Weathering of Asia
 Changes in Land Surface. The earth's surface has not always been the same. It has been undergoing constant change.
In the course of ages various agents have been working at the relief of the surface, and what were seas yesterday have become land today. The gulfs of the seas are slowly filled with river silt, and lowlands take their place.
The mountains are undergoing change, too. Young mountains have tall, jagged ridges, steep slopes, sharp-pointed peaks They
become worn down, however, and their appearance is slowly changed. They become lower, their tops rounded, their slopes more gentle. This process continues until finally a hilly plain remains instead of the mountains. Thus, for example, a hilly upland now separates the West Siberian and Turan Lowlands, where formerly lofty peaks towered.
 What Weathering Is. What forces of nature destroy stony mountains and flatten the earth's surface? These forces are dependant on solar energy and are, therefore, called exterior forces, in contrast to in-terior forces that originate deep inside the earth and are revealed in the form of secular movements, earthquakes and volcanic eruptions.
In the day-time the earth's surface is warmed by the sun, at night it cools. All substances expand when heated and contract when cooled. The alternate expansion and contraction leads to the breaking-up of the rocks. There appear cracks in them, small at first, but gradually becoming wider and deeper. Rocks consisting of a number of components (granite, for example) are most easily broken, as each component expands and contracts in a different way. Water enters
the cracks in the rocks where it freezes and expands, so that the cracks are gradually widened and pieces of rock broken off. Water, besides, dissolves rocks. Rock salt, gypsum and limestone are most easily dissolved. In limestone districts water cuts ruts and grooves on the surface of the rock and, penetrating deeper and deeper, forms great caves.
Plants and animals play an important part in the destruction of rocks. There appear sorts of lichen rooted fast to the surface of the rocks. These are followed by moss and grass. Their roots produce an acid that eats away the rock.
A seed from a tree may fall into a crack. It begins to grow, and as the root thickens, it widens the crack.
Earth-worms, moles, gophers—all creatures that live in burrows— help to loosen the soil.
This process by which rocks are broken by changes of tempera-lure and under the influence of water or plant and animal life is called weathering. This is a very gradual process, but in the course of many thousands of years the most solid rocks split and break into pieces.  Such fragments tumble down the slopes and collect together to form piles of loose rock, known as screes. These gradually fill the mountain valleys. Thus, the lowering of the mountain tops and the rising of the valley bottoms occur at one and the same time, the surface being gradually levelled. Such was the case with the Pamir and Tibet Plateaus, for instance.
Not all rocks undergo the same changes. The more solid rocks often take on the forms of pillars, towers, mushrooms, etc.
 Weathering is going on everywhere, but it tells most on tall mountains where the temperature changes are most extreme. When fragments of rock tumble down the slopes, the weathering of the exposed cliffs continues with the same force.
Weathering is very noticeable in the desert. Under the clear and cloudless sky the range of temperature is unusually great. As a result of weathering, deserts are covered with stones, broken rock and sand.
Questions and Assignments.
- Make the following experiment: heat a piece of granite, then
cool it quickly in cold water and watch the results.
- Fill a bottle with water, cork it tightly and let it stand in the
frost. What happens to the bottle? Explain this phenomenon.
- Find traces of weathering on a big stone or cliff in your neigh
bourhood (if it is mountainous).
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