Divergent boundary

In plate tectonics, a divergent boundary or divergent plate boundary (also known as a constructive boundary or an extensional boundary) is a linear feature that exists between two tectonic plates that are moving away from each other. Divergent boundaries within continents initially produce rifts which eventually become rift valleys. Most active divergent plate boundaries occur between oceanic plates and exist as mid-oceanic ridges. Divergent boundaries also form volcanic islands which occur when the plates move apart to produce gaps which molten lava rises to fill.

Current research indicates that complex convection within the Earth's mantle allows material to rise to the base of the lithosphere beneath each divergent plate boundary.[1] This supplies the area with vast amounts of heat and a reduction in pressure that melts rock from the asthenosphere (or upper mantle) beneath the rift area forming large flood basalt or lava flows. Each eruption occurs in only a part of the plate boundary at any one time, but when it does occur, it fills in the opening gap as the two opposing plates move away from each other.

Over millions of years, tectonic plates may move many hundreds of kilometers away from both sides of a divergent plate boundary. Because of this, rocks closest to a boundary are younger than rocks further away on the same plate.

Continental-continental constructive plate boundary
Continental-continental divergent/constructive boundary

Description

Bridge across continents iceland
Bridge across the Álfagjá rift valley in southwest Iceland, that is part of the boundary between the Eurasian and North American continental tectonic plates.

At divergent boundaries, two plates move away from each other and the space that this creates is filled with new crustal material sourced from molten magma that forms below. The origin of new divergent boundaries at triple junctions is sometimes thought to be associated with the phenomenon known as hotspots. Here, exceedingly large convective cells bring very large quantities of hot asthenospheric material near the surface and the kinetic energy is thought to be sufficient to break apart the lithosphere. The hot spot which may have initiated the Mid-Atlantic Ridge system currently underlies Iceland which is widening at a rate of a few centimeters per year.

Divergent boundaries are typified in the oceanic lithosphere by the rifts of the oceanic ridge system, including the Mid-Atlantic Ridge and the East Pacific Rise, and in the continental lithosphere by rift valleys such as the famous East African Great Rift Valley. Divergent boundaries can create massive fault zones in the oceanic ridge system. Spreading is generally not uniform, so where spreading rates of adjacent ridge blocks are different, massive transform faults occur. These are the fracture zones, many bearing names, that are a major source of submarine earthquakes. A sea floor map will show a rather strange pattern of blocky structures that are separated by linear features perpendicular to the ridge axis. If one views the sea floor between the fracture zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center, will be older and deeper... (from thermal contraction and subsidence).

It is at mid-ocean ridges that one of the key pieces of evidence forcing acceptance of the seafloor spreading hypothesis was found. Airborne geomagnetic surveys showed a strange pattern of symmetrical magnetic reversals on opposite sides of ridge centers. The pattern was far too regular to be coincidental as the widths of the opposing bands were too closely matched. Scientists had been studying polar reversals and the link was made by Lawrence W. Morley, Frederick John Vine and Drummond Hoyle Matthews in the Morley–Vine–Matthews hypothesis. The magnetic banding directly corresponds with the Earth's polar reversals. This was confirmed by measuring the ages of the rocks within each band. The banding furnishes a map in time and space of both spreading rate and polar reversals.

Examples

Other plate boundary types

See also

  • Seafloor spreading – A process at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge
  • Continental drift – The movement of the Earth's continents relative to each other
  • Subduction zone – A geological process at convergent tectonic plate boundaries where one plate moves under the other

References

  1. ^ Toshiro Tanimoto, Thorne Lay (November 7, 2000). "Mantle dynamics and seismic tomography". Proc. Natl. Acad. Sci. U.S.A. 97 (23): 12409–10. Bibcode:2000PNAS...9712409T. doi:10.1073/pnas.210382197. PMC 34063. PMID 11035784.
Aegean Sea Plate

The Aegean Sea Plate (also called the Hellenic Plate or Aegean Plate) is a small tectonic plate located in the eastern Mediterranean Sea under southern Greece and far western Turkey. Its southern edge is a subduction zone south of Crete, where the African Plate is being swept under the Aegean Sea Plate. To the north is the Eurasian Plate, which is a divergent boundary responsible for the formation of the Gulf of Corinth.

Balmoral Reef Plate

The Balmoral Reef Plate is a small tectonic plate (microplate) located in the south Pacific north of Fiji. Clockwise from the north, it borders the Pacific Plate, Australian Plate, Conway Reef Plate, and the New Hebrides Plate. The northern and western borders are a divergent boundary while the rest of the borders are transform and convergent boundaries. The Balmoral Reef Plate's ocean crust is less than 12 million years old and is spreading between the New Hebrides and Tonga subduction.

Bird's Head Plate

The Bird's Head Plate is a minor tectonic plate incorporating the Bird's Head Peninsula, at the western end of the island of New Guinea. Hillis and Müller consider it to be moving in unison with the Pacific Plate. Bird considers it to be unconnected to the Pacific Plate.The plate is separating from the Australian Plate and the small Maoke Plate along a divergent boundary to the southeast. Convergent boundaries exist along the north, between the Bird's Head and the Caroline Plate, the Philippine Sea Plate and the Halmahera Plate to the northwest. A transform boundary exists between the Bird's Head and the Molucca Sea Collision Zone to the southwest. Another convergent boundary exists between the Bird's Head and the Banda Sea Plate to the south.

Easter Microplate

Easter Plate is located to the west of Easter Island off the west coast of South America in the middle of the Pacific Ocean, bordering the Nazca plate to the east and the Pacific plate to the west. It was discovered from looking at earthquake distributions that were offset from the previously perceived Nazca-Pacific Divergent boundary. This young plate is 5.25 million years old and is considered a microplate because it is small with an area of approximately 160,000 km2. Seafloor spreading along the Easter microplate's borders have some of the highest global rates, ranging from 50 to 140 mm/yr.

Eurasian Plate

The Eurasian Plate is a tectonic plate which includes most of the continent of Eurasia (a landmass consisting of the traditional continents of Europe and Asia), with the notable exceptions of the Indian subcontinent, the Arabian subcontinent, and the area east of the Chersky Range in East Siberia. It also includes oceanic crust extending westward to the Mid-Atlantic Ridge and northward to the Gakkel Ridge.

The eastern side is a boundary with the North American Plate to the north and a boundary with the Philippine Sea Plate to the south and possibly with the Okhotsk Plate and the Amurian Plate. The southerly side is a boundary with the African Plate to the west, the Arabian Plate in the middle and the Indo-Australian Plate to the east. The westerly side is a divergent boundary with the North American Plate forming the northernmost part of the Mid-Atlantic Ridge, which is straddled by Iceland. All of the volcanic eruptions in Iceland, such as the 1973 eruption of Eldfell, the 1783 eruption of Laki, and the 2010 eruption of Eyjafjallajökull, are caused by the North American and the Eurasian plates moving apart, which is a result of divergent plate boundary forces.

The geodynamics of central Asia is dominated by the interaction between the Eurasian and Indian Plates. In this area, many subplates or crust blocks have been recognized, which form the Central Asian and the East Asian transit zones.

Galapagos Rise

The Galapagos Rise is a divergent boundary located between the South American coast and the triple junction of the Nazca Plate, the Cocos Plate, and the Pacific Plate. The volcanically active Galapagos Islands exist on the Galápagos hotspot above the Galapagos Rise. The Galapagos Microplate is a small separate plate on the rise just to the southeast of the triple junction.

The Cocos Ridge trends northeasterly from the Galapagos to the coast of Costa Rica and Panama. The Carnegie Ridge trends almost due east to the Ecuadorian coast.

The Galapagos Rise is a currently active ridge. Fernandina Volcano on Fernandina Island, the most westerly island of the chain erupted on May 12, 2005 ejecting a column of ash that rose to a height of seven km from a fissure on the west side of the volcano. Volcanic ash fell on neighboring Isabela Island. Alcedo Volcano on Isabela Island last erupted in the 1950s.

Geology of Iceland

The geology of Iceland is unique and of particular interest to geologists. Iceland lies on the divergent boundary between the Eurasian plate and the North American plate. It also lies above a hotspot, the Iceland plume. The plume is believed to have caused the formation of Iceland itself, the island first appearing over the ocean surface about 16 to 18 million years ago. The result is an island characterized by repeated volcanism and geothermal phenomena such as geysers.

The eruption of Laki in 1783 caused much devastation and loss of life, leading to a famine that killed approximately 25% of the island's human population and resulting in a drop in global temperatures, as sulfur dioxide was spewed into the Northern Hemisphere. This caused crop failures in Europe and may have caused droughts in India. The eruption has been estimated to have killed over six million people globally, making it one of the deadliest volcanoes, with Tambora being number one.In the period from 1963 to 1967, the new island of Surtsey was created on the southwest coast by a volcanic eruption.

Gorda Plate

The Gorda Plate, located beneath the Pacific Ocean off the coast of northern California, is one of the northern remnants of the Farallon Plate. It is sometimes referred to (by, for example, publications from the USGS Earthquake Hazards Program) as simply the southernmost portion of the neighboring Juan de Fuca Plate, another Farallon remnant.

Unlike most tectonic plates, the Gorda Plate experiences significant intraplate deformation inside its boundaries. Numerous faults have been mapped in both the sediments and basement of the Gorda Basin, which is in the interior of the plate south of 41.6°N. Stresses from the neighboring North American Plate and Pacific Plate cause frequent earthquakes in the interior of the plate, including the 1980 Eureka earthquake (also known as the Gorda Basin event).The easterly side is a convergent boundary subducting under the North American Plate in northern California. The southerly side is a transform boundary with the Pacific Plate along the Mendocino Fault. The westerly side is a divergent boundary with the Pacific Plate forming the Gorda Ridge. This ridge provides morphological evidence of differing spreading rates, with the northern portion of the ridge being narrow, and the southern portion being wide. The northerly side is a transform boundary with the Juan de Fuca Plate, the Blanco Fracture Zone.

The subducting Gorda Plate is connected with the volcanoes in northern California, namely, Mount Shasta and Lassen Peak. Lassen Peak last erupted in 1914–1917.

Interplate earthquake

An interplate earthquake is an earthquake that occurs at the boundary between two tectonic plates. Earthquakes of this type account for more than 90 percent of the total seismic energy released around the world. If one plate is trying to move past the other, they will be locked until sufficient stress builds up to cause the plates to slip relative to each other. The slipping process creates an earthquake with relative displacement on either side of the fault, resulting in seismic waves which travel through the Earth and along the Earth's surface. Relative plate motion can be lateral as along a transform fault boundary, vertical if along a convergent boundary (i.e. subduction or thrust/reverse faulting) or a divergent boundary (i.e. rift zone or normal faulting), and oblique, with horizontal and lateral components at the boundary. Interplate earthquakes associated at a subduction boundary are called megathrust earthquakes, which are the most powerful earthquakes.

Intraplate earthquakes are often confused with interplate earthquakes, but are fundamentally different in origin, occurring within a single plate rather than between two tectonic plates on a plate boundary. The specifics of the mechanics by which they occur, as well as the intensity of the stress drop which occurs after the earthquake also differentiate the two types of events. Intraplate earthquakes have, on average, a higher stress drop than that of an interplate earthquake and generally higher intensity.

Kermadec Plate

The Kermadec Plate is a very long and narrow tectonic plate located west of the Kermadec Trench in the south Pacific Ocean. Also included on this tectonic plate is a small portion of the North Island of New Zealand and the Kermadec Islands. It is separated from the Australian Plate by a long divergent boundary which forms a back-arc basin. This area is highly prone to earthquakes and tsunami.

Mariana Plate

The Mariana Plate is a micro tectonic plate located west of the Mariana Trench which forms the basement of the Mariana Islands which form part of the Izu-Bonin-Mariana Arc. It is separated from the Philippine Sea Plate to the west by a divergent boundary with numerous transform fault offsets. The boundary between the Mariana and the Pacific Plate to the east is a subduction zone with the Pacific Plate subducting beneath the Mariana. This eastern subduction is divided into the Mariana Trench, which forms the southeastern boundary, and the Izu-Ogasawara Trench the northeastern boundary. The subduction plate motion is responsible for the shape of the Mariana plate and back arc.

Mendocino Triple Junction

The Mendocino Triple Junction (MTJ) is the point where the Gorda plate, the North American plate, and the Pacific plate meet, in the Pacific Ocean near Cape Mendocino in northern California. This triple junction is the location of a change in the broad plate motions which dominate the west coast of North America, linking convergence of the northern Cascadia subduction zone and translation of the southern San Andreas Fault system. The Gorda plate is subducting, towards N50ºE, under the North American plate at 2.5 – 3 cm/yr, and is simultaneously converging obliquely against the Pacific plate at a rate of 5 cm/yr in the direction N115ºE. The accommodation of this plate configuration results in a transform boundary along the Mendocino Fracture Zone, and a divergent boundary at the Gorda Ridge.Due to the relative plate motions, the triple junction has been migrating northwards for the past 25–30 million years, and assuming rigid plates, the geometry requires that a void, called slab window, develop southeast of the MTJ. At this point, removal of the subducting Gorda lithosphere from beneath North America causes asthenospheric upwelling. This instigates different tectonic processes, which include surficial uplift, crustal deformation, intense seismic activity, high heat flow, and even the extrusion of volcanic rocks. This activity is centred on the current triple junction position, but evidence for its migration is found in the geology all along the California coast, starting as far south as Los Angeles.

Nazca Plate

The Nazca Plate, named after the Nazca region of southern Peru, is an oceanic tectonic plate in the eastern Pacific Ocean basin off the west coast of South America. The ongoing subduction, along the Peru–Chile Trench, of the Nazca Plate under the South American Plate is largely responsible for the Andean orogeny. The Nazca Plate is bounded on the west by the Pacific Plate and to the south by the Antarctic Plate through the East Pacific Rise and the Chile Rise respectively. The movement of the Nazca Plate over several hotspots has created some volcanic islands as well as east-west running seamount chains that subduct under South America. Nazca is a relatively young plate both in terms of the age of its rocks and its existence as an independent plate having been formed from the break-up of the Farallon Plate about 23 million years ago. The oldest rocks of the plate are about 50 million years old.

North Bismarck Plate

The North Bismarck Plate is a small tectonic plate located in the Bismarck Sea off the northeast coast of New Guinea.

Pacific Plate

The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At 103 million square kilometres (40,000,000 sq mi), it is the largest tectonic plate.The Pacific Plate contains an interior hot spot forming the Hawaiian Islands.Hillis and Müller are reported to consider the Bird's Head Plate to be moving in unison with the Pacific Plate. Bird considers them to be unconnected.

Philippine Sea Plate

The Philippine Sea Plate or the Philippine Plate is a tectonic plate comprising oceanic lithosphere that lies beneath the Philippine Sea, to the east of the Philippines. Most segments of the Philippines, including northern Luzon, are part of the Philippine Mobile Belt, which is geologically and tectonically separate from the Philippine Sea Plate.

Philippine Sea plate is bordered mostly by convergent boundaries:

To the north, the Philippine Sea Plate meets the Okhotsk Plate at the Nankai Trough. The Philippine Sea Plate, the Amurian Plate, and the Okhotsk Plate meet at Mount Fuji in Japan. The thickened crust of the Izu-Bonin-Mariana arc colliding with Japan constitutes the Izu Collision Zone.

To the east, Philippine Sea Plate meets the Pacific Plate, subducting at the Izu-Ogasawara Trench. The east of the plate includes the Izu-Ogasawara (Bonin) and the Mariana Islands, forming the Izu-Bonin-Mariana Arc system. There is also a divergent boundary between the Philippine Sea Plate and the small Mariana Plate which carries the Mariana Islands.

To the south, the Philippine Sea Plate is bounded by the Caroline Plate and Bird's Head Plate.

To the west, the Philippine Sea Plate subducts under the Philippine Mobile Belt at the Philippine Trench and the East Luzon Trench. (The adjacent rendition of Prof. Peter Bird's map is inaccurate in this respect.)

To the northwest, the Philippine Sea Plate meets Taiwan and the Nansei islands on the Okinawa Plate, and southern Japan on the Amurian Plate.

Rift

In geology, a rift is a linear zone where the lithosphere is being pulled apart and is an example of extensional tectonics.Typical rift features are a central linear downfaulted depression, called a graben, or more commonly a half-graben with normal faulting and rift-flank uplifts mainly on one side. Where rifts remain above sea level they form a rift valley, which may be filled by water forming a rift lake. The axis of the rift area may contain volcanic rocks, and active volcanism is a part of many, but not all active rift systems.

Major rifts occur along the central axis of most mid-ocean ridges, where new oceanic crust and lithosphere is created along a divergent boundary between two tectonic plates.

Failed rifts are the result of continental rifting that failed to continue to the point of break-up. Typically the transition from rifting to spreading develops at a triple junction where three converging rifts meet over a hotspot. Two of these evolve to the point of seafloor spreading, while the third ultimately fails, becoming an aulacogen.

South American Plate

The South American Plate is a major tectonic plate which includes the continent of South America as well as a sizable region of the Atlantic Ocean seabed extending eastward to the African Plate, with which it forms the southern part of the Mid-Atlantic Ridge.

The easterly edge is a divergent boundary with the African Plate; the southerly edge is a complex boundary with the Antarctic Plate, the Scotia Plate, and the Sandwich Plate; the westerly edge is a convergent boundary with the subducting Nazca Plate; and the northerly edge is a boundary with the Caribbean Plate and the oceanic crust of the North American Plate. At the Chile Triple Junction, near the west coast of the Taitao–Tres Montes Peninsula, an oceanic ridge known as the Chile Rise is actively subducting under the South American Plate.

Geological research suggests that the South American Plate is moving westward away from the Mid-Atlantic Ridge: "Parts of the plate boundaries consisting of alternations of relatively short transform fault and spreading ridge segments are represented by a boundary following the general trend." As a result, the eastward-moving and more dense Nazca Plate is subducting under the western edge of the South American Plate, along the continent's Pacific coast, at a rate of 77 mm (3.0 in) per year. The collision of these two plates is responsible for lifting the massive Andes Mountains and for creating the numerous volcanoes which are strewn throughout them.

Timor Plate

The Timor Plate is a microplate in southeast Asia carrying the island of Timor and surrounding islands. The Australian Plate is subducting under the southern edge of the plate, while a small divergent boundary is located on the eastern edge. Another convergent boundary exists with the Banda Sea Plate to the north, and to the west is a transform boundary.

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