plate tectonics | Definition, Theory, Facts, & Evidence | misjon.info
Plate tectonics is a scientific theory describing the large-scale .. concept was of continents plowing through oceanic crust. of the relationships recognized during this pre-plate tectonics. Dec 1, It is now common to recognise continental shelves (land that is below water level cover all of the planet's surface, but can move a bit in relation to each other. Sometimes you will find a tectonic plate has a continent in the middle of it, but. Tectonic Plates, fossil correlations in Pangea and notional diagram of plate motions. Plate tectonics is a scientific theory describing how continents move around.
The deepest ocean trench is the Mariana Trench just east of Guam. It is located at the subduction zone where the Pacific plate plunges underneath the edge of the Filipino plate. Subduction zones are also sites of deepwater earthquakes. Transform faults are found where two tectonic plates move past each other.
As the plates slide past one another, there is friction, and great tension can build up before slippage occurs, eventually causing shallow earthquakes. People living near the San Andreas Fault, a transfom fault in California, regularly experience such quakes. Hot Spots Recall that some volcanoes form near plate boundaries, particularly near subduction zones where oceanic crust moves underneath continental crust Fig. However, some volcanoes form over hot spots in the middle of tectonic plates far away from subduction zones Fig.
When magma erupts and flows at the surface, it is called lava. The basalt lava commonly found at hot spots flows like hot, thick syrup and gradually forms shield volcanoes. A shield volcano is shaped like a dome with gently sloping sides. These volcanoes are much less explosive than the composite volcanoes formed at subduction zones.
- Plate tectonics
Some shield volcanoes, such as the islands in the Hawaiian archipelago, began forming on the ocean floor over a hot spot. Each shield volcano grows slowly with repeated eruptions until it reaches the surface of the water to form an island Fig. Almost all of the mid-Pacific and mid-Atlantic ocean basin islands formed in a similar fashion over volcanic hot spots.
Over millions of years as the tectonic plate moves, a volcano that was over the hot spot moves away, ceases to erupt, and becomes extinct Fig. Islands can erode through natural processes such as wind and water flow. Eventually all that remains of the island is a ring of coral reef. An atoll is a ring-shaped coral reef or group of coral islets that has grown around the rim of an extinct submerged volcano forming a central lagoon Fig.
Atoll formation is dependent on erosion of land and growth of coral reefs around the island. Coral reef atolls can only occur in tropical regions that are optimal for coral growth. This process can lead to collision between the approaching continentswhich eventually terminates subduction.
Mountain building can occur in a number of ways at a convergent margin: Many mountain belts were developed by a combination of these processes. For example, the Cordilleran mountain belt of North America —which includes the Rocky Mountains as well as the Cascadesthe Sierra Nevadaand other mountain ranges near the Pacific coast—developed by a combination of subduction and terrane accretion. As continental collisions are usually preceded by a long history of subduction and terrane accretion, many mountain belts record all three processes.
Over the past 70 million years the subduction of the Neo-Tethys Seaa wedge-shaped body of water that was located between Gondwana and Laurasialed to the accretion of terranes along the margins of Laurasia, followed by continental collisions beginning about 30 million years ago between Africa and Europe and between India and Asia.
These collisions culminated in the formation of the Alps and the Himalayas. Jurassic paleogeographyDistribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the late Jurassic Period.
Included in the paleogeographic reconstruction are the locations of the interval's subduction zones. Subduction results in voluminous magmatism in the mantle and crust overlying the subduction zoneand, therefore, the rocks in this region are warm and weak. Although subduction is a long-term process, the uplift that results in mountains tends to occur in discrete episodes and may reflect intervals of stronger plate convergence that squeezes the thermally weakened crust upward.
For example, rapid uplift of the Andes approximately 25 million years ago is evidenced by a reversal in the flow of the Amazon River from its ancestral path toward the Pacific Ocean to its modern path, which empties into the Atlantic Ocean.
In addition, models have indicated that the episodic opening and closing of back-arc basins have been the major factors in mountain-building processes, which have influenced the plate-tectonic evolution of the western Pacific for at least the past million years. Mountains by terrane accretion As the ocean contracts by subduction, elevated regions within the ocean basin—terranes—are transported toward the subduction zone, where they are scraped off the descending plate and added—accreted—to the continental margin.
Since the late Devonian and early Carboniferous periods, some million years ago, subduction beneath the western margin of North America has resulted in several collisions with terranes. The piecemeal addition of these accreted terranes has added an average of km miles in width along the western margin of the North American continentand the collisions have resulted in important pulses of mountain building.
The more gradual transition to the abyssal plain is a sediment-filled region called the continental rise. The continental shelf, slope, and rise are collectively called the continental margin.
During these accretionary events, small sections of the oceanic crust may break away from the subducting slab as it descends. Instead of being subducted, these slices are thrust over the overriding plate and are said to be obducted.
Where this occurs, rare slices of ocean crust, known as ophiolitesare preserved on land. They provide a valuable natural laboratory for studying the composition and character of the oceanic crust and the mechanisms of their emplacement and preservation on land. A classic example is the Coast Range ophiolite of Californiawhich is one of the most extensive ophiolite terranes in North America. These ophiolite deposits run from the Klamath Mountains in northern California southward to the Diablo Range in central California.
This oceanic crust likely formed during the middle of the Jurassic Periodroughly million years ago, in an extensional regime within either a back-arc or a forearc basin. In the late Mesozoicit was accreted to the western North American continental margin. Because preservation of oceanic crust is rare, the recognition of ophiolite complexes is very important in tectonic analyses. Until the mids, ophiolites were thought to represent vestiges of the main oceanic tract, but geochemical analyses have clearly indicated that most ophiolites form near volcanic arcs, such as in back-arc basins characterized by subduction roll-back the collapse of the subducting plate that causes the extension of the overlying plate.
The recognition of ophiolite complexes is very important in tectonic analysis, because they provide insights into the generation of magmatism in oceanic domains, as well as their complex relationships with subduction processes.
See above back-arc basins. Mountains by continental collision Continental collision involves the forced convergence of two buoyant plate margins that results in neither continent being subducted to any appreciable extent. A complex sequence of events ensues that compels one continent to override the other.
The subducted slab still has a tendency to sink and may become detached and founder submerge into the mantle. The crustal root undergoes metamorphic reactions that result in a significant increase in density and may cause the root to also founder into the mantle.
Both processes result in a significant injection of heat from the compensatory upwelling of asthenosphere, which is an important contribution to the rise of the mountains. Continental collisions produce lofty landlocked mountain ranges such as the Himalayas. Much later, after these ranges have been largely leveled by erosionit is possible that the original contact, or suture, may be exposed. The balance between creation and destruction on a global scale is demonstrated by the expansion of the Atlantic Ocean by seafloor spreading over the past million years, compensated by the contraction of the Pacific Oceanand the consumption of an entire ocean between India and Asia the Tethys Sea.
The northward migration of India led to collision with Asia some 40 million years ago. Since that time India has advanced a further 2, km 1, miles beneath Asia, pushing up the Himalayas and forming the Plateau of Tibet.
Exploring Our Fluid Earth
Pinned against stable SiberiaChina and Indochina were pushed sideways, resulting in strong seismic activity thousands of kilometres from the site of the continental collision. Transform faults are so named because they are linked to other types of plate boundaries. The majority of transform faults link the offset segments of oceanic ridges. However, transform faults also occur between plate margins with continental crust—for example, the San Andreas Fault in California and the North Anatolian fault system in Turkey.
These boundaries are conservative because plate interaction occurs without creating or destroying crust. Because the only motion along these faults is the sliding of plates past each other, the horizontal direction along the fault surface must parallel the direction of plate motion. The fault surfaces are rarely smooth, and pressure may build up when the plates on either side temporarily lock.
This buildup of stress may be suddenly released in the form of an earthquake.
Geological Survey Many transform faults in the Atlantic Ocean are the continuation of major faults in adjacent continents, which suggests that the orientation of these faults might be inherited from preexisting weaknesses in continental crust during the earliest stages of the development of oceanic crust.
On the other hand, transform faults may themselves be reactivated, and recent geodynamic models suggest that they are favourable environments for the initiation of subduction zones.
Linear chains of islandsthousands of kilometres in length, that occur far from plate boundaries are the most notable examples. These island chains record a typical sequence of decreasing elevation along the chain, from volcanic island to fringing reef to atoll and finally to submerged seamount.
An active volcano usually exists at one end of an island chain, with progressively older extinct volcanoes occurring along the rest of the chain. Tuzo Wilson and American geophysicist W. Jason Morgan explained such topographic features as the result of hotspots. The principal tectonic plates that make up Earth's lithosphere.
Comparing the Continents with Plate Boundaries
Also located are several dozen hot spots where plumes of hot mantle material are upwelling beneath the plates. Black dots indicate active volcanoes, whereas open dots indicate inactive ones. The number of these hotspots is uncertain estimates range from 20 tobut most occur within a plate rather than at a plate boundary.
Hotspots are thought to be the surface expression of giant plumes of heat, termed mantle plumesthat ascend from deep within the mantle, possibly from the core-mantle boundary, some 2, km 1, miles below the surface. These plumes are thought to be stationary relative to the lithospheric plates that move over them. A volcano builds upon the surface of a plate directly above the plume. As the plate moves on, however, the volcano is separated from its underlying magma source and becomes extinct.
Extinct volcanoes are eroded as they cool and subside to form fringing reefs and atollsand eventually they sink below the surface of the sea to form a seamount. At the same time, a new active volcano forms directly above the mantle plume. Diagram depicting the process of atoll formation. Atolls are formed from the remnant parts of sinking volcanic islands.
The best example of this process is preserved in the Hawaiian-Emperor seamount chain. The plume is presently situated beneath Hawaii, and a linear chain of islandsatollsand seamounts extends 3, km 2, miles northwest to Midway and a further 2, km 1, miles north-northwest to the Aleutian Trench.
The age at which volcanism became extinct along this chain gets progressively older with increasing distance from Hawaii —critical evidence that supports this theory. Hotspot volcanism is not restricted to the ocean basins ; it also occurs within continents, as in the case of Yellowstone National Park in western North America.
Measurements suggest that hotspots may move relative to one another, a situation not predicted by the classical model, which describes the movement of lithospheric plates over stationary mantle plumes. This has led to challenges to this classic model. Furthermore, the relationship between hotspots and plumes is hotly debated. All of the plates are moving. They are slow, moving at speeds of centimeters to tens of centimeters per year. They slide along on top of an underlying mantle layer called the asthenosphere, which contains a little magma molten rock.
The plates are layers of rigid, solid rock. However, as they move, plates interact at their edges or boundaries. There are three basic directions or types of boundary interactions. In some places, two plates move apart from each other; this is called a diverging plate boundary. Elsewhere two plate move together; this is a converging plate boundary. Finally plates can also slide past each other horizontally.
This is called a transform plate boundary. Volcanoes and earthquakes help define the boundaries between the plates. Volcanoes form mostly at converging and diverging plate boundaries, where much magma is generated. Earthquakes occur at all three types of boundaries.