational level by displacing a volume of water equal to its mass, so must the lighter crustal rocks displace a volume of denser mantle rocks to reach their buoyancy level. Thus the thickest crust is found under the highest mountainous regions because they have such deep roots, while the thinnest crust is found under ocean basins.

Toward the center of continental landmasses are large areas of old rocks known as cratons. The ages of rocks found in the cratons range from about 600 million to as much as 4 billion years. The cratons have been free of deformation and mountain-building for at least the last 600 million years. Consequently, their surfaces tend to be relatively flat as a result of surface processes such as weathering and erosion acting on the exposed rocks over geologically long periods of time. Cratons have long and complex histories. Over large areas of them, highly deformed rocks from the deep roots of ancient mountain regions are exposed, and in other regions the ancient mountain roots are covered by more recent layers of sedimentary rocks.

Methods of Study

Seismic waves created by both earthquakes and artificial explosions are used to probe the interior of the Earth, including its top layer, the crust. One type of seismic wave that travels through the Earth is a longitudinal compressional wave called a P wave (for primary wave); this type of wave is analogous to an acoustic or sound wave, alternately compressing and stretching the material it travels through. The density, rigidity, and compressibility of the material determine the wave speed in it. P waves travel at speeds between about 2 and 6 kilometers per second (1.2 and 3.7 miles per second) near the surface, because of the wide range of compositions of surface rocks as well as the presence of open space and fluids within them. (For comparison, sound waves travel at about 0.3 kilometers per second, or 0.18 miles per second, through the lower atmosphere.) P-wave speed reaches about 6 to 7 kilometers per second (3.7 to 4.3 miles per second) in the lower crust just above the Moho. It has been found in the laboratory that metamorphic rocks known as granulites, which can form when basalts and gabbros are subjected to the pressures and temperatures of the lower crust, have P-wave speeds in the proper range. Furthermore, granulites are similar to some rock samples brought to the surface in volcanic pipes that are thought to have originated in the lower crust.

The thickness of continental crust has been determined by the study of seismic waves that are reflected off the Moho, as well as those that refracted by it. The increase in density and the resulting increase in speed of when they cross the Moho from crust to mantle cause their wave path to bend or refract. Waves that cross the Moho at what is termed the critical angle will travel along right beneath the Moho and be refracted back to the surface at the same angle. By Snell’s law of refraction, the sine of the critical angle equals the ratio between the crust and mantle speeds. The travel time of these refracted seismic waves from their source till they are recorded on a seismograph is determined by the crust and mantle speeds, and the thickness of the crust. Using critically refracted P waves, thicknesses have been estimated for much of the crust. It is possible to check the crustal thickness determined from critically refracted waves by using reflected waves. This has been applied with particular success in continental areas with artificial acoustic wave sources such as explosives and vibrator trucks.

Context

The Earth’s crust is in a state of dynamic evolution, with rock materials being created, deformed, and destroyed. This dynamic evolution of the crust is brought about by the movement of large lithospheric plates composed of the crust and upper mantle, up to 100 kilometers (62 miles) thick. Processes at plate boundaries can produce volcanism and earthquakes.

Volcanic activity over the billions of years of the Earth’s existence has provided the water vapor and other gases necessary to form the oceans and atmosphere by the release of gases trapped in lavas that reach the surface. An understanding of volcanoes, including how, why, and where they occur, requires an understanding of the Earth’s crust and crustal dynamics. It is especially important in the areas with active volcanoes to be able to assess the hazards they pose.

Plate motion also produces earthquakes where two plates rub against each other. Usually the strongest occur at transform plate boundaries and at subduction zones along convergent boundaries. Eventually, knowledge of how crustal rocks change and respond before impending earthquakes may allow their prediction.

Exploration for important economic minerals is guided by knowledge about the evolution and composition of the crust. The concentration of valuable metal deposits, such as gold and copper, occurs during volcanic activity at ocean ridge sites where new oceanic crustal rocks are being created. Consequently, exploration efforts for such metallic ores can be directed toward identifying ancient ridge site locations. The formation of continental sedimentary rocks in the Gulf of Mexico traps organic materials that will be turned into oil and natural gas. Looking for similar types of sedimentary rocks in ancient crustal environments aids in the search for petroleum and natural gas.

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