Rocks become weathered in several ways.
Over the course of time, rocks naturally disintegrate and decompose through a process known as weathering. Weathering is the breakdown of exposed rock and soil on the earth’s surface through various means. There are two different types of weathering—chemical and physical. Physical weathering, also known as mechanical weathering, occurs in three different ways.
Frost Wedging and Thermal Expansion
Every day, rock on the earth’s surface expands and contracts due to heating and cooling. When rock is heated, it expands; upon cooling, it contracts. This constant expansion and contraction can cause the rock to break up and become weathered. Different types of minerals have different rates of expansion and contraction; where there are different minerals in the rock, there is likely to see more weathering.
Water from precipitation, such as rain or melting snow, can seep into cracks and chips in the rock. The water can freeze if the temperature is cold enough; once it freezes, it expands and causes rock to break apart. This expansion pushes the rock out of place, which can wear away and chip the rock. This stage of mechanical weathering occurs most often in areas where there are a large number of freeze-thaw cycles throughout the year. Mountainous areas and areas closer to the North and South Poles experience frost wedging, also known as ice wedging, more often than other areas. In areas with cliffs that experience frost wedging, the chips of rock break off and collect at the base, forming what is known as a talus slope.
Unloading, also known as exfoliation, is the second stage of mechanical weathering. Exfoliation is the process of rock breaking off into sheets or leaves parallel with the ground surface. The rocks can become cracked; when soil beneath the rock begins to erode or expand, it pushes the rock upwards. The rock then becomes stressed and can crack into large sheets known as exfoliation sheets.
Plants and animals can cause rock to crack and break into sheets. Plant roots can cause the rock to form cracks, known as joints, and the roots pushing up soil contribute to the rocks breaking up. Some plants can also produce chemical components that lead to the breakdown of rock. Animals contribute to the weathering of rock when they burrow into the soil. This burrowing helps push soil upward, which can damage the rock above. Animals also contribute to weathering by exposing covered rock.