Monday, November 29, 2010

The Face of Earth

The Earth’s surface is the scene of a constant battle between the upward forces of mountain building and the erosional forces of wind, water and waves, aided and abetted by gravity. In the midst of all this, Human too have made their mark, in their attempts to hold back the sea, concrete over the surface and reclaim land. By removing vegetation, humans aid erosion rather than prevent it.

The features of landscape can be classified according to the predominant forces at work and the timescale during which they have been at work. In Andes and Himalayas, mountain building still dominates over erosion: parts of Scotland and Canada where once the mountains were higher are now eroded into old age. The form erosion takes depends very much on the climate. Where temperatures frequently drop below freezing, ice can act like a wedge, chiseling great boulders from the mountains.

 Glaciers grind out broad valleys and transport the debris far away. Rivers cut into the hill sides and wash millions of tons of rock and soil away, depositing them on wide flood plains, in deltas and in deep sedimentary basins out to the sea. Wind scours deserts with blown sand and spreads dunes far and wide. Eventually all this material gets pressed into rock and pushed back into mountains.