Soil formation occurs over a period of thousands of years. It is a result of the continuous breaking down of rocks into smaller pieces by the forces of nature. The process by which rocks on the Earth's surface disintegrate due to the action of natural forces such as strong winds, flowing water, ice, roots of plants and so on is called weathering. Extreme differences in temperatures cause rocks to expand when the weather is hot and contract when it is very cold. Alternate expansion and contraction weakens the rocks, causing them to disintegrate into small pieces. In cold weather, water flowing through the cracks of rocks freeze and expand to form ice. The ice also causes rocks to crack and break down.