The mantle sits between Earth's dense, very hot core and its thin outer layer, the crust. Plate tectonics has become the unifying theory of geology. Watch this video animation for more information. Tectonic plates are gigantic pieces of the Earth's crust and uppermost mantle.
They are made up of oceanic crust and continental crust. Earthquakes occur around mid-ocean ridges and the large faults which mark the edges of the plates. The Earth is always on the move due to the motion of the tectonic plates. Seven of the major plates make up most of the seven continents and the Pacific Ocean.
They are named after nearby landmasses, oceans, or regions. The Ring of Fire is in the Pacific Ocean. It is made up of a string of volcanoes, deep ocean trenches, and high mountain ranges. 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 two tectonic plates meet and one moves underneath the other Fig. Oceanic crust is primarily composed 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 crust on one plate with the continental crust of a second plate can result in the formation of volcanoes Fig.
As the oceanic crust enters the mantle, pressure breaks the crustal rock, heat 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 eventually flows along weaker crustal channels toward the surface. The magma periodically breaks through the crust to form great, violently explosive composite volcanoes —steep-sided, cone-shaped mountains like those in the Andes at the margin of the South American Plate Fig.
Continental collision occurs when two plates carrying continents collide. Because continental crusts are composed of the same low-density material, one does not sink under the other.
During collision, the crust moves upward, and the crustal material folds, buckles, and breaks Fig. The Himalayan Mountains were formed by the collision between Indian and Eurasian tectonic plates. Ocean trenches are steep depressions in the seafloor formed at subduction zones where one plate moves downward beneath another Fig. These trenches are deep up to 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.
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. Some shield volcanoes, such as the islands in the Hawaiian archipelago, began forming on the ocean floor over a hot spot.
Many spectacular volcanoes are found along subduction zones, such as the " Ring of Fire " that surrounds the Pacific Ocean. When two oceanic plates converge, a deep trench forms, such as the Mariana Trench in the North Pacific Ocean which is believed to be the deepest point on Earth. These types of collisions can also lead to underwater volcanoes. As the name suggests, divergent boundaries are tectonic boundaries where plates "diverge" or are tugged apart.
This motion creates giant troughs on land, such as the East Africa Rift. In the ocean, this same process creates mid-ocean ridges. Underwater mountains and volcanoes can rise along this seam, in some cases forming islands. For example, the Mid-Atlantic Ridge runs directly through Iceland. The final type of plate boundary, transform boundaries, exist where plates move sideways in relation to each other.
California's San Andreas Fault , where the North American and Pacific tectonic plates grind past each other with a mostly horizontal motion, is one famous example of a transform boundary.
Tectonic plates move at a rate of one to 2 inches 3 to 5 centimeters per year, according to National Geographic. That's about as fast as your fingernails grow! Because Earth is spherical, its tectonic or lithospheric plates are fractured into dozens of curved sections.
Imagine it like a cracked egg shell. Each plate ranges from a few hundred to thousands of kilometers in size, according to the U. Download image jpg, 76 KB. A divergent boundary occurs when two tectonic plates move away from each other.
The Mid-Atlantic Ridge is an example of divergent plate boundaries. When two plates come together, it is known as a convergent boundary. The impact of the colliding plates can cause the edges of one or both plates to buckle up into a mountain ranges or one of the plates may bend down into a deep seafloor trench.
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