plate tectonics | Definition, Theory, Facts, & Evidence | relax-sakura.info
Each tectonic plate is free-floating and can move independently. Collision: when two continental plates are shoved together; Subduction: when one plate. Learn about how plates move and their impact on the Earth's surface. divergent , where plates move apart; and transform, where plates move sideways in relation to each other. Quake split a tectonic plate in two, and geologists are shaken. 2. Divergent boundary - two plates are moving in opposite directions as in a When the plates move they collide or spread apart allowing the very hot molten This drives the oceanic plates deep into the mantle destroying the oceanic plates.
A relatively thin crustwhich typically varies from a few kilometres to 40 km about 25 miles in thickness, sits on top of the mantle. Beneath the mantle is the core, which extends to the centre of Earth, some 6, km nearly 4, miles below the surface. Geologists maintain that the core is made up primarily of metallic iron accompanied by smaller amounts of nickelcobaltand lighter elements, such as carbon and sulfur. There are two types of crust, continental and oceanicwhich differ in their composition and thickness.
The distribution of these crustal types broadly coincides with the division into continents and ocean basins, although continental shelveswhich are submerged, are underlain by continental crust. The continents have a crust that is broadly granitic in composition and, with a density of about 2. Continental crust is typically 40 km 25 miles thick, while oceanic crust is much thinner, averaging about 6 km 4 miles in thickness. These crustal rocks both sit on top of the mantle, which is ultramafic in composition i.
The Moho is clearly defined by seismic studies, which detect an acceleration in seismic waves as they pass from the crust into the denser mantle.
The boundary between the mantle and the core is also clearly defined by seismic studies, which suggest that the outer part of the core is a liquid. The four main types of seismic waves are P waves, S waves, Love waves, and Rayleigh waves. The effect of the different densities of lithospheric rock can be seen in the different average elevations of continental and oceanic crust. The less-dense continental crust has greater buoyancy, causing it to float much higher in the mantle.
Its average elevation above sea level is metres 2, feetwhile the average depth of oceanic crust is 3, metres 12, feet.240 million years ago to 250 million years in the future
The lithosphere itself includes all the crust as well as the upper part of the mantle i. However, as temperatures increase with depth, the heat causes mantle rocks to lose their rigidity. This process begins at about km 60 miles below the surface. This change occurs within the mantle and defines the base of the lithosphere and the top of the asthenosphere. This upper portion of the mantle, which is known as the lithospheric mantle, has an average density of about 3.
How Earth's Plates Move Lesson #3 | Volcano World | Oregon State University
The asthenosphere, which sits directly below the lithospheric mantle, is thought to be slightly denser at 3. In contrast, the rocks in the asthenosphere are weaker, because they are close to their melting temperatures. As a result, seismic waves slow as they enter the asthenosphere. With increasing depth, however, the greater pressure from the weight of the rocks above causes the mantle to become gradually stronger, and seismic waves increase in velocity, a defining characteristic of the lower mantle.
The lower mantle is more or less solid, but the region is also very hot, and thus the rocks can flow very slowly a process known as creep.
During the late 20th and early 21st centuries, scientific understanding of the deep mantle was greatly enhanced by high-resolution seismological studies combined with numerical modeling and laboratory experiments that mimicked conditions near the core-mantle boundary.
At a depth of about 5, km 3, milesthe outer core transitions to the inner core. The polarity of the iron crystals of the OIC is oriented in a north-south direction, whereas that of the IIC is oriented east-west. Earth's coreThe internal layers of Earth's core, including its two inner cores. Plate boundaries Lithospheric plates are much thicker than oceanic or continental crust.
Their boundaries do not usually coincide with those between oceans and continentsand their behaviour is only partly influenced by whether they carry oceans, continents, or both. The Pacific Plate, for example, is entirely oceanic, whereas the North American Plate is capped by continental crust in the west the North American continent and by oceanic crust in the east and extends under the Atlantic Ocean as far as the Mid-Atlantic Ridge.
A general discussion of plate tectonics. In a simplified example of plate motion shown in the figure, movement of plate A to the left relative to plates B and C results in several types of simultaneous interactions along the plate boundaries. At the rear, plates A and B move apart, or diverge, resulting in extension and the formation of a divergent margin. At the front, plates A and B overlap, or converge, resulting in compression and the formation of a convergent margin.
Along the sides, the plates slide past one another, a process called shear. As these zones of shear link other plate boundaries to one another, they are called transform faults.
Theoretical diagram showing the effects of an advancing tectonic plate on other adjacent, but stationary, tectonic plates. At the advancing edge of plate A, the overlap with plate B creates a convergent boundary. In contrast, the gap left behind the trailing edge of plate A forms a divergent boundary with plate B. As plate A slides past portions of both plate B and plate C, transform boundaries develop.
Divergent margins As plates move apart at a divergent plate boundarythe release of pressure produces partial melting of the underlying mantle.
This molten material, known as magmais basaltic in composition and is buoyant. As a result, it wells up from below and cools close to the surface to generate new crust. Because new crust is formed, divergent margins are also called constructive margins. Continental rifting Upwelling of magma causes the overlying lithosphere to uplift and stretch. Whether magmatism [the formation of igneous rock from magma] initiates the rifting or whether rifting decompresses the mantle and initiates magmatism is a matter of significant debate.
If the diverging plates are capped by continental crust, fractures develop that are invaded by the ascending magma, prying the continents farther apart. Settling of the continental blocks creates a rift valleysuch as the present-day East African Rift Valley.
As the rift continues to widen, the continental crust becomes progressively thinner until separation of the plates is achieved and a new ocean is created. The ascending partial melt cools and crystallizes to form new crust. Because the partial melt is basaltic in composition, the new crust is oceanic, and an ocean ridge develops along the site of the former continental rift.
Consequently, diverging plate boundaries, even if they originate within continents, eventually come to lie in ocean basins of their own making.
The Thingvellir fracture lies in the Mid-Atlantic Ridge, which extends through the centre of Iceland. Samples collected from the ocean floor show that the age of oceanic crust increases with distance from the spreading centre —important evidence in favour of this process. These age data also allow the rate of seafloor spreading to be determined, and they show that rates vary from about 0.
Seafloor-spreading rates are much more rapid in the Pacific Ocean than in the Atlantic and Indian oceans. At spreading rates of about 15 cm 6 inches per year, the entire crust beneath the Pacific Ocean about 15, km [9, miles] wide could be produced in million years. Divergence and creation of oceanic crust are accompanied by much volcanic activity and by many shallow earthquakes as the crust repeatedly rifts, heals, and rifts again. Brittle earthquake -prone rocks occur only in the shallow crust.
Deep earthquakes, in contrast, occur less frequently, due to the high heat flow in the mantle rock. These regions of oceanic crust are swollen with heat and so are elevated by 2 to 3 km 1. The elevated topography results in a feedback scenario in which the resulting gravitational force pushes the crust apart, allowing new magma to well up from below, which in turn sustains the elevated topography.
Its summits are typically 1 to 5 km 0. This is accomplished at convergent plate boundaries, also known as destructive plate boundaries, where one plate descends at an angle—that is, is subducted—beneath the other.
Because oceanic crust cools as it ages, it eventually becomes denser than the underlying asthenosphere, and so it has a tendency to subduct, or dive under, adjacent continental plates or younger sections of oceanic crust. The life span of the oceanic crust is prolonged by its rigidity, but eventually this resistance is overcome.
Experiments show that the subducted oceanic lithosphere is denser than the surrounding mantle to a depth of at least km about miles. The mechanisms responsible for initiating subduction zones are controversial. During the late 20th and early 21st centuries, evidence emerged supporting the notion that subduction zones preferentially initiate along preexisting fractures such as transform faults in the oceanic crust.
Irrespective of the exact mechanism, the geologic record indicates that the resistance to subduction is overcome eventually. Where two oceanic plates meet, the older, denser plate is preferentially subducted beneath the younger, warmer one. Where one of the plate margins is oceanic and the other is continental, the greater buoyancy of continental crust prevents it from sinking, and the oceanic plate is preferentially subducted.
Continents are preferentially preserved in this manner relative to oceanic crust, which is continuously recycled into the mantle. This explains why ocean floor rocks are generally less than million years old whereas the oldest continental rocks are more than 4 billion years old. Before the middle of the 20th century, most geoscientists maintained that continental crust was too buoyant to be subducted. However, it later became clear that slivers of continental crust adjacent to the deep-sea trenchas well as sediments deposited in the trench, may be dragged down the subduction zone.
The recycling of this material is detected in the chemistry of volcanoes that erupt above the subduction zone. Two plates carrying continental crust collide when the oceanic lithosphere between them has been eliminated. Eventually, subduction ceases and towering mountain ranges, such as the Himalayasare created.
See below Mountains by continental collision. Because the plates form an integrated system, it is not necessary that new crust formed at any given divergent boundary be completely compensated at the nearest subduction zone, as long as the total amount of crust generated equals that destroyed. Subduction zones The subduction process involves the descent into the mantle of a slab of cold hydrated oceanic lithosphere about km 60 miles thick that carries a relatively thin cap of oceanic sediments.
The factors that govern the dip of the subduction zone are not fully understood, but they probably include the age and thickness of the subducting oceanic lithosphere and the rate of plate convergence.
Most, but not all, earthquakes in this planar dipping zone result from compressionand the seismic activity extends to km to miles below the surface, implying that the subducted crust retains some rigidity to this depth.
At greater depths the subducted plate is partially recycled into the mantle. The site of subduction is marked by a deep trench, between 5 and 11 km 3 and 7 miles deep, that is produced by frictional drag between the plates as the descending plate bends before it subducts. The overriding plate scrapes sediments and elevated portions of ocean floor off the upper crust of the lower plate, creating a zone of highly deformed rocks within the trench that becomes attached, or accreted, to the overriding plate.
This chaotic mixture is known as an accretionary wedge. The rocks in the subduction zone experience high pressures but relatively low temperatures, an effect of the descent of the cold oceanic slab.
Under these conditions the rocks recrystallize, or metamorphose, to form a suite of rocks known as blueschists, named for the diagnostic blue mineral called glaucophanewhich is stable only at the high pressures and low temperatures found in subduction zones. See also metamorphic rock. At deeper levels in the subduction zone that is, greater than 30—35 km [about 19—22 miles]eclogiteswhich consist of high-pressure minerals such as red garnet pyrope and omphacite pyroxeneform. The formation of eclogite from blueschist is accompanied by a significant increase in density and has been recognized as an important additional factor that facilitates the subduction process.
Island arcs When the downward-moving slab reaches a depth of about km 60 milesit gets sufficiently warm to drive off its most volatile components, thereby stimulating partial melting of mantle in the plate above the subduction zone known as the mantle wedge. Melting in the mantle wedge produces magmawhich is predominantly basaltic in composition.
This magma rises to the surface and gives birth to a line of volcanoes in the overriding plate, known as a volcanic arctypically a few hundred kilometres behind the oceanic trench.
The distance between the trench and the arc, known as the arc-trench gap, depends on the angle of subduction. Steeper subduction zones have relatively narrow arc-trench gaps. A basin may form within this region, known as a fore-arc basin, and may be filled with sediments derived from the volcanic arc or with remains of oceanic crust. If both plates are oceanic, as in the western Pacific Ocean, the volcanoes form a curved line of islandsknown as an island arcthat is parallel to the trench, as in the case of the Mariana Islands and the adjacent Mariana Trench.
If one plate is continental, the volcanoes form inland, as they do in the Andes of western South America. Though the process of magma generation is similar, the ascending magma may change its composition as it rises through the thick lid of continental crust, or it may provide sufficient heat to melt the crust. In either case, the composition of the volcanic mountains formed tends to be more silicon -rich and iron - and magnesium -poor relative to the volcanic rocks produced by ocean-ocean convergence.
Back-arc basins Where both converging plates are oceanic, the margin of the older oceanic crust will be subducted because older oceanic crust is colder and therefore more dense. This results in a process known as back-arc spreading, in which a basin opens up behind the island arc. The crust behind the arc becomes progressively thinner, and the decompression of the underlying mantle causes the crust to melt, initiating seafloor-spreading processessuch as melting and the production of basalt; these processes are similar to those that occur at ocean ridges.
The geochemistry of the basalts produced at back-arc basins superficially resembles that of basalts produced at ocean ridgesbut subtle trace element analyses can detect the influence of a nearby subducted slab.
This style of subduction predominates in the western Pacific Oceanin which a number of back-arc basins separate several island arcs from Asia. However, if the rate of convergence increases or if anomalously thick oceanic crust possibly caused by rising mantle plume activity is conveyed into the subduction zone, the slab may flatten.
Such flattening causes the back-arc basin to close, resulting in deformationmetamorphismand even melting of the strata deposited in the basin. Mountain building If the rate of subduction in an ocean basin exceeds the rate at which the crust is formed at oceanic ridges, a convergent margin forms as the ocean initially contracts. The island of Iceland, located in the North Atlantic, is still being formed at this Mid-Atlantic ridge. The Pacific Ocean, on the other hand, is becoming smaller and smaller.
The North and South American plates are crashing into the thinner and denser oceanic plates of the Pacific. This drives the oceanic plates deep into the mantle destroying the oceanic plates. This boundary in which an oceanic plate is driven down and destroyed by a continental plate is called a subduction zone.
This Pacific Ocean region has more earthquakes and volcanic activity than any other area of the world. Because of all the volcanoes this region has been given the nickname of "The ". When the less dense, lighter continental plate overrides the oceanic plate a subduction zone forms.
Because the oceanic plate is bent and driven down, a deep trench forms at this collison point. These trenches are the lowest points on the Earth's crust. One trench is a mile deeper than Mount Everest is tall!
As the oceanic plate descends into the mantle some of it melts.
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This material moves into the mantle above the plate and causes the mantle to melt. This liquid rock, called magma, rises to the surface because it is less dense then the surrounding rock. If the magma reaches the surface of the Earth, a volcano forms. As the mantle rocks melt they form magma. The magma collects in a magma pool. Because the magma is less dense than the surrounding mantle material it will rise.