what causes plates to move on the mantles surface
World'south Tectonic Plates
The earth's crust is broken into separate pieces called tectonic plates (Fig. 7.14). Recall that the crust is the solid, rocky, outer shell of the planet. Information technology is composed of two distinctly different types of fabric: the less-dense continental crust and the more-dense oceanic crust. Both types of crust rest atop solid, upper drapery material. The upper mantle, in plough, floats on a denser layer of lower curtain that is much like thick molten tar.
Each tectonic plate is costless-floating and can move independently. Earthquakes and volcanoes are the direct consequence of the movement of tectonic plates at fault lines. The term fault is used to describe the boundary between tectonic plates. Virtually of the earthquakes and volcanoes around the Pacific bounding main basin—a pattern known equally the "ring of fire"—are due to the movement of tectonic plates in this region. Other observable results of short-term plate movement include the gradual widening of the Slap-up Rift lakes in eastern Africa and the ascent of the Himalayan Mountain range. The motility of plates can be described in four general patterns:
- Collision: when two continental plates are shoved together
- Subduction: when one plate plunges beneath some other (Fig. 7.15)
- Spreading: when two plates are pushed apart (Fig. seven.xv)
- Transform faulting: when ii plates slide past each other (Fig. 7.xv)
The rise of the Himalayan Mountain range is due to an ongoing collision of the Indian plate with the Eurasian plate. Earthquakes in California are due to transform fault motion.
Geologists accept hypothesized that the movement of tectonic plates is related to convection currents in the world's mantle. Convection currents describe the rising, spread, and sinking of gas, liquid, or molten material caused by the application of heat. An instance of convection current is shown in Fig. 7.sixteen. Within a beaker, hot water rises at the indicate where heat is practical. The hot water moves to the surface, then spreads out and cools. Cooler h2o sinks to the bottom.
Earth's solid crust acts as a heat insulator for the hot interior of the planet. Magma is the molten rock beneath the chaff, in the mantle. Tremendous estrus and pressure within the earth crusade the hot magma to catamenia in convection currents. These currents cause the motion of the tectonic plates that make up the earth's crust.
Activeness: Modeling Plate Spreading
Simulate tectonic plate spreading by modeling convection currents that occur in the mantle.
Action: Earth'south Plates
Examine a map of the earth'south tectonic plates. Based on prove that has been institute at plate boundaries, brand some hypotheses about the movement of those plates.
The earth has inverse in many means since it first formed 4.5 billion years ago. The locations of Earth'southward major landmasses today are very different from their locations in the past (Fig. 7.18). They have gradually moved over the form of hundreds of millions of years—alternately combining into supercontinents and pulling apart in a procedure known every bit continental drift. The supercontinent of Pangaea formed as the landmasses gradually combined roughly between 300 and 100 mya. The planet'southward landmasses eventually moved to their current positions and will continue to move into the future.
Plate tectonics is the scientific theory explaining the movement of the earth's crust. It is widely accepted past scientists today. Think that both continental landmasses and the ocean floor are part of the world's crust, and that the chaff is cleaved into private pieces chosen tectonic plates (Fig. 7.14). The motility of these tectonic plates is likely caused by convection currents in the molten stone in World's drapery below the crust. Earthquakes and volcanoes are the brusque-term results of this tectonic movement. The long-term upshot of plate tectonics is the movement of entire continents over millions of years (Fig. vii.18). The presence of the same type of fossils on continents that are now widely separated is evidence that continents accept moved over geological history.
Activity: Continental Motility over Long Time Scales
Evaluate and translate several lines of show for continental drift over geological time scales.
Evidence for the Movement of Continents
The shapes of the continents provide clues virtually the past movement of the continents. The edges of the continents on the map seem to fit together similar a jigsaw puzzle. For example, on the west coast of Africa, at that place is an indentation into which the bulge along the east coast of Southward America fits. The shapes of the continental shelves—the submerged landmass around continents—shows that the fit between continents is even more striking (Fig. 7.19).
Some fossils provide evidence that continents were once located nearer to one another than they are today. Fossils of a marine reptile called Mesosaurus (Fig. vii.20 A) and a state reptile called Cynognathus (Fig. 7.twenty B) accept been found in South America and Due south Africa. Some other case is the fossil plant called Glossopteris, which is found in India, Australia, and Antarctica (Fig. 7.20 C). The presence of identical fossils in continents that are at present widely separated is one of the main pieces of evidence that led to the initial idea that the continents had moved over geological history.
Evidence for continental drift is also found in the types of rocks on continents. There are belts of stone in Africa and South America that match when the ends of the continents are joined. Mountains of comparable age and structure are constitute in the northeastern function of Northward America (Appalachian Mountains) and across the British Isles into Norway (Caledonian Mountains). These landmasses tin can be reassembled so that the mountains class a continuous chain.
Paleoclimatologists (paleo = ancient; climate = long term temperature and weather condition patterns) study evidence of prehistoric climates. Prove from glacial striations in rocks, the deep grooves in the land left by the movement of glaciers, shows that 300 mya in that location were large sheets of ice covering parts of Southward America, Africa, India, and Commonwealth of australia. These striations betoken that the direction of glacial movement in Africa was toward the Atlantic body of water basin and in South America was from the Atlantic ocean basin. This evidence suggests that South America and Africa were once connected, and that glaciers moved across Africa and South America. There is no glacial testify for continental move in North America, because there was no ice roofing the continent 300 million years ago. Due north America may have been nearer the equator where warm temperatures prevented ice sail germination.
Seafloor Spreading at Mid-Ocean Ridges
Convection currents bulldoze the movement of Earth'due south rigid tectonic plates in the planet's fluid molten drape. In places where convection currents rise up towards the chaff's surface, tectonic plates move away from each other in a procedure known as seafloor spreading (Fig. 7.21). Hot magma rises to the crust's surface, cracks develop in the bounding main flooring, and the magma pushes upwardly and out to class mid-ocean ridges. Mid-ocean ridges or spreading centers are fault lines where two tectonic plates are moving away from each other.
Mid-body of water ridges are the largest continuous geological features on Globe. They are tens of thousands of kilometers long, running through and connecting most of the body of water basins. Oceanographic data reveal that seafloor spreading is slowly widening the Atlantic ocean basin, the Red Sea, and the Gulf of California (Fig. 7.22).
The gradual procedure of seafloor spreading slowly pushes tectonic plates apart while generating new rock from cooled magma. Bounding main floor rocks close to a mid-ocean ridge are non merely younger than distant rocks, they also display consistent bands of magnetism based on their age (Fig. seven.22.1). Every few hundred chiliad years the world's magnetic field reverses, in a process known as geomagnetic reversal. Some bands of stone were produced during a time when the polarity of the earth'south magnetic field was the reverse of its current polarity. Geomagnetic reversal allows scientists to study the movement of ocean floors over fourth dimension.
Paleomagnetism is the study of magnetism in ancient rocks. As molten rock cools and solidifies, particles within the rocks align themselves with the world'southward magnetic field. In other words, the particles volition point in the direction of the magnetic field present as the rock was cooling. If the plate containing the rock drifts or rotates, then the particles in the stone volition no longer be aligned with the earth'southward magnetic field. Scientists tin can compare the directional magnetism of stone particles to the direction of the magnetic field in the rock's current location and estimate where the plate was when the rock formed (Fig. seven.22.one).
Seafloor spreading gradually pushes tectonic plates apart at mid-ocean ridges. When this happens, the opposite edge of these plates push against other tectonic plates. Subduction occurs when 2 tectonic plates see and one moves underneath the other (Fig. 7.23). Oceanic crust is primarily composed of basalt, which makes information technology slightly denser than continental chaff, which is equanimous primarily of granite. Because it is denser, when oceanic crust and continental crust meet, the oceanic crust slides below the continental crust. This collision of oceanic chaff on one plate with the continental crust of a 2d plate can result in the germination of volcanoes (Fig. 7.23). As the oceanic crust enters the mantle, pressure breaks the crustal stone, rut from friction melts it, and a pool of magma develops. This thick magma, called andesite lava, consists of a mixture of basalt from the oceanic crust and granite from the continental crust. Forced by tremendous pressure, it somewhen flows along weaker crustal channels toward the surface. The magma periodically breaks through the crust to grade great, violently explosive composite volcanoes—steep-sided, cone-shaped mountains similar those in the Andes at the margin of the Due south American Plate (Fig. 7.23).
Continental collision occurs when ii plates conveying continents collide. Because continental crusts are equanimous of the same low-density material, 1 does non sink under the other. During collision, the crust moves upward, and the crustal fabric folds, buckles, and breaks (Fig. 7.24 A). Many of the earth'south largest mount ranges, similar the Rocky Mountains and the Himalayan Mountains, were formed past the collision of continents resulting in the upward movement of the globe's crust (Fig. vii.24 B). The Himalayan Mountains were formed by the standoff betwixt Indian and Eurasian tectonic plates.
Ocean trenches are steep depressions in the seafloor formed at subduction zones where 1 plate moves downwards beneath another (Fig. 7.24 C). These trenches are deep (up to x.8 km), narrow (about 100 km), and long (from 800 to five,900 km), with very steep sides. 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 border of the Filipino plate. Subduction zones are also sites of deepwater earthquakes.
Transform faults are found where two tectonic plates move by each other. Every bit the plates slide past i some other, there is friction, and bang-up tension can build up before slippage occurs, eventually causing shallow earthquakes. People living near the San Andreas Mistake, a transfom fault in California, regularly experience such quakes.
Hot Spots
Call up that some volcanoes form near plate boundaries, particularly virtually subduction zones where oceanic crust moves underneath continental chaff (Fig. seven.24). However, some volcanoes class over hot spots in the center of tectonic plates far away from subduction zones (Fig. seven.25). A hot spot is a identify where magma rises up from the earth's mantle toward the surface crust. When magma erupts and flows at the surface, information technology is called lava. The basalt lava normally 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.
Some shield volcanoes, such every bit the islands in the Hawaiian archipelago, began forming on the ocean flooring 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. 7.25). The highest peak on the island of Hawai'i reaches 4.2 km above body of water level. Notwithstanding, the base of this volcanic island lies almost 7 km below the water surface, making Hawai'i's peaks some of the tallest mountains on Earth—much college than Mount Everest. Nigh all of the mid-Pacific and mid-Atlantic body of water bowl 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 abroad, ceases to erupt, and becomes extinct (Fig. seven.25). Erosion and subsidence (sinking of the earth'due south crust) eventually causes older islands to sink below sea level. Islands can erode through natural processes such as wind and water flow. Reefs continue to grow around the eroded land mass and course fringing reefs, equally seen on Kauaʻi in the master Hawaiian Islands (Fig. vii.26).
Eventually all that remains of the island is a band of coral reef. An atoll is a ring-shaped coral reef or group of coral islets that has grown effectually the rim of an extinct submerged volcano forming a central lagoon (Fig. 7.27). Atoll formation is dependent on erosion of land and growth of coral reefs around the isle. Coral reef atolls can but occur in tropical regions that are optimal for coral growth. The main Hawaiian Islands will all likely get coral atolls millions of years into the future. The older Northwestern Hawaiian Islands, many of which are now atolls, were formed by the aforementioned volcanic hot spot as the younger main Hawaiian Islands.
Source: https://manoa.hawaii.edu/exploringourfluidearth/node/1348
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