Continental crust

Continental crust underlies the continents, their margins, and isolated regions of the oceans. Continental crust is distinguished from its counterpart oceanic crust by its physical properties, chemical composition, topography, and age. The creation and eventual modification of continental crust is a direct function of plate tectonics.

Physical Properties and Topography

The Earth's crust exists in two forms—continental crust, or sial, and oceanic crust, or sima. Oceanic crust is characterized by dense, basic, igneous rock, basalt, while continental crust is an assemblage of sedimentary, metamorphic, and less dense, silicon-rich igneous (granitic) rocks. Scientists posit these rocks formed during the Archean geological eon. The oceanic crust makes up the floors of the Earth's ocean basins. The continental crust underlies the continents and their margins and small, isolated regions within the oceans. The total area of all existing continental crust is 150 × 10⁶ square kilometers. In total, continental crust covers about 43 percent of the Earth's surface and makes up about 0.3 percent of its mass.

The continental crust is distinguished from its counterpart, the oceanic crust, and underlying mantle by its physical properties and chemical composition. Additionally, the continental crust and oceanic crust contrast in topography. The Earth's major topographic features range from the highest mountain on the continental crust (Mount Everest, at 8,848 meters) to the deepest ocean trench (the Mariana Trench at 10,912 meters). The difference in average elevation between the two crustal forms is quite pronounced. The continental crust varies in thickness from ten kilometers along the Atlantic margin to more than ninety kilometers beneath the Himalayan mountain system. On average, continental crust is thirty-five to fifty kilometers thick. Seismic studies of the Mohorovičić (Moho) discontinuity indicate the oceanic crust averages five to ten kilometers thick. The continental crust averages a height of 0.9 kilometers above mean sea level, while the oceanic crust averages a depth of 3.8 kilometers below that datum.

This difference in levels is attributed to the fact that despite being thin, oceanic crust accounts for the majority of the Earth's crust and has a density (3.0-3.1 grams per cubic centimeter) greater than that of continental crust. While continental crust is thicker than oceanic crust, it is less dense (2.7-2.8 grams per cubic centimeter) and represents less crustal surface area. For the most part, continental crust lies near or above sea level. It is thickest where it underlies places of great elevation, such as mountain ranges. It is thinnest where it lies below sea level, such as along continental shelves. There are exceptions to this pattern of thickening and thinning. The relatively flat basins of the oceans are transversed by two-kilometer-high ridge systems, and areas of continents where intraplate volcanism is active often display thinning where the crust is stretched by rising hot mantle material. Rising hot material also makes for more buoyancy and raises the surface elevation yet maintains a thin crust. The Basin and Range Province of the western United States is a good example; the crust beneath the mountains is of relatively normal thickness, yet the elevation is high.

Composition of Oceanic and Continental Crusts

Differences in the vertical structure and rock composition between the oceanic and continental crust are pronounced. The structure of the oceanic crust has a prominent layered effect that seismic waves can readily detect. The layers are attributed to petrologic differences between basalt, gabbro, and peridotites that constitute the layers. The continental crust has a more complex layered structure, and the contacts between layers are not well defined. The continental crust is separated into upper and lower zones. The upper zone is usually highly variable in composition, with the top few kilometers of material being any combination of unmetamorphosed volcanic or sedimentary rocks, ranging to medium-grade metamorphics such as quartzites and greenschists. Below this immediate layer, the upper zone of continental crust is typically regarded as either granodiorite or quartz diorite. This assumption is based on seismic wave travel times. The upper zone of the crust is separated from the lower zone by a change in seismic velocity similar to that which separates the asthenosphere from the crust itself. This intracrustal boundary is called the Conrad discontinuity. The composition of the lower continental crust is less well known because of the relatively few places where outcrops are available for study. Observations made on rocks in the most deeply eroded regions of Precambrian shields led researchers to believe that the lower zone is composed of granulite. Granulite is a rock of intermediate-to-basic composition, containing mainly pyroxene and calcium feldspars. The velocities of seismic waves through granulite compare favorably with seismic velocities observed passing through rocks of the lower zone. Such circumstantial evidence favors granulite as the composition of the lower continental crust.

Crustal Rocks

Rocks of the continental crust formed throughout nearly the complete 4.6-billion-year history of the Earth. These rocks can be grouped into three main componentsorogenic belts, Precambrian shields, and continental platforms. Orogenic belts (orogens) are long, broad, linear-to-arcuate (curved) areas of deformed rocks. The deformation occurs to the crust during uplift and usually includes faulting and sometimes the formation of plutons and volcanoes. The result of the deformation is the creation of a mountain system. The deformations affect thick sections through the crust and leave permanent scars that can be recognized long after the uplifted mountains are eroded away.

Precambrian shields consist of deformed crystalline igneous and high-grade metamorphic rocks more than 544 million years old. These shields are the eroded roots of ancient orogens. Continental platforms are regions of relatively underformed, younger sedimentary or volcanic rocks overlying Precambrian basement. These platforms, while nestled within the continental interior and isolated from internal strain, still typically warp into broad regional structures, usually basins or domes. Shields and platforms can form a stable nucleus to continental masses. These stable regions are called cratons. Examples of a shield and sedimentary platforms forming stable craton regions are the Canadian Shield, the Michigan Basin, and the Ozark Uplift.

Crustal Plates

The Earth's crust is a solid, rigid layer of mobile plates that make up the uppermost part of the lithosphere. There are seven major plates, seven minor plates, and numerous microplates. These plates appear to float on the plastic upper mantle of the Earth, called the asthenosphere. The vertical boundary between the asthenosphere and the crust is called the Mohorovičić discontinuity, or the Moho. The Moho is a zone less than one kilometer thick in some places but several kilometers thick in others, where the velocity of seismic waves changes from about seven kilometers per second in the crust to about eight kilometers per second in the mantle. This change in seismic velocity is caused largely by a change in composition between the crust and mantle. Rocks of the mantle are rich in iron and magnesium but poor in silicon, making them denser than the silicon-enriched overlying crust.

The movement of crustal plates upon the denser asthenosphereere is believed to be caused by complex convection currents deep within the mantle. The upper zone of the Earth's crust, in which plate movement takes place, is called the tectonosphere. As the plates move about the tectonosphere, they interact with one another. The plates tear apart (rift), collide, slide under (subduct), or slide against each other (transform fault). The active edges of the plates are called plate boundaries. The interaction of crustal plates at plate boundaries, in addition to the cyclic phenomena of sedimentation, metamorphism, and igneous activity, makes the crust the most complex region of the Earth. These activities process and reprocess crustal material and lead to the diversity of physical and chemical properties observed in crustal rocks. The rocks of the crust indicate that these processes have taken place throughout geological time and further suggest that the crust has grown in bulk at the expense of the upper mantle.

Plate Margins

The oceanic and continental crusts interact along their margins. The margin may be passive, in that stresses are no longer deforming it, or the margin may be active, in which case it is a zone of seismic and tectonic activity. Along passive margins, the transition between continental and oceanic crust is gradual; a good example is the Atlantic shelf along the eastern coast of North America. The best example of an active margin between continental and oceanic crustal plates is the Pacific basin. Around the margin of the Pacific basin, relatively dense oceanic crust is being actively subducted beneath the lighter continental crust. On the continental side, mountain ranges rise (the Andes) and island arcs are formed (the Aleutians), both dominated by active volcanism. Active margins also exist where two continental plates collide. When continents come together, there is little subduction because both plates are of low density. While igneous activity is less prominent than along convergent plate boundaries, the degree of deformation along the margins can be extreme; there is often considerable uplift involved. The contacting continental plates can either slide past each other along a transform fault (such as the San Andreas fault) or act like two cars in a head-on collision. As the plates collide, the crust shortens and the intervening seafloor is uplifted, folded, faulted, and overthrust. The most dramatic example of such an interaction of continental masses is the Himalaya mountain system.

Regions such as the Himalayas of Asia and the Alps of Europe are known as suture zones. They mark the boundaries where two plates of continental crust have collided. At suture zones, the oceanic crust is subducted until the ocean basin separating the two continents disappears, and a violent collision takes place. Moving only centimeters per year, the two continents ram into each other. The deformation to the plates during such a collision can be quite dramatic. The two continental plate margins that collide already have thick, mountainous continental margins along their active subduction zones. As the collision occurs, mountain range meets mountain range, and a new, higher, and more complex set of mountains is created. The already thicker-than-average crusts beneath the two colliding continental edges combine to form an even thicker crust to support the newly uplifted mountains. This process is complicated by secondary magmatic activity that adds buoyancy and uplift. At such suture zones, the continental crust is at its thickest, and mountain peaks reach their most spectacular heights. Continental crust thus forms convergent (subducting) boundaries with oceanic crust and can also collide or slide alongside other continental plates at transform boundaries.

Rifting

The continental crust can form one other tectonic boundary within its plate margin: It can split and form a spreading zone (rift) similar to the spreading ridges of oceanic crust. Plate interaction at a continental margin may influence the crust hundreds of kilometers inland. If forces within the plate work to stretch the crust, thinning it markedly, crustal faults may develop along the thinning zone. The crustal fault blocks that form will begin to subside as the crust continues to be stretched. Because the upper mantle is also being stretched, material from the lower mantle rises to take its place. This material is hotter and raises the temperature of the surrounding rock. The result is the formation of a magma zone beneath the thin crust of the rift zone. If the magma reaches the surface, volcanic activity similar to that seen at ocean ridge systems develops. Basaltic lava flows to the surface and begins to force the sides of the rift apart. Sometimes, the divergence ends as a result of a shift in the overall dynamics of the plate. If that happens, the rift may leave a scar only a few tens of kilometers wide. Some examples are the Midcontinent Rift system of North America, the Rhine Valley of Europe, and the East African Rift Valley. If the rift continues to expand, a new ocean basin/plate is formed. The Atlantic basin is an example of continued rifting of a continental plate to form an active oceanic plate.

In some instances, rifting of two continental bodies occurs near the margin of an older continental margin, and fragments are rifted away from the main continental body. When that happens, small plateaus of continental crust (microplates) become partially submerged in the ocean or become surrounded by oceanic crust. One example is the Lord Howe Rise of the South Pacific. The highest part of the Lord Howe microplate surfaces above the ocean as New Zealand.

Andesite Model

Earth scientists have used studies of seismic velocity waves to define the boundaries and limits of continental crust and, through exhaustive field investigations and geophysical analysis, have made reliable estimates of the crust's composition. The processes responsible for the formation and dynamic nature of continental crust, however, have remained elusive. To explain their observations, Earth scientists have come to rely on their understanding of plate tectonics and the related processes of volcanism and orogenesis.

The andesite model is a tectonics-based explanation for the formation and growth of continental crust. The model can be stated as follows: The growth of continental crust results from the emplacement or extrusion of largely mantle-derived magmas formed at destructive plate margins. The process begins at the ocean ridges, where melted mantle peridotite rises to the surface as basaltic lava, forming new oceanic crust. The oceanic crust moves away from the ridge by way of seafloor spreading. The spreading is caused by the constant extrusion of more basaltic lava at the ridge. Eventually, the oceanic plate encounters a continental plate and, because of the oceanic crust's greater density, descends below the continental plate. The oceanic crust descends at an angle of 30-60 degrees, forming deep trenches along the continental margin. The descending plate eventually reaches a seismically active region of the mantle known as the Benioff zone. At the Benioff zone, the subducting plate melts, producing a chemically complex, destructive margin magma (andesitic). This lighter, less dense andesitic melt rises through the mantle and into the overriding continental crust. The rising melt creates large plutons within the crust or breaks through to the surface to form andesitic volcanoes. Many large andesitic stratovolcanoes surround the Pacific basin and form the Ring of Fire. Around the Ring of Fire, andesitic lava is erupted and added to the surface of the continents. The volcanoes of the Andes, Cascades, Indonesian Arc, Japan, and Alaska, having such familiar names as Krakatoa, Rainier, and Fujiyama, are the birthplaces of new continental crust.

Research and Safety Applications

Studying the continental crust and its related processes is important to Earth scientists because the development of the continental crust appears to be a terrestrial phenomenon—one not observed on other planets in the solar system. Furthermore, the Earth’s continental crust provided a platform on which the later stages of animal and plant life evolution occurred. Without it, life would have been restricted to ocean basins and isolated volcanic islands, and evolution would have taken a drastically different course.

The memory of early Earth's history can only be found in the continental crust. For decades, the oceanic crust recorded an age no older than 200 million years due to subduction, and the continental crust was scientists' only link with the 4.28-billion-year-old geological record of the Earth. Studies of the continental crust allow scientists to venture educated speculations as to the beginnings of the solar system some 4.6 billion years ago. In one of the oldest mountains in Europe in 2024, scientists discovered a hidden portion of the Earth's crust in Finland dating back three billion years, indicating the Earth is unique in the solar system. These findings suggest the Earth's crust beneath Denmark and Scandinavia was once part of Greenland and accounts for the oldest known portion of the Earth—250 million years older than scientists previously believed the Earth to be.

Although the mass of continental crust is small compared to Earth's overall mass, it contains substantial amounts of minerals and elements necessary for life on Earth. Additionally, continued investigations into the dynamic processes that form continental crust aid scientists in understanding many of the geological hazards that plague humans. Earthquakes and volcanoes, two of the Earth's most destructive forces, are directly related to the processes that form and shape continental crust. By establishing a more complete understanding of the nature and functions of continental crust, scientists can better prepare and warn citizens of impending geological hazards.

Principal Terms

asthenosphere: a layer of the Earth's mantle at the base of the lithosphere

crust: the outermost shell of the lithosphere

lithosphere: the outer, rigid shell of the Earth, overlying the asthenosphere

mantle: the region of the Earth's interior between the crust and the outer core

Mohorovičić (Moho) discontinuity: the seismic discontinuity, or physical interface, between the Earth's crust and mantle

orogenesis: the process of mountain-range formation

pluton: a deep-seated igneous intrusion

tectonics: the study of the assembling, deformation, and structure of the Earth's crust

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