Plate tectonics

Plate tectonics theory holds that the earth's surface is composed of major and minor plates that are being created at one edge by the formation of new igneous rocks and consumed at another edge as one plate is thrust, or subducted, below another. This theory accounts for the cause of earthquakes, the formation of volcanoes and mountain belts, the growth and fracturing of continents, and many types of ore deposits.

Plate Boundaries

According to plate tectonics theory, the earth's crust is composed of seven major rigid plates and numerous minor plates with three types of boundaries. The divergent plate margin is a tensional boundary in which basaltic magma (molten rock material that will crystallize to become calcium-rich plagioclase, pyroxene, and olivine-rich rock) is formed so that the plate grows larger along this boundary. The rigid plate, or lithosphere, moves in conveyer-belt fashion in both directions away from a divergent boundary across the ocean floor at rates of 0–18 centimeters per year. The lithosphere consists of the crust and part of the upper mantle and averages about 100 kilometers thick. It is thicker under the continental crust than over oceanic crust. The lithosphere seems to slide over an underlying plastic layer of rock and magma called the asthenosphere. Eventually, the lithosphere meets a second type of plate boundary, called a convergent plate margin. If a lithospheric plate containing oceanic crust collides with another lithospheric plate containing either oceanic or continental crust, then the oceanic lithospheric plate is thrust or subducted below the second plate. If both intersecting lithospheric plates contain continental crust, they crumple and form large mountain ranges, such as the Himalayas or the Alps. Much magma is also produced along convergent boundaries. A third type of boundary, called a transform fault, may develop along divergent or compressional plate margins. Transform faults develop as fractures transverse to the sinuous margins of plates, in which they move horizontally so that the plate margins may be displaced many tens or even hundreds of kilometers.

Oceanic Rises

Divergent plate margins in ocean basins occur as long, sinuous mountain chains called oceanic rises that are many thousands of kilometers long. The rises are often discontinuous, as they are displaced over long distances by transform faults. The two longest oceanic rises are the East Pacific Rise, which runs from the Gulf of California south and west into the Antarctic, and the Mid-Atlantic Ridge, which runs more or less north-south across the middle of the Atlantic Ocean. The oceanic rises are deep-sea mountain ranges, and there is a rift valley that runs down the middle of the highest part of the mountain chain. The rift valley apparently forms along the ocean rises as the plates move outward from the rises in both directions and pull apart the lithosphere. The oceanic floor descends from a maximum elevation at the oceanic rises to a minimum in the deepest trenches along subduction zones. Thus, the lithosphere moves downhill from the oceanic rises to the convergent plate margins. It is thought that the lithosphere gradually cools and contracts as it moves from the oceanic rises to the convergent margins.

The oceanic rises are composed of piles of basalts forming gentle extrusions. There is high heat flow out of oceanic rises because of the large volume of magma carried up toward the surface. The magnetic minerals in the lavas are frozen into alignment with the earth's magnetic field. Half the magnetized lavas move out from the oceanic rises in one direction, and the other half move out in the opposite direction. The magnetic field of the earth appears to reverse itself periodically over geologic time. The last magnetic reversal occurred about 730,000 years ago. This last reversal can now be observed at the same distance in both directions away from the oceanic rises. A series of such magnetic reversals can be traced back across the Pacific Ocean floor for a period of about 165 million years. Many shallow-focus earthquakes occur at depths of up to 100 kilometers below the surface, along the rises and transform faults. Presumably they result from periodic movement that releases tension in the lithosphere.

Continental Rifts

A second type of divergent plate margin, called a continental rift zone, occurs in continents. Examples are the Rio Grande Rift, occurring as a sinuous north-south belt in central New Mexico and southern Colorado, and the East African Rift, occurring as a sinuous north-south belt across eastern Africa. These rift zones occur as down-dropped blocks forming narrow, elongate valleys that fill with sediment. The rift valleys often contain rivers or elongated lakes. They are characterized by abundant basalts with high potassium content and, often, smaller amounts of more silica-rich rocks called rhyolites. Rhyolites are light-colored volcanic rocks containing the minerals alkali feldspar (potassium, sodium, and aluminum silicate), quartz (silica), sodium-rich plagioclase, and often minor dark-colored minerals. Shallow-focus earthquakes result in these areas from the tension produced as the continental crust is stretched apart, much as taffy is pulled.

Many rift valleys never become very large. Others grow and may actually rip apart the continents to expose the underlying oceanic crust and rise, as is occurring in the Red Sea. There, the oceanic crust is near enough to the continents that it is covered with sediment. Eventually, the continents on both sides of the Red Sea may be pulled apart so far that the underlying oceanic floor will be exposed, with no sediment cover. About 240 million years ago, the continents of North and South America, Europe, and Africa were joined in an ancient landmass called Pangaea. They slowly broke apart along the north-south Mid-Atlantic Ridge from about 240 to 70 million years ago. At first, only a rift valley similar to the East African Rift was formed. Later it opened, much like the area of the Red Sea today. Finally, the continents drifted far enough apart during the last 70 million years to form a full-fledged ocean basin, the Atlantic Ocean.

Hot Spots

As the lithosphere moves slowly across the ocean floor, volcanic activity is generated over hot spots on the ocean floor. The Hawaiian Islands are situated over one of these hot spots. The basalts produced there are much richer in potassium than are those formed over oceanic rises. The Hawaiian Islands are part of a linear, northwest-trending chain of islands, about 2,000 kilometers long, that extends to the island of Midway. The volcanic rocks become progressively older from the Hawaiian Islands to Midway Island. Presumably, Midway Island formed first as the plate slid over the hot spot. As the plate moved to the northwest, the source of magma was removed from Midway, and newer volcanoes began progressively to form over the same hot spot.

Subduction Zones

A lithospheric plate with oceanic crust eventually reaches a compressional plate boundary and may slip below other oceanic crust in the process called subduction. One result is the island arcs in the western Pacific Ocean, such as Japan. Alternatively, the plate may be pulled below continental crust, often at an angle of 20–60 degrees to the horizontal (the Andes in western South America are the result of such movement). The intersection of the two colliding plates is marked by a sinuous, deep trench forming the deepest portions of the ocean floors. Sediment collects along the slopes of the trench, carried down from the topographic highs of the upper plate. Mountain belts are built up on the nonsubducted plate, as a result of the tremendous amounts of igneous rock that form and of the compressional forces of the plate collision, which throw much sediment and metamorphic rock in the nonsubducted plate to higher elevations.

The subducted plate can be traced to depths as great as 700 kilometers. Some of the sediments collecting along the trench are carried rapidly to great depths, where they undergo a very high-pressure and low-temperature metamorphism. (Metamorphism is the transformation of minerals in response to high temperatures and pressures deep within the earth.) Studies in 2004 indicated that large lithospheric slabs could be subducted as deep as 2,900 kilometers. Some rocks are carried more slowly to great depths and have a more normal, higher-temperature metamorphism. During metamorphism, many minerals containing water along the subducted plate gradually break down and give off water vapor, which moves up into the overlying plate. The water vapor is believed to lower the melting point of these rocks within the subducted and overlying plates so that widespread melting takes place, producing the abundant basalts and andesites that build up island arcs or continental masses above the subducted plate. In addition, much rhyolitic magma is formed in the continental crust, presumably through the melting of some of the higher-silica rocks in the continents.

Earthquake Zones

Sometimes a continent is carried by an oceanic plate into another continent at a subduction zone, which is what happened when India collided with the Asian continent. Such a collision crumples the continents into very high mountains; the Himalaya Mountains were formed in this way. This process produces an earthquake zone that is more diffuse (with foci to depths up to 300 kilometers) than are those along subducted plates. No volcanic rocks are produced in these continental-continental plate collisions. Instead, abundant granites crystallize below the surface. Granites contain the same minerals as rhyolites. Rhyolites form small crystals by quick cooling when they crystallize rapidly in volcanic rocks; granites form larger crystals from magma of the same composition by slow cooling below the earth's surface.

Development of the Plate Tectonic Model

Plate tectonics is a major, unifying theory that clarifies how many large-scale processes on the earth work. These include the formation of volcanoes, earthquakes, mountain belts, and many types of ore deposits, as well as the growth, drift, and fracturing of continents. The major concepts to support the theory were put together only in the late 1950s and the 1960s, yet many of the keys to developing the theory had been known for many years. Beginning in the sixteenth century, a number of scientists noticed the remarkable “fit” in the shape of the continents on opposing sides of the Atlantic Ocean and suggested that the continents could have been joined at one time. It was not until the early twentieth century that Alfred Wegener put many pieces of this puzzle together. Wegener noticed the remarkable similarity of geological structures, rocks, and especially fossils that were currently located on opposite sides of the Atlantic Ocean. Most notably, land plants and animals that predated the hypothesized time of the breakup of the continents, at about 200 million years before the present, were remarkably similar on all continents. Subsequently, their evolution in North and South America was quite different from their development in Europe and Africa. Climates could also be matched across the continents. For example, when the maps of the continents were reassembled into their predrift positions, the glacial deposits in southern Africa, southern South America, Antarctica, and Australia could be explained as having originated as one large continental glacier in the southern polar region.

One of the biggest problems with Wegener's concept of continental drift at that time was the lack of understanding of a driving force to explain how the continents could have drifted away from one another. However, in 1928, British geologist Arthur Holmes proposed a mechanism that foreshadowed the explanation that was later adopted by science. He suggested that the mantle material upwelled under the continents and pulled them apart as it spread out laterally and produced tension. The basaltic oceanic crust would then carry the continents out away from one another, much like rafts. When the mantle material cooled, Holmes believed, it descended back into the mantle and produced belts along these areas.

From the 1920s to the early 1960s, continental drift theories had no currency, for despite Holmes's idea there was no real evidence for driving forces that might move the continents. It was not until the ocean floors began to be mapped that evidence was found to support a plate tectonic model. The topography of the ocean floor was surveyed, and large mountain ranges, such as the Mid-Atlantic Ridge with its rift valleys, and the deep ocean trenches were discovered. Harry Hess suggested in the early 1960s that the oceanic ridges were areas where mantle material upwelled, melted, and spread laterally. Evidence for this seafloor spreading hypothesis came from the mirror-image pattern of the periodically reversed magnetic bands found in basalts on either side of the ridges. The symmetrical magnetic bands could be explained only by the theory that they were originally produced at the ridges, as the earth's magnetic field periodically reversed, and then were spread laterally in both directions at the same rate.

Supporting evidence for plate tectonics began to accumulate during the 1960s. Further magnetic pattern surveys on ocean floors confirmed that the symmetrical pattern of matching magnetic bands could be found everywhere around ridges. Also, earthquake, volcanic rock, and heat-flow patterns were discovered to be consistent with the concept of magma upwelling along rises and seafloor material being subducted along oceanic trenches. Oceanic and lithospheric plates could then be defined, and the details of the interaction of the plate boundaries could be understood. With this overwhelming evidence, most geologists became convinced that the plate tectonic model was valid. However, debate continued regarding the actual forces driving the whole process. By the early 2020s, the most popular theories held that the heat emanating from the earth's mantle is the primary energy source for tectonic motion by subduction, although other forces are needed to account for all types of movement. Other theories include movement due to various mantle dynamics, gravity, or the rotation of the earth. The field remains the subject of considerable research and speculation.

Economic Applications

Plate tectonics is important economically because of the theory's usefulness in predicting and explaining the occurrence of ore deposits. Plate boundaries such as the mid-oceanic rises are areas of high temperature in which hot waters are driven up toward the surface. These hot waters are enriched in copper, iron, zinc, and sulfur, so sulfide minerals such as pyrite (iron sulfide), chalcopyrite (copper and iron sulfide), and sphalerite (zinc sulfide) form along oceanic rises. One such deposit in Cyprus has been mined for many centuries. Tensional zones sometimes created in basins behind subduction zones may form deposits similar to those at oceanic rises. In addition, ferromanganese nodules form in abundance in some places by chemical precipitation from seawater. These nodules are enriched in cobalt and nickel, as well as in iron and manganese as complex oxides and hydroxides. They could potentially be mined from ocean floors.

Deposits enriched in chromium occur in folded and faulted rocks on the nonsubducted plate next to the oceanic trench in subduction zones. This deposit is found in some peridotites (olivine, pyroxene, and garnet rocks) or dunites (olivine rock) that have been ripped out of the upper mantle and thrust up into these areas. The ore mineral chromite (magnesium and chromium oxide) is found in pods and lenses that range in size from quite small to massive. Many intrusions of silica-rich magma above subduction zones contain water-rich fluids that have moved through the granite after it solidified. The water-rich fluids deposit elements such as copper, gold, silver, tin, mercury, molybdenum, tungsten, and bismuth throughout a large volume of the granite in low concentrations. Hundreds of these deposits have been found around subduction zones in the Pacific Ocean.

Principal Terms

andesite: a volcanic rock that occurs in abundance only along subduction zones

basalt: a dark-colored, fine-grained igneous rock

continental rift: a divergent plate boundary at which continental masses are being pulled apart

convergent plate margin: a compressional plate boundary at which an oceanic plate is subducted or two continental plates collide

divergent plate margin: a tensional plate boundary where volcanic rocks are being formed

earthquake focus: the area below the surface of the earth where active movement occurs to produce an earthquake

oceanic rise: a type of divergent plate boundary that forms long, sinuous mountain chains in the oceans

subduction zone: a convergent plate boundary where an oceanic plate is being thrust below another plate

transform fault: a large fracture transverse to a plate boundary that results in displacement of oceanic rises or subduction zones

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