The Movement of These Pushes Rocks Back Into the Mantle Where They Melt and Become Magma Again
Globe's Tectonic Plates
The earth's crust is cleaved into divide pieces called tectonic plates (Fig. 7.14). Call up that the crust is the solid, rocky, outer shell of the planet. It is composed of ii distinctly different types of textile: the less-dense continental crust and the more-dense oceanic crust. Both types of chaff rest atop solid, upper drapery material. The upper mantle, in turn, floats on a denser layer of lower drape that is much like thick molten tar.
Each tectonic plate is costless-floating and can move independently. Earthquakes and volcanoes are the direct result of the movement of tectonic plates at fault lines. The term fault is used to describe the boundary between tectonic plates. Near of the earthquakes and volcanoes effectually the Pacific body of water basin—a pattern known as the "ring of fire"—are due to the movement of tectonic plates in this region. Other observable results of short-term plate move include the gradual widening of the Groovy Rift lakes in eastern Africa and the rising of the Himalayan Mountain range. The motion of plates can exist described in four general patterns:
- Standoff: when two continental plates are shoved together
- Subduction: when one plate plunges beneath another (Fig. 7.15)
- Spreading: when ii plates are pushed apart (Fig. vii.15)
- Transform faulting: when two plates slide by each other (Fig. 7.fifteen)
The ascension 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 have hypothesized that the movement of tectonic plates is related to convection currents in the earth's mantle. Convection currents draw the rising, spread, and sinking of gas, liquid, or molten textile caused by the application of heat. An example of convection current is shown in Fig. 7.xvi. Inside a beaker, hot water rises at the point where oestrus is practical. The hot h2o moves to the surface, so spreads out and cools. Cooler water sinks to the bottom.
Earth's solid crust acts equally a heat insulator for the hot interior of the planet. Magma is the molten rock below the chaff, in the mantle. Tremendous heat and pressure within the earth cause the hot magma to catamenia in convection currents. These currents crusade the motion of the tectonic plates that make up the world's chaff.
Activeness: Modeling Plate Spreading
Simulate tectonic plate spreading by modeling convection currents that occur in the drapery.
Activity: Earth's Plates
Examine a map of the globe'southward tectonic plates. Based on evidence that has been found at plate boundaries, make some hypotheses nigh the move of those plates.
The earth has changed in many ways since information technology first formed 4.five billion years agone. 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 course of hundreds of millions of years—alternately combining into supercontinents and pulling apart in a process known as continental drift. The supercontinent of Pangaea formed as the landmasses gradually combined roughly betwixt 300 and 100 mya. The planet'south landmasses somewhen moved to their electric current positions and will continue to movement into the future.
Plate tectonics is the scientific theory explaining the motility of the earth's chaff. It is widely accepted past scientists today. Recollect that both continental landmasses and the ocean flooring are part of the earth's crust, and that the chaff is broken into individual pieces called tectonic plates (Fig. 7.14). The movement of these tectonic plates is probable caused by convection currents in the molten rock in World's pall below the crust. Earthquakes and volcanoes are the short-term results of this tectonic movement. The long-term effect of plate tectonics is the movement of entire continents over millions of years (Fig. 7.18). The presence of the same type of fossils on continents that are at present widely separated is evidence that continents have moved over geological history.
Activity: Continental Movement over Long Fourth dimension Scales
Evaluate and interpret several lines of evidence for continental migrate over geological time scales.
Bear witness for the Movement of Continents
The shapes of the continents provide clues nigh the past movement of the continents. The edges of the continents on the map seem to fit together like a jigsaw puzzle. For example, on the west declension of Africa, there is an indentation into which the bulge along the east declension of South America fits. The shapes of the continental shelves—the submerged landmass effectually continents—shows that the fit between continents is fifty-fifty more hit (Fig. vii.nineteen).
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. seven.20 A) and a land reptile called Cynognathus (Fig. vii.twenty B) have been constitute in South America and Due south Africa. Another example is the fossil plant called Glossopteris, which is found in India, Commonwealth of australia, and Antarctica (Fig. seven.xx C). The presence of identical fossils in continents that are now widely separated is one of the principal pieces of show that led to the initial thought that the continents had moved over geological history.
Prove for continental drift is besides found in the types of rocks on continents. At that place are belts of rock in Africa and South America that match when the ends of the continents are joined. Mountains of comparable historic period and structure are found in the northeastern role of North America (Appalachian Mountains) and across the British Isles into Norway (Caledonian Mountains). These landmasses can be reassembled so that the mountains class a continuous chain.
Paleoclimatologists (paleo = ancient; climate = long term temperature and weather patterns) study evidence of prehistoric climates. Evidence from glacial striations in rocks, the deep grooves in the land left past the movement of glaciers, shows that 300 mya there were big sheets of ice covering parts of South America, Africa, Bharat, and Australia. These striations indicate that the direction of glacial move in Africa was toward the Atlantic ocean bowl and in S America was from the Atlantic bounding main bowl. This evidence suggests that South America and Africa were once connected, and that glaciers moved across Africa and South America. There is no glacial evidence for continental motility in N America, because there was no water ice roofing the continent 300 million years ago. North America may have been nearer the equator where warm temperatures prevented ice sheet formation.
Seafloor Spreading at Mid-Ocean Ridges
Convection currents bulldoze the movement of Globe's rigid tectonic plates in the planet'due south fluid molten mantle. In places where convection currents rise upward towards the chaff's surface, tectonic plates move abroad from each other in a procedure known every bit seafloor spreading (Fig. 7.21). Hot magma rises to the crust's surface, cracks develop in the ocean floor, and the magma pushes up and out to form mid-sea ridges. Mid-bounding main ridges or spreading centers are error lines where two tectonic plates are moving away from each other.
Mid-sea ridges are the largest continuous geological features on World. They are tens of thousands of kilometers long, running through and connecting nigh 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 process of seafloor spreading slowly pushes tectonic plates apart while generating new rock from cooled magma. Ocean flooring rocks close to a mid-ocean ridge are not only younger than distant rocks, they as well display consistent bands of magnetism based on their age (Fig. seven.22.ane). Every few hundred one thousand years the earth'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'due south magnetic field was the opposite of its current polarity. Geomagnetic reversal allows scientists to study the movement of ocean floors over fourth dimension.
Paleomagnetism is the written report of magnetism in ancient rocks. As molten rock cools and solidifies, particles within the rocks align themselves with the world's magnetic field. In other words, the particles volition signal in the direction of the magnetic field nowadays as the rock was cooling. If the plate containing the rock drifts or rotates, and so the particles in the rock will no longer be aligned with the earth's magnetic field. Scientists can compare the directional magnetism of stone particles to the direction of the magnetic field in the rock's current location and approximate where the plate was when the rock formed (Fig. 7.22.1).
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. seven.23). Oceanic crust is primarily equanimous of basalt, which makes it slightly denser than continental crust, which is composed 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 second plate can result in the germination of volcanoes (Fig. 7.23). As the oceanic crust enters the mantle, pressure breaks the crustal rock, heat from friction melts it, and a puddle of magma develops. This thick magma, called andesite lava, consists of a mixture of basalt from the oceanic crust and granite from the continental chaff. Forced by tremendous pressure level, it eventually flows along weaker crustal channels toward the surface. The magma periodically breaks through the crust to course cracking, violently explosive composite volcanoes—steep-sided, cone-shaped mountains like those in the Andes at the margin of the South American Plate (Fig. 7.23).
Continental standoff occurs when two plates carrying continents collide. Considering continental crusts are equanimous of the same low-density textile, one does not sink nether the other. During standoff, the crust moves upward, and the crustal material folds, buckles, and breaks (Fig. vii.24 A). Many of the world's largest mountain ranges, like the Rocky Mountains and the Himalayan Mountains, were formed by the collision of continents resulting in the upward motion of the earth's crust (Fig. 7.24 B). The Himalayan Mountains were formed by the collision between Indian and Eurasian tectonic plates.
Bounding main trenches are steep depressions in the seafloor formed at subduction zones where one plate moves downwards below another (Fig. seven.24 C). These trenches are deep (up to 10.8 km), narrow (nigh 100 km), and long (from 800 to 5,900 km), with very steep sides. The deepest sea 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 plant where two tectonic plates move past each other. As the plates slide past one some other, there is friction, and peachy tension can build upwardly earlier slippage occurs, somewhen causing shallow earthquakes. People living well-nigh the San Andreas Error, a transfom error in California, regularly feel such quakes.
Hot Spots
Recall that some volcanoes form most plate boundaries, particularly nearly subduction zones where oceanic crust moves underneath continental crust (Fig. 7.24). However, some volcanoes form over hot spots in the middle of tectonic plates far abroad from subduction zones (Fig. 7.25). A hot spot is a place where magma rises upwardly 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 commonly establish 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 as 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 isle of Hawai'i reaches 4.2 km to a higher place body of water level. However, the base of this volcanic island lies well-nigh seven km below the h2o surface, making Hawai'i'southward peaks some of the tallest mountains on Globe—much higher than Mount Everest. About all of the mid-Pacific and mid-Atlantic bounding main basin islands formed in a similar fashion over volcanic hot spots. Over millions of years every bit the tectonic plate moves, a volcano that was over the hot spot moves away, ceases to erupt, and becomes extinct (Fig. 7.25). Erosion and subsidence (sinking of the earth's crust) eventually causes older islands to sink below sea level. Islands can erode through natural processes such equally air current and water menstruation. Reefs continue to grow effectually the eroded land mass and grade fringing reefs, equally seen on Kauaʻi in the main Hawaiian Islands (Fig. 7.26).
Eventually all that remains of the island is a ring of coral reef. An atoll is a band-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 country and growth of coral reefs effectually the island. Coral reef atolls can simply occur in tropical regions that are optimal for coral growth. The main Hawaiian Islands volition all likely get coral atolls millions of years into the time to come. The older Northwestern Hawaiian Islands, many of which are at present atolls, were formed by the same volcanic hot spot as the younger main Hawaiian Islands.
braunsteinfinece1954.blogspot.com
Source: https://manoa.hawaii.edu/exploringourfluidearth/node/1348
0 Response to "The Movement of These Pushes Rocks Back Into the Mantle Where They Melt and Become Magma Again"
Post a Comment