1. Briefly outline the main elements that constitude the theory of plate tectonics. 9mks

      b. How this theory helps us to understand the origin of;

      i. Mid-oceanic Ridges and 

      ii. submarine or deep sea trenches.    CAMEROON GCE MOCK, MARCH 2012.

   

Describe and explain the origin of

a. Mid-oceanic ridges and 

b. Deep see trenches as a major landforms resulting from the theory of  plate tectonics. CAMEROON GCE JUNE 2006.

        ANSWER

  Plate tectonics is the the bending and breaking of segments of the crust under gravitational forces. this crustal segments or spherical caps are about 100km thick. Plate tectonics also involve mountain building. two main ideas are contained in the theory of plate tectonics that is the GEOMETRIC AND THE KENETIC PRINCIPLE.

  The geometric concept or principle explains the whole lithosphere is fragmented into a series of rigid blocks, firm segments called plates. These plates are of different geometric shapes and sizes. Some of tthe plates are semi-circular, some triangular, some roughly rectangular. Most of the plates are partly land or continental crust (SIAL) or partly ocean or simatic crust (SIMA). Similarly, the earth's outer crust resembles an egg shell ctracked in several places.

  There are seven major plates of continental proportion, six of which are partly land and one is principally made up of the ocean crust namely: (Major plates). THE AFRICAN PLATE, INDO-AUSTRALIAN PLATE, THE ANTARCTICA PLATE, THE EURASIAN PLATES, THE NORTH AMERICAN PLATE, THE SOUTH AMERICAN PLATE AND THE PACIFIC PLATE which is completely oceanic or simatic.

   The minor plates are the Nazca plate, coco's plate, arabian plate, Bismark plate, the Aegean plate, the Iranian, Phillipine plate, Juan de fuca. The plate ride or move gradually on a weak sorf gradually melted upper mantle known as the asthenosphere. They interlock with each other like jigsaw pieces causing great intabilities at their margins (plate margins).

   The kinetic or kinetic concept explains plate motion (movement). This means that plates possess energy especially at their plates margins. These plates are in constant relative motion floating and moving on the soft upper most zone of the mantle known as the asthenosphere. Plate's movements are averagely 5-13 cm per year. There are main types of plate margins depending on the maner of movements that is; Divergent plate margins or spreading ridges, convergent plate margins or subduction zones, collision plate margins, conservating plate boundary or neutral zones.