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Translations of Encyclopedia about Geology

 

Tectonic boundaries

According to the plate tectonics theory, the lithosphere consists of about one dozen of moving rigid plates. The movement is produced by convection flow in the earth's interior. These movements of the plate boundaries may be categorised into three types: converging boundary, diverging boundary, and transform faults.

In the case of converging boundaries, two plates collide. In the case of ocean plates we call it subductive region. One plate is subducted, which means it is pushed under another lithospheric plates into the underlying earth's mantle. There, the plate melts and becomes magma. Among other things, magma forms volcanoes.

The action of collision and subduction results in the formation of fold and fault mountain ranges, mid-ocean rifts, and magma zones. It also results in the formation of volcanic crests on the ocean floor, which is also called island arc formation. A good example of subduction region on the converging boundary is the Nazca plate and the South American plate. Alongside the coast there is the Atacama deep-ocean ridge, while on the coast there are the andes that include numerous volcanoes. Many of the strongest earthquakes in these regions indicate a great release of energy when the plates collide.

An example of a collision of two continental plates is the one that occurred between India and Eurasia. Here, 60 million years ago, began a subduction of the Indo-Australian plate under the Eurasian plate. The thickness of the earth's crust doubled and the ascending magma from the subducted plate joined to form the highest mountain range of the world, the Himalayas. This process still continues.

In the case of the diverging plate boundary the plates drift away from each other. The molten material rises from the interior of the mantle filling up the forming gap. New lithosphere, which forms, is deposited on the retreating edges.

We recognize the retreating plates on the ocean floor, because they form mid-ocean ridges. The forces originating as a result of the plate divergence cause earthquakes and volcanic eruptions. The process during which the ocean crust is being formed is called "seafloor spreading."

Centres of such seafloor spreading are Eastern Pacific Ridge and the Mid-Atlantic Ridge. These were the place where during a long time gigantic areas of ocean floor were forming. Iceland is also one of such places. Iceland is a part of the Mid-Atlantic Ridge, but it is the part that is above the sea level.

When the plates drift away from each other, it lead to formation of rift valleys on land. This is also accompanied by volcanic explosions and earthquakes. It is assumed, that the East African Rift Valley was formed by such a process.

When the plates drift alongside each other (there is no convergence or divergence), the so-called transform faults will occur. No new lithosphere is created and the original one remains in place. San andreas fault in California is an example of such a situation. Here, the Pacific plate passes the North American plate. Each one of these lithospheric plates combines in itself all three types of boundaries.

The North American plate, for example, is surrounded by the diverging boundary of the Mid-Atlantic ridge in the east, the San andreas fault in the west, and the converging boundary and transform faults in the northwest. Analogously, the same occurs in the case of other plate boundaries.

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