Every single rock on the surface of the Earth-whether it is gravestone, a piece of solidified lava from a volcano or a boulder that has broken from a cliff or mountain-is slowly being broken down. This breaking down of rocks at or near the Earth's surface is called weathering. The word is used because the weather is mainly responsible. Weathering turns solid rock into soft materials that may eventually form soil.

    Air and water are the cause of most weathering. Sometimes they change the chemical in the rocks, and sometimes they just break apart the rock physically. If water seeps into the cracks in the rocks, for example, it may later freeze if the temperature falls below 0 degrees Celsius. As the water turns to ice, it expands, pushing against the sides of the rock with a pressure believe to be as much as 2,100 kilograms per square centimeter. This forces the cracks open. Repeated freezing and thawing causes the fragments to break away from the original rock, these may slide down a cliff or mountain and form a sloping mass of fragments at the bottom, called scree.

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