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Plate tectonics is a scientific theory describing the large-scale motion of seven large plates and the movements of a larger number of smaller plates of Earth's lithosphere, since tectonic processes began on Earth between 3.3 and 3.5 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century. The geoscientific community accepted plate-tectonic theory after seafloor spreading was validated in the late 1950s and early 1960s.
An orogeny is an event that leads to both structural deformation and compositional differentiation of the Earth's lithosphere at convergent plate margins. An orogen or orogenic belt develops when a continental plate crumples and is uplifted to form one or more mountain ranges; this involves a series of geological processes collectively called orogenesis.
Oceanic trenches are topographic depressions of the seafloor, relatively narrow in width, but very long. These oceanographic features are the deepest parts of the ocean floor. Oceanic trenches are a distinctive morphological feature of convergent plate boundaries, along which lithospheric plates move towards each other at rates that vary from a few millimeters to over ten centimeters per year. A trench marks the position at which the flexed, subducting slab begins to descend beneath another lithospheric slab. Trenches are generally parallel to a volcanic island arc, and about 200 km (120 mi) from a volcanic arc. Oceanic trenches typically extend 3 to 4 km below the level of the surrounding oceanic floor. The greatest ocean depth measured is in the Challenger Deep of the Mariana Trench, at a depth of 11,034 m (36,201 ft) below sea level. Oceanic lithosphere moves into trenches at a global rate of about 3 km2/yr.
A lithosphere is the rigid, outermost shell of a terrestrial-type planet or natural satellite. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater. The crust and upper mantle are distinguished on the basis of chemistry and mineralogy.
The asthenosphere is the highly viscous, mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at depths between approximately 80 and 200 km below the surface. The lithosphere–asthenosphere boundary is usually referred to as LAB. The asthenosphere is almost solid, although some of its regions could be molten. The lower boundary of the asthenosphere is not well defined. The thickness of the asthenosphere depends mainly on the temperature. However, the rheology of the asthenosphere also depends on the rate of deformation, which suggests that the asthenosphere could be also formed as a result of a high rate of deformation. In some regions the asthenosphere could extend as deep as 700 km (430 mi). It is considered the source region of mid-ocean ridge basalt (MORB).
Subduction is a geological process in which oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the heavier plate dives beneath the second plate and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. The process of subduction has created most of the Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with the average rate of convergence being approximately two to eight centimeters per year along most plate boundaries.
Obduction is the overthrusting of continental crust by oceanic crust or mantle rocks at a convergent plate boundary, such as closing of an ocean or a mountain building episode. This process is uncommon because the denser oceanic lithosphere usually subducts underneath the less dense continental plate.
The Basin and Range Province is a vast physiographic region covering much of the inland Western United States and northwestern Mexico. It is defined by unique basin and range topography, characterized by abrupt changes in elevation, alternating between narrow faulted mountain chains and flat arid valleys or basins. The physiography of the province is the result of tectonic extension that began around 17 million years ago in the early Miocene epoch.
A convergent boundary is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Wadati–Benioff zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.
Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle along the subduction zone. They are the principal way by which continental growth is achieved.
Earth's mantle is a layer of silicate rock between the crust and the outer core. It has a mass of 4.01 × 1024 kg and thus makes up 67% of the mass of Earth. It has a thickness of 2,900 kilometres (1,800 mi) making up about 84% of Earth's volume. It is predominantly solid but in geological time, it behaves as a viscous fluid. Partial melting of the mantle at mid-ocean ridges produces oceanic crust, and partial melting of the mantle at subduction zones produces continental crust.
The outer trench swell, outer trench high, or outer rise is a subtle ridge on the seafloor near an oceanic trench, where a descending plate begins to flex and fault in preparation for its descent into the mantle at a subduction zone. The lithosphere is bent upwards by plate stresses, and is not in isostatic equilibrium.
The Farallon Trench was a subduction related tectonic formation located off the coast of the western California continental margin during the late to mid Cenozoic era, around 50 miles southeast of modern-day Monterey Bay. The time duration of subduction began from around 165 Ma when the Farallon Plate replaced the Mezczlera promontory, until the San Andreas Fault straightening around 35 Ma. As data accumulated over time, a common view developed that one large oceanic plate, the Farallon Plate, acted as a conveyor belt, conveying accreted terranes onto the North American west coast. As the continent overran the subducting Farallon Plate, the denser plate became subducted into the mantle below the continent. When the plates converged, the dense oceanic plate sank into the mantle to form a slab below the lighter continent. Rapid subduction under the southwestern North America continent began 40 to 60 million years ago (Ma), during the mid Paleocene to mid Eocene epochs. This convergent subduction margin created a distinctive geomorphologic feature called an oceanic trench, which occurs at a convergent plate boundaries as a heavy metal rich, lithospheric plate moves below a light silica rich continental plate. The trench marks the position at which the flexed subducting slab begins to descend beneath and deform the continental plate margin. By 43 Ma, during the Eocene, worldwide plate motions changed and the Pacific Plate began to move away from North America and subduction of the Farallon Plate slowed dramatically. By around 36 Ma, the easternmost part of the East Pacific Rise, located between the Pioneer and Murray fracture zones at that time, approached the trench and the young, hot, buoyant lithosphere appears to have clogged part of the subduction zone, resulting in widespread dramatic uplift on land. The eventual complete subduction of this plate, consequential contact of the Pacific Plate with the California continental margin, and creation of the Mendocino Triple Junction (MTJ), took place around 30 to 20 Ma. The partial complete subduction and division of the Farallon Plate by the Pacific Plate, created the Juan de Fuca Plate to the north and the Cocos Plate to the south. The final stages of the evolution of California's continental margin was the growth of the San Andreas transform fault system, which formed as the Pacific Plate came into contact with the continental margin and the MTJ was formed. As subduction of the Pacific Plate continued along this margin, and the contact zone grew, the San Andreas proportionally grew as well.
The Izu–Bonin–Mariana (IBM) arc system is a tectonic plate convergent boundary. The IBM arc system extends over 2800 km south from Tokyo, Japan, to beyond Guam, and includes the Izu Islands, the Bonin Islands, and the Mariana Islands; much more of the IBM arc system is submerged below sealevel. The IBM arc system lies along the eastern margin of the Philippine Sea Plate in the Western Pacific Ocean. It is the site of the deepest gash in Earth's solid surface, the Challenger Deep in the Mariana Trench.
In geodynamics, delamination refers to the loss and sinking (foundering) of the portion of the lowermost lithosphere from the tectonic plate to which it was attached.
Non-volcanic passive margins (NVPM) constitute one end member of the transitional crustal types that lie beneath passive continental margins; the other end member being volcanic passive margins (VPM). Transitional crust welds continental crust to oceanic crust along the lines of continental break-up. Both VPM and NVPM form during rifting, when a continent rifts to form a new ocean basin. NVPM are different from VPM because of a lack of volcanism. Instead of intrusive magmatic structures, the transitional crust is composed of stretched continental crust and exhumed upper mantle. NVPM are typically submerged and buried beneath thick sediments, so they must be studied using geophysical techniques or drilling. NVPM have diagnostic seismic, gravity, and magnetic characteristics that can be used to distinguish them from VPM and for demarcating the transition between continental and oceanic crust.
In geology and geophysics, thermal subsidence is a mechanism of subsidence in which conductive cooling of the mantle thickens the lithosphere and causes it to decrease in elevation. This is because of thermal contraction: as mantle material cools and becomes part of the mechanically rigid lithosphere, it becomes more dense than the surrounding material. Additional material added to the lithosphere thickens it and further causes a buoyant decrease in the elevation of the lithosphere. This creates accommodation space into which sediments can deposit, forming a sedimentary basin.
Cape Verde is a volcanic archipelago situated above an oceanic rise that puts the base of the islands 2 kilometers (1.2 mi) above the rest of the seafloor. Cape Verde has been considered to be a hotspot and it has been argued that the archipelago may be underlain by a mantle plume and that the unusual geochemical signatures of its lavas derive from recycled oceanic crust and lithosphere in the source material. The lack of any clear age progression in the Cape Verde Archipelago, however, is inconsistent with a mantle plume origin, and various lines of evidence suggest that the islands are a product rather of sub-lithospheric small-scale convection and instabilities arising from heterogeneities in the asthenosphere and variations in lithospheric thickness. The geochemistry of the lavas has been explained in terms of remnants of subcontinental lithospheric mantle left in the oceanic lithosphere when the Atlantic Ocean opened.
The subcontinental lithospheric mantle (SCLM) is the uppermost solid part of Earth's mantle associated with the continental lithosphere.
Heat-pipe tectonics is a cooling mode of terrestrial planets in which the main heat transport mechanism in the planet is volcanism through the outer hard shell, also called the lithosphere. Heat-pipe tectonics initiates when volcanism becomes the dominant surface heat transfer process. Melted rocks and other more volatile planetary materials are transferred from the mantle to surface via localised vents. Melts cool down and solidify forming layers of cool volcanic materials. Newly erupted materials deposit on top of and bury older layers. The accumulation of volcanic layers on the shell and the corresponding evacuation of materials at depth cause the downward transfer of superficial materials such that the shell materials continuously descend toward the planet's interior.
References
- ↑ National Science Foundation (May 27, 2009). "Dripping 'Blob' Under Western U.S.: A Hidden Drip, Drip, Drip Beneath Earth's Surface". Science Daily. Retrieved January 12, 2016.
- ↑ West, John D.; Fouch, Matthew J.; Roth, Jeffrey B.; Elkins-Tanton, Linda T. (2009). "Vertical mantle flow associated with a lithospheric drip beneath the Great Basin" (PDF). Nature Geoscience. 2 (6): 439–444. doi:10.1038/ngeo526. ISSN 1752-0894 . Retrieved January 12, 2016.
- ↑ Saleeby, Jason; Foster, Zorka (2004). "Topographic response to mantle lithosphere removal in the southern Sierra Nevada region, California". Geology. 32 (3): 245. doi:10.1130/G19958.1. ISSN 0091-7613 . Retrieved January 12, 2016.
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