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Mechanical Weathering (Physical Weathering)

Mechanical weathering takes place when rocks are broken down without any change in the chemical nature of the rocks. The rocks are essentially torn apart by physical force, rather than by chemical breakdown.

Mechanical Weathering

The forces that break rocks down can be numerous, and include such things as pent up energy as the Earth’s crust slowly moves. When great amounts of pressure build up, the resulting mechanical effect can be that very large joints, or faults are created.

The most common type of mechanical weathering is the constant freezing, and thawing of water. In liquid form, water is able to penetrate the many holes, joints, and fissures within a rock. As the temperature drops below 32 ° F, this water freezes. As water freezes, it expands, becoming about 10% larger than it was in liquid form. The result is that the holes and cracks in rocks are pushed outward. Even the strongest rocks are no match for this force.

As the water thaws, it is then able to penetrate further into the widened space, where it later freezes yet again. The expansion of holes and cracks is very slow. However, water does not mind. It is very patient. Month after month, year after year, water freezes and thaws over and over, creating larger and larger holes and cracks in the rocks.

Another important type of mechanical weathering is salt wedging. As water enters the holes and cracks in the surface of rocks, it often carries salt with it. As the water later evaporates, the salt is left behind. Over time, these salt deposits build up, creating pressure that can cause rocks to split and weaken.

Temperature changes also effect mechanical weathering. As temperatures heat up, the rocks themselves expand somewhat. As the temperatures cool down, rocks contract slightly. The effect can be the weakening of the rock itself.