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Plate Tectonics

According to the theory of plate tectonics, the earth’s crust is broken up into at least a dozen rigid plates that move independently of one another. These slabs rest upon a layer of heated, pliable rock called the asthenosphere, which flows slowly like hot tar. Geologists have not yet determined exactly how these two layers interact, but a popular theory is that the movement of the thick, molten material in the asthenosphere forces the upper plates to shift, sink, or rise.

The basic concept behind plate tectonics is simply that heat rises. Hot air rises above cool air, and warm water currents flow above cold water. The same is true of the heated rock below the earth’s surface. The asthenosphere’s molten material, or magma, pushes upwards, while cooler, hardened matter sinks deeper into the mantle

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Sinking rock eventually reaches the extremely hot temperatures of the lower asthenosphere, heats up, and begins to rise again. This continual, roughly circular motion is called convection. At diverging plate boundaries and at hot spots in the otherwise solid lithosphere, molten material wells up to the surface, forming a new crust.

Continental Drift

The theory of plate tectonics was not widely accepted until the 1960s and 1970s. Before that time, most scientists believed the earth’s continents and oceans to be stationary. At the beginning of the 20th century, German meteorologist Alfred Wegener suggested that all continents had been part of one huge supercontinent, Pangaea. According to Wegener, about 200 million years ago Pangaea broke into separate plates that slowly drifted away from each other, leading to today’s continental arrangement.

One of Wegener’s most convincing pieces of evidence was the almost perfect fit between the eastern coast of South America and the western coast of Africa. To support his theory, he pointed out that rock formations on opposite sides of the Atlantic—in Brazil and West Africa—match in age, type, and structure. Also, the formations often contain fossils of the same terrestrial creatures, indicating that South America and Africa must have previously been connected.

In subsequent years, scientific discoveries steadily began to support the fundamental aspects of Wegener’s theory. Geologists demonstrated the existence of the slowly moving asthenosphere, underlying the crust at depths of 50 to 150 kilometres (30 to 80 miles). In addition, scientists in the 1920s used sonar, an echo-sounding device, to determine ocean depths and map the seafloor. They concluded that the Mid-Atlantic Ridge, detected in the 19th century, was part of a worldwide ocean ridge system.

Seafloor Spreading

Additional evidence for plate tectonics came in the 1950s and 1960s. During this period, scientists discovered that all rock fragments maintain a set magnetic pattern based on when the rocks formed. Geophysicists also learned that the earth’s magnetic field had reversed between north and south dozens of times over millions of years. With this knowledge, they examined both sides of ocean ridges and found that the rocks on one side of the ridge produced a mirror-image geomagnetic pattern of the rocks on the other side.

The rocks nearest the ridge were relatively young, but the rocks aged as the distance from the ridge increased. In addition, marine sediment was thicker and older further from the ridge, whereas the ridge itself had virtually no deposits of sediment. These observations, added to those of the heat flow at the ridge, confirmed the creation of new crust at mid-ocean ridges and the mechanism of seafloor spreading. platetectonics2.gif (103736 bytes)
After molten rock reaches the seafloor as lava, deep ocean water quickly cools and consolidates the material. To make room for this continual addition of new crust, the plates on either side of the ridge must constantly move apart. In the North Atlantic, the rate of movement of each plate is only about 1 to 2 centimetres (0.4 to 0.8 of an inch) per year. In the Pacific, the rate can be more than 10 centimetres (about 4 inches) annually.
The Marianas Trench, just east of the Mariana Islands in the western Pacific, is the deepest seafloor depression in the world at 11,033 metres (36,198 feet). The Marianas Trench is one of many deepwater trenches formed by the geologic process of subduction. During subduction, the edges of plates are subducted, or forced under, other plates. Ocean crust is drawn down into the mantle and partially melted. wpe3.jpg (12229 bytes)
An important effect of the melting of subducted ocean crust is the production of new magma. When subducted ocean crust melts, the magma that forms may rise from the plane of subduction deep within the mantle, erupting on the earth’s surface. Eruption of magma melted by subduction has created long, arc-shaped chains of volcanic islands, such as Japan, the Philippines, and the Aleutians. Where an oceanic plate is subducted beneath continental crust, the magma produced by subductive melting erupts from volcanoes situated among long, linear mountain chains, such as the Andes in South America.

Plate Boundaries

Plate boundaries do not necessarily match the coastlines of continents. A plate can consist of continental crust, oceanic crust, or both. In most cases, continents are part of larger plates that extend for hundreds of miles offshore. Many plate boundaries are far out in the middle of the ocean. There are three types of plate boundaries: divergent, convergent, and transform.

Divergent boundaries exist where plates move away from each other, pushed apart by heated, material moving upwards from the asthenosphere. An additional force involved in divergence may be the subduction of the heavier, older, and thicker crust at the opposite ends of each diverging plate. As the heavy edge sinks, it pulls the rest of the plate with it, away from the divergent boundary. Magma at the divergent boundary hardens, adding new crust to the edges of the separating plates. Scientists often refer to these as constructive boundaries, due to the construction of new material. Mid-ocean ridges are examples of this type of boundary. These ridges frequently resemble submarine mountain ranges, portions of which are high enough to break the ocean’s surface, in places such as Iceland in the North Atlantic.

Divergent boundaries also exist within continents. The Great Rift Valley, which extends for more than 4,830 kilometres (3,000 miles) from Syria to Mozambique, is a well-known example. Divergence has caused the earth’s crust to thin and drop along this plate boundary. A boundary where two plates collide is a convergent boundary.

When an oceanic plate, such as the Nazca Plate which moves eastwards under the southeastern Pacific Ocean, meets a continental edge such as South America, the denser and heavier oceanic crust is normally subducted and partially melted beneath the continental plate. Ocean trenches at the boundary of the plate and mountain chains on the continental plate often result. 

Earthquakes can occur at these plate margins, shifting plates by up to 5 metres (about 15 feet) at once. Such faults exist in Chile, Japan, Taiwan, the Philippines, New Zealand, and Sumatra. When two continental plates collide, the crust from both plates thrusts upwards, creating mountain chains. The collision of India with the Asian continent formed the Himalayas. In fact, the mountain range is still growing in height today because India and Asia are still converging. platetectonics3.gif (86215 bytes)
At a transform boundary, plates move past each other in opposite directions. Little volcanic activity accompanies transform boundaries, but large, shallow earthquakes can occur. The San Andreas Fault in California (USA), is the most famous example of this type of boundary. Mid-ocean ridges are offset by hundreds of small transforms.
The revolutionary theory of plate tectonics forms the basis of modern geologic thought and explains many of today’s landforms and the movement of continents. This theory also provides an explanation for many of the world’s earthquakes and volcanoes. Most earthquakes and volcanic eruptions take place near plate margins.  platetectonics4.gif (83214 bytes)
Unfortunately, many large cities exist along plate margins, such as along the Ring of Fire, a zone of intense volcanic and seismic activity surrounding the Pacific Ocean. Humans repeatedly suffer the effects of these often catastrophic manifestations of tectonic activity.

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