Subduction and orogeny

Subduction and orogeny are fundamental consequences of plate tectonics and are the two processes that build mountains on the edges of continents. Through the recognition of subduction, scientists have been better able to determine regions where risks of earthquakes and volcanic explosions are significant.

Theory of Geosynclines

Subduction and orogeny are two processes that are fundamental to the evolution of continents. All continents contain long, narrow mountain chains near their edges that are composed of folded and faulted rocks that are younger than the rocks in the continental interiors. The event that formed the mountains is termed an orogeny, and the mountain chain itself is called an orogenic belt. Because of the proximity of mountain chains to the edges of continents, scientists have believed for centuries that orogenies reflected movements localized along continental margins. It is only lately, however, that orogeny has been coupled with subduction, the process in which seafloor descends below a continent or another piece of seafloor. Earlier views of orogeny were part of the theory of geosynclines. Geosynclines are linear basins that form on subsiding regions of the earth's surface adjacent to continental margins, fill with sediment, and evolve into mountains composed of folded and faulted sedimentary strata. The origin of the compressive forces responsible for the creation of the mountains was not known. Erosion of the newly created mountains provides sediment for new geosynclines that develop seaward of the mountain belt, thereby completing one geosynclinal cycle, which typically lasts on the order of a few hundred million years. The advent of plate tectonics theory in the 1960s led the majority of the scientific community to abandon the geosynclinal cycle in favor of the subduction process as an explanation for orogenies. Subduction was attractive because it readily provided a mechanism by which the large compressive forces needed to form mountains could be produced.

Orogenic belts are characterized by the folding and faulting of layers of rock, by the intrusion of magma, and by volcanism. Folds and faults form parallel to the continental margin and extend hundreds of kilometers toward the continental interior. Folding bends layers of rocks, whereas faulting takes rocks that were side by side and stacks them on top of each other in sheets up to 20 kilometers thick. Both processes significantly shorten the horizontal and thicken the vertical dimensions of the continents. At the same time as they are folded and faulted, the rocks are intruded by magmas derived from tens of kilometers below the surface. Some of the magmas eventually erupt, building volcanoes on the deformed rocks. An additional feature of orogenic belts is the juxtaposition of sequences of rock that have nothing in common with each other. The rocks in the two sequences may be different in age, composition, or style of folding. The origin of this juxtaposition was unreconciled by the theory of geosynclines, which holds that all rocks in a mountain belt were originally deposited near one another and were derived from the same source. The theory of plate tectonics, however, easily explains the juxtaposition.

Theory of Plate Tectonics

To understand subduction and orogeny, one must have a clear grasp of the theory of plate tectonics. This theory states that the surface of the earth is composed of about twelve rigid plates, which are less than 100 kilometers thick. Plates are either oceanic or continental. Below the plates is a partially molten layer that allows the rigid plates to float and move relative to each other at speeds between two and ten centimeters per year. The motions are defined primarily by the oceanic plates; the continental plates drift passively.

The relative motions of the plates define three types of boundaries: convergent, where plates move toward one another; divergent, where plates spread apart; and transcurrent, or conservative, where plates slide smoothly past each other. Convergent boundaries are frequently along the margins of continents, and divergent boundaries are commonly in the ocean basins. For example, the west coast of South America is a convergent boundary, and the Mid-Atlantic Ridge—the mountain range that runs down the middle of the Atlantic Ocean—is a divergent boundary. Divergent boundaries are zones along which two plates separate. This type of plate boundary is typically demarcated by a linear ridge system in an ocean basin where magma rises from deep in the earth to fill the gap created by the diverging plates. When the hot magma contacts the cold seawater, it solidifies into new oceanic crust. As the plates continue to separate, additional magma wells up from the earth's interior, allowing the continuous creation of oceanic crust at the ridge. This process is known as seafloor spreading and is responsible for the drifting of the continents on the surface of the earth.

Convergent boundaries are where two plates move toward each other and one plate subducts, or descends below, the other. The subducting plate is always oceanic, but the overriding plate may be either oceanic or continental. This reflects the greater density of oceanic crust relative to continental crust, which allows the oceanic plates to sink readily into the earth's interior, whereas the continental plates remain afloat. When two continental plates collide, neither plate subducts—they are too light—but the plates push against each other with tremendous force, such that their edges buckle and huge mountain ranges grow. This process built the world's tallest mountains, the Himalaya, which are the result of the collision between the subcontinent of India and the continent of Asia.

Subduction Zones

Subduction zones are characterized by a progression from the subducting to the overriding plate of deep trenches, high mountains, and many volcanoes that occupy an area hundreds of kilometers wide and thousands of kilometers long. The deep trench, frequently filled with sediments eroded from the adjacent mountains, marks the point in the ocean floor where the subducting plate bends to descend below the overriding plate. As the oceanic plate descends, these sediments are scraped onto the overriding plate. Slivers of oceanic crust may also scrape off and mix with the sediments. The off-scraped rocks form an intricately folded and faulted region tens of kilometers wide and several kilometers high at the edge of the overriding plate. These complexly deformed mixtures of sediments and slivers of oceanic crust are called mélanges and are characteristic of most ancient subduction zones now exposed on land.

Another important feature of subduction zones is the linear belt of volcanoes on the overriding plate that parallels the plate boundary. The volcanoes grow from the eruption of magma that is generated at the interface between the subducting and overriding plates at depths between 100 and 200 kilometers. At these depths, the temperature of the earth is high enough to melt small areas of either the subducting or the overriding plate. The magma rises, intruding the rocks at the surface and eventually erupting to build the volcanic belt. Some of the magma, however, may solidify between the top of the oceanic plate and the surface.

The similarity of features in orogenic belts and subduction zones is striking and forces the obvious conclusion that subduction leads directly to orogeny. An orogeny can occur either during the subduction of an oceanic plate below a continental plate, such as on the west coast of South America, or during the collision of two continental plates, such as in the Himalaya. Because continents do not subduct, the compressive forces are much greater in a continent-continent collision than in seafloor subduction. The mountains produced during collision (Himalaya), therefore, are much taller than those generated during subduction (Andes).

Consequences of Plate Tectonics

The theory of plate tectonics elucidates important differences between the oceans and the continents and provides a mechanism by which different rock sequences can be juxtaposed in orogenic belts. The ocean basins are transient features that are constantly modified by the growth and destruction of new seafloor at divergent and convergent boundaries, respectively. In contrast, the continents are too light to be subducted and are permanent features of the earth's surface. This consequence of plate tectonics is supported by the 200 million-year age of the oldest seafloor and the four billion-year age of the most ancient rocks on the continents. Continents, therefore, drift, fragment, and collide as relative plate motions change through geologic time. The collision of continents that were once widely separated allows the bringing together of rocks that have had very different histories. As the collision leads to orogeny, these different sequences of rock may be juxtaposed in the same mountain belt.

The difference between the age of orogenic belts and the interiors of continents implies that the continents have evolved through time by the addition of material at their edges during orogenies. Orogenic belts are also of different ages, ranging from a billion years to zero (actively forming). Two or three belts whose ages decrease away from the continental interior may define one edge of a continent. This suggests that orogenies have occurred repeatedly through geologic time and that continents have added material continuously to their margins since the formation of their interiors. Because the ocean floor is so young, orogenic belts are the only record of subduction and collision events prior to 200 million years ago. If subduction is the only mechanism responsible for orogeny, plate tectonics must have been active since early in the history of the earth.

Analysis of Earthquakes

Subduction and orogeny are studied by hundreds of scientists, each of whom looks at only a small part of the picture. One may determine the composition of volcanic rocks that are characteristic of subduction zones; another may examine the styles of folds and faults in orogenic belts. Three techniques, however, are dominant in the study of subduction and orogeny: the analysis of the locations and sizes of earthquakes, the discrimination of relationships between different types of rocks in the field, and the investigation of features in deep-sea trenches and in the submerged region of folded and faulted rocks. The first defines where subduction and orogeny occur in the present, whereas the second determines what the physiographic expressions of these processes are, how they are preserved in the rocks, and where they were active in the past. The third technique provides a direct link between subduction and orogeny and illustrates the early stages of development of a mountain belt.

One of the most important discoveries of plate tectonics was that earthquake zones define plate boundaries. Earthquakes occur when a fracture, or fault, forms in the earth's crust, and the two pieces on either side of the fault move, or slip, past each other. For large earthquakes, the slip is on the order of 10–20 meters. The forces responsible for faulting are simply the result of the relative motions of the plates at the plate boundaries. The motion can accumulate in the rocks for hundreds of years prior to causing a rupture. When the crust finally breaks, the energy stored by the rocks is released suddenly as waves that travel through the earth and generate the intense vibrations associated with an earthquake. The rupture continues for as much as 1,000 kilometers and moves at speeds in excess of 10,000 kilometers per hour.

The energy carried by the waves is recorded on seismographs, which are instruments that monitor ground motion. Seismographs are composed of a mass attached to a pendulum. The mass remains still during an earthquake, measuring the amount the earth moves around it. The motion is recorded on a chart as a series of sharp peaks and valleys that deviate from the background value measured during times of no earthquake activity. The arrival of the waves at different times at different places allows the geophysicist to calculate the location, or epicenter, of the earthquake. The amount of the deviation of the peaks and valleys from the background noise is an estimate of the magnitude of the earthquake.

Earthquakes near mountain belts define zones that extend at an angle from the surface of the earth at the deep-sea trench to depths of hundreds of kilometers below the continents. This zone corresponds to the subducting plate at a convergent boundary. As a result, the locations of subduction zones that are currently active are very well known. The descent of a subducting plate below an overriding continent has triggered some of the deepest and largest earthquakes ever recorded. Continued motion of the plate and rupturing of the earth's crust in response translate into mountain ranges on the earth's surface.

Study of Rocks and Deep-Sea Trenches

Analysis of earthquakes is essential to evaluate the modern plate tectonic setting of the earth, but it reveals nothing about the geologic past. Information about the plate tectonics of the past must be obtained from looking at ancient mountain belts. Recognition of relationships among rocks in the field involves determining the ages, compositions, and histories of the rocks. This process led to the discovery that mountain belts on different continents contained rock sequences that were very similar. For example, rocks in the Appalachian Mountains of the east coast of North America were found to match closely those in the Atlas Mountains of the west coast of Africa. Conversely, recognition of relationships determined that dissimilar rock sequences frequently are adjacent to each other in the same orogenic belt. Both phenomena are most readily explained by continental drift, seafloor spreading, and subduction.

The critical link between ancient orogenic belts and modern subduction zones, identified by earthquake activity, was provided by deep-sea trenches. Using highly sophisticated techniques to “see” the ocean floor, scientists discovered the region of offscraped rocks that lies on the overriding plate in a subduction zone. These regions sometimes continue to the continental margin, where they are exposed on land as mountains. Thus, subduction was observed to cause folding and faulting in rocks and to build mountains—both important processes in orogenic belts.

Earthquake and Volcano Hazard Assessment

The theory of plate tectonics provides scientists with a process that can be observed—subduction—to explain the origin of mountains. Because young mountain chains are the locus of most of the large earthquakes that occur in the present, understanding subduction yields insight into the potential for destructive earthquakes in any given area. This is extremely important because most of the global population lives along convergent plate boundaries. The identification of subduction zones at the margins of the Pacific Ocean has explained the “Ring of Fire,” a region of abundant earthquakes and volcanoes that had long puzzled the scientific community. Restriction of most earthquakes to plate boundaries allows the assessment of earthquake hazards anywhere in the world if the locations of plate boundaries are known. For example, the city of Santiago in Chile, which is above a subduction zone, has a high risk, whereas the city of Chicago in the United States, which is in the continental interior, has a low risk.

Additional information can also be gathered about the type of earthquakes that may occur. In subduction zones, the piece of the crust that is above the rupture typically moves upward relative to the piece below, which generates waves that shake the ground in certain directions. At divergent plate boundaries, however, the piece of crust that is above the rupture moves downward relative to the piece below. This motion produces waves that vibrate the ground in directions different from those generated by earthquakes in subduction zones. Additional differences between convergent and divergent boundaries that may affect ground motion include the depth and size of the earthquakes. Subduction zones generate the deepest and largest earthquakes; earthquakes at divergent plate boundaries are more frequent, smaller, and shallower. Knowledge of the way the ground may move helps civil engineers to design and construct buildings able to withstand large earthquakes.

Eruptions of volcanoes that lie above subduction zones can be devastating. These volcanoes typically erupt violently and explosively in contrast to volcanoes near mid-ocean ridges, which erupt quietly and smoothly. This reflects the greater viscosity (resistance to flow) of magmas at convergent boundaries relative to those at divergent boundaries. Because of their greater viscosity, the magmas above subduction zones tend to plug the volcanoes at the surface, preventing any eruptions. Finally, when the pressure below the plug is great enough, the volcano erupts with such force that cities nearby are damaged considerably. For example, in 79 CE, the entire city of Pompeii, Italy, was destroyed, and hundreds of people were killed by the volcano Vesuvius. Clearly, the investigation of subduction and orogeny is beneficial to understanding the forces of nature that are harmful to humankind. Perhaps someday in the future, large earthquakes and violent volcanic eruptions may be predicted far enough in advance that precautions can be taken to prevent the loss of human life.

Modern advances

Modern advances in seismic imaging, deep-sea exploration, and collaborative programs further refined understanding of subduction and orogeny. Detailed studies of the Cascadia Subduction Zone revealed that the megathrust fault was segmented into distinct sections, potentially limiting the extent of future earthquakes. The SZ4D initiative strengthened research on fluid migration, earthquake cycles, and volcanic hazards.

Breakthroughs in deep-sea geology were also significant. Seismic studies around the Mariana Trench showed that tectonic plates subducted roughly three times more water into Earth’s mantle than previously estimated. Using advanced submersibles such as Fendouzhe and drilling vessels such as Chikyu, scientists directly sampled exposed mantle rocks, including serpentinite and peridotite, near the crust–mantle boundary. Furthermore, the IODP JTRACK Expedition recovered critical cores from the Japan Trench, providing new records of past mega-earthquakes and tsunamis to improve hazard predictions.

Principal Terms

continental margin: the edge of a continent that is both exposed on land and submerged below the water that marks the transition to the ocean basin

crust: the outermost layer of the earth, which consists of materials that are relatively light; the continental crust is lighter than oceanic crust, which allows it to float while oceanic crust sinks

faulting: the process of fracturing the earth such that rocks on opposite sides of the fracture move relative to one another; faults are the structures produced during the process

folding: the process of bending initially horizontal layers of rock so that they dip; folds are the features produced by folding and can be as small as millimeters and as big as kilometers long

geosynclines: major depressions in the surface of the earth where sediments accumulate; geosynclines lie parallel to the edges of continents and are long and narrow

intrusion: the process of forcing a body of molten rock generally derived from depths of tens of kilometers in the earth into solidified rock at the surface

magma: molten rock that is the source for volcanic eruptions

orogeny: mountain building by tectonic forces through the folding and faulting of rock layers

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