Wednesday, October 28, 2009

Distribution of Continents and Oceans

About 71 per cent of the globe is water and about 29 per cent is land surface. The continents into which the land surface is broadly divided are: Asia, Africa, North America, South America, Europe, Australia, and Antarctica. If the seas were to drain away, it appears that broad areas lying close to continental shores are actually covered by shallow water less than 180 m deep. Disregarding the earth's curvature, the continents can be visualised as platform-like masses; the oceans as broad, flat-floored basins. Most of the land surface of the continents is less than 1 km above sea-level, while a predominant part of the ocean floor lies between 3 and 6 km below sea level.
Relief features of continents are being discussed here, while those of oceans will be dealt with in a separate chapter Ocean and Oceanography.
RELIEF FEATURES OF CONTINENTS
The continents consist of two fundamental kinds of geological units: (i) the shields, and (ii) the mountain belts or orogenic belts.
The shields are extensive, rigid and permanent part of the earth's crust, composed of Precambrian rocks (more than 590 million years old). These are the oldest rocks on the earth's surface. The mountains of earlier periods were eroded to their roots. The shields today are, therefore, largely plains and low plateaus and are highly stable parts of the earth's continental crust. Crustal movement of
1. Laurentian Shield 2. Brazilian Shield 3. Baltic Shield
4. African Shield
5. Arabian Shield
6. Siberian Shield 7. Deccan Shield
8. China Shield
9. Austra.lian Shield

shields in the later geologic time have been epeirogenic movements, i.e., rise and fall of the crust over broad areas without breaking or bending the rocks. Negative epeirogenic movement resulted in submerging of shields, shallow seas and continental shelves. Later, positive epeirogenic move­ment brought the sedimentary cover that had gathered over time above sea level where it has been carved up by streams into hills and plateaus. The Canadian Shield in North America and the Russian-Baltic Shield of Europe are well-known shields of the northern hemisphere. Similar shield areas occupy parts of Australia, Africa, South America, India and Antarctica. Rocks in these areas date back to the oldest geological time from one to three billion years ago.

Mountain belts of continents are along narrow a genic zones in which crust has been compressed and fon to buckle into tight folds and at the same time to
strongly elevated. The older, inactive mountain ranges he usually been reduced to hill belts. In some places, they i still moderately elevated possessing a rugged relief. 1 young ranges are still active zones of crustal deformati and volcanic activity in some areas. They rise as speetacu alpine mountains.

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