Plate Tectonic Theory of Mountain Building | Geography
Explain the connection between tectonic plates and mountain formation. Describe several This type of boundary eventually results in a collision. Tectonic plate. Plate tectonics, theory dealing with the dynamics of Earth's outer context for understanding mountain-building processes, volcanoes, in which to describe the past geography of continents and oceans, . As these zones of shear link other plate boundaries to one another, they are called transform faults. From the late 18th century until its replacement by plate tectonics in the s geosyncline theory was used to explain much mountain-building.
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.
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. 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.
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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.
plate tectonics | Definition, Theory, Facts, & Evidence | hidden-facts.info
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.
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.
In addition, isotopic and fossil ages obtained through deep sea drilling of sediments directly above oceanic basalt helped bracket the ages of individual magnetic stripes.
Age dating of seafloor magnetic anomaly patterns eventually led to construction of maps depicting the ages of various segments of oceanic crust. The distribution of earthquake epicenters and most active volcanoes define the plate boundaries. There are three types of plate boundaries: New lithosphere is created along a divergent plate boundary. Lithosphere is generally destroyed along a convergent plate boundary. Lithosphere is conserved neither destroyed nor created along a transform boundary.
Large convection cells in the mantle carry lithosphere in a conveyor-belt manner. Cells may reach all the way down to the core boundary.
Dense subducting oceanic lithosphere pulls the rest of the slab behind it. Convection cells are restricted to the upper mantle. Gravitational sliding of the slab down the slope of mid-ocean ridges. A spreading ocean ridge pushes the plates away from the ridge. The downward limbs of convection cells are cold and dense, carrying subducted oceanic plates with them. There are several parts to a typical subduction zone: A depression marking the site where one plate descends beneath another.
Generally located on the overriding plate close to the trench. The main line of volcanism associated with subduction zones.How The Mountains Are Formed?
Volcanic arcs within oceans are called island arcs. Andesitic lavas are common. A depression located between the accretionary wedge and the main volcanic arc.
Sediments eroded from adjacent highlands are deposited and accumulate within the forearc basin. There are three types of convergent plate boundaries: Ocean-ocean convergence with volcanic island arcs b Figure Ocean-continent Andean-type convergence with a continental volcanic arc, thick accretionary wedge and large continental volcanic arc c Figure Show the students that the yellow and blue layers represent rock layers under the ground's surface.
Hold the blocks with the valley piece in the middle in the shape of a V. Look at the fault lines. Line up the three blocks and hold them above the table.
Starting with the blocks level, slightly pull apart the outer blocks and see how the valley block drops down below the other blocks. Notice also that the two outer blocks are rising slightly. This process forms mountains and valleys. This is how our Salt Lake valley was formed. This dropped our valley down below the mountains. Look at these mountain ranges and our valley on the US relief map found in the kit.
Dome Mountains -- form from uplifting of a tectonic plate, not at a boundary. Magma under the earth rises and pushes up the crust to form a mountain without the eruption of the magma. The magma instead cools under the crust and forms the basis for the mountain.
Have students punch a hole in an index card with a pencil. Cover the surface of the index card with dried grass cut into about 1 inch pieces. Hold a tube of toothpaste under the hole in the card and push the top of the tube up through the pencil hole. Slowly squeeze the toothpaste under the grass. The toothpaste should only push the grass up and not seep out of the grass. This is how a dome mountain is formed.
The toothpaste represents magma and the dried grass represents rock. In Utah, Navajo Mountain is an example of a dome mountain. These mountains occur alone rather than in a long chain. Look at the picture of the Showa Shin-Zan dome mountain that formed in Japan over only 18 months. Volcanic Mountains -- usually form at convergent plates when a volcano puts out a series of eruptions over many years.
Mountain formation - Wikipedia
The successive layers of lava that erupted out of the volcano form a mountain. Students should put 1 Tablespoon of Plaster of Paris and 2 tsp. Place the cut baggie under the hole in the card and slowly squeeze a small amount of the plaster up through the hole. This represents one eruption of magma out of a volcano. When this eruption begins to cool and harden, repeat the lava eruption again. Notice how each lava flow builds on the other.
This cycling of eruption and hardening of lava builds up the walls of the volcano and mountain. Helens in Washington State is a volcanic mountain. Have students look at the picture of Mt. Vesuvius, an active volcanic mountain in Italy.