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The movement of these crustal plates causes the formation of various landforms and is the principal cause of all earth movements.
Crustal Plates:
There are six major plates and many minor ones. The major plates are American, Eurasian, African, Indo-Australian, Pacific and the Antarctic. The minor plates include North American, South American, Nazca, Turkish, Aegean, and Arabian, Caribbean, Philippine, Juan de Fuca and the Iranian plate.
The average thickness of these plates is around 100 km and the entire thickness is involved in crustal spreading and down-bending. The oceanic plates contain mainly the Simatic crust and are relatively thinner, while the continental plates contain Sialic material and are relatively thicker (Fig. 1.29).
Interaction of Plates and the Results:
These crustal plates are borne along a worldwide system of oceanic ridges, oceanic trenches, great faults and active fold belts. The plates migrate away from ridges and inevitably collide.
There are three ways in which the plates meet each other:
1. Divergent Edge:
Or the constructive edge can be seen along ridges. Here, the basaltic magma erupts and moves apart. Such edges are sites of earth crust formation (thus constructive) and volcanic earth-forms are common along such edges. For example, Surtsey Island (south of Iceland) was ‘born’ along mid-Atlantic ridge on November 14th, 1968. Even earthquakes are common along divergent edges.
2. Convergent Edge:
Or destructive edge is formed when two plates meet head on. The zone of collision may undergo crumpling and folding and folded mountains may emerge. This is an orogenic collision. Himalayan Boundary Fault is one such example [Fig. 1.30 (i)]. When one of the plates is an oceanic plate, it gets embedded in the softer asthenosphere of the continental plate and as a result, trenches are formed at the zone of subduction. The subducted material gets heated up and is thrown out forming volcanic islands [Fig. 1.30 (ii)]. The intrusive activity may result in the formation of batholiths.
Thus, a dynamic equilibrium is achieved as the material uplifted and folded at ridges and mountains is balanced by subduction of material at trenches. Most of the Pacific Ring of Fire is dotted with volcanic islands.
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3. Transcurrent Edge:
Or a conservative is formed when two plates move past each other without directly interacting with each other. San Andreas Fault along the western coast of USA is an example.
Thus, all landforms are influenced directly or indirectly by tectogenesis which provides energy for formation of landforms. Because of interplay of the factors discussed above, the plate margins are areas of intense volcanic and earthquake activity.
Evidence in Support of Plate Tectonics:
1. Older rocks form the continents while younger rocks are present on the ocean floor. On continents, rocks of upto 3.5 billion years old can be found while the oldest rock found on the ocean floor is not more than 75 million years old (western part of Pacific floor). As we move, towards ridges, still younger rocks appear. This joint to an effective spread of sea floor along oceanic ridges which are also the plate margins.
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2. The normal temperature gradient on the sea floor is 9.4°C/300 m but near the ridges it becomes higher, indicating an upwelling of magmatic material from the mantle.
3. In trenches, where subduction has taken place (convergent edge), the value of gravitational constant ‘g’ is less. This indicates a loss of material. For instance, gravity measurements around the Indonesian islands have indicated that large gravity anomalies are associated with the oceanic trench bordering Indonesia. The mass deficiency which caused this negative gravity anomaly can be explained by the lighter earth’s crust being pulled down into the mantle by sinking convection currents.
4. The orientation of iron grains on older rocks (for instance, investigations revealing such an evidence on the ocean floor south west of Iceland in the 1960s) shows an orientation which points to the existence of the South Pole, once upon a time, somewhere between the present-day Africa and Antarctica.
5. The fact that all plate boundary regions are areas of earthquake and volcanic disturbances goes to prove the theory of plate tectonics.
Significance of Plate Tectonics:
1. For the earth scientists, it is a fundamental principle for study. For physical geographers, this approach is an aid in interpretation of landforms.
2. New minerals are thrown up from the core with the magmatic eruptions. Economically valuable minerals like copper and uranium are found more frequently near the plate boundaries. Valuable mineral zone in one continent can thus be compared with its historical cousin continent.
3. On the basis of present knowledge of crustal plate movement, the shape of landmasses in future can be guessed. For instance, if the present trends continue, North and South American will separate. A piece of land will separate from the east coast of Africa. Australia will move closer to Asia. (See Fig. 1.31)
