1. A plate is a large, rigid slab of rock which moves slowly over the asthenosphere.
2. The thickness of a plate is from '0 to 10 Kms at the ridges and 100 to 150 kms elsewhere.
3. The plates are of continental dimensions.
4. So far as the number of lithospheric plates (of which the earth's surface is considered to be made up) is concerned, there exists a divergence of opinions among the prominent geologists in this field.
While W.J. Morgan believes that the earth is composed of 20 lithospheric plates, Le Pichon throught it to be composed of six major plates and the smaller plates were incorporated into these chosen six. This number is commonly accepted by most of the workers in the field. These major plates are:
(a) the Pacific plate.
(b) The American plate.
(c) The African plate.
(d) The Eurasian plate.
(e) The Indian, Plate and
(f) the Antarctic plate.
According to the U.S. Geological Survey, there are eight large lithospheric plates and a few dozen smaller plates which constitute the crust of the earth.
5. The plates are continuously in motion both with respect to each other and to the earth's axis of rotation. Each plate is capable of moving independently of its surrounding plates.
6. The plates may contain continental as well as oceanic surface. Of the six major plates, five contain continents or parts of the continents in the lithosphere.
Only the Pacific-plate is made up mostly of sea-floor. Some of the smaller plates may be entirely continental but all the large plates contain some sea-floor.
7. Virtually all seismicity, volcanicity and tectonic activity is localized around plate margins and is associated with differential motion between adjacent plates.
8. Plate interiors generally are devoid of earthquakes, volcanoes, young mountain ranges and other significant geologic activities.
9. These plates are small and large, separated by faults and thrusts, lying mostly across ridges or parallel to borders (trenches).
10. Plates move with velocities ranging from 1 to 6 cm per year. Plates move away from one another, past one another or towards one another.
Where two plates diverge, we find extensional features, typically the oceanic-ridges, symmetrical about the vertical axis.
Where two plates converge and one is thrust beneath the other, we find the island arcs-the huge asymmetric features which are the sites of greatest earthquakes, explosive volcanism, great topographic relief and many other distinctive features.
Where two plates slide past each other, there occurs trans- current-faults i.e. the large strike-slip faults joining segments of ocean-ridges or arcs.
11. Plate boundary is the surface trace of the zone of motion between two plates. While plates are located by- outlining the
inactive regions of a map, plate boundaries are located and defind by mapping narrow bands of tectonic activities such as belts of earthquakes and volcanism etc.
Depending upon whether plates move away from each other, move towards each other or move past each other, plate boundaries are classified into three types as Constructive, Destructive and Conservative plate boundary, respectively.
12. Plate margin is the marginal part of a particular plate. Two plate margins meet at a common plate boundary. Where three plate boundaries meet, it is termed as a Triple-Junctions.
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