Mechanical weathering refers to physical processes that break down the structure of rocks. It differs from chemical weathering, by which rocks are broken down by reactions among chemicals inside and outside the rock. You can observe mechanical weathering's effects nearly anywhere: In addition to producing some of the most impressive rock formations on Earth, mechanical weathering is responsible for the cracked and smoothed rocks you see in your daily life.
Frost and Salt Wedging
One of the most common forms of mechanical weathering is frost wedging. This occurs when water gets into the small holes and gaps in rocks. If the water in the gap freezes, it expands, splitting the existing gaps into wider cracks. When the water thaws, the wider gaps allow even more water to enter the rock and freeze. Frost wedging can repeat over months or years, turning microscopic gaps in the rock into large cracks.
Salt wedging is also caused by water intruding into rocks. When water containing salt evaporates from within a gap in a rock, the salt is left behind. Over time, salt builds up, creating pressure that will split the gaps in the rock.
Unloading and Exfoliation
Many rocks form deep beneath the surface of the Earth under conditions of intense pressure; hundreds of tons of rock or ice often press down on them. If the rocks above these rocks erode, or the ice above them melts, the release of this weight causes the rock to expand upward and crack at its top. The release of weight is known as unloading. When a rock expands and cracks this way, the top of the rock may split into sheets that slide off the exposed rock. This process is called exfoliation.
Water and Wind Abrasion
Abrasion occurs when the surface of rocks is exposed to water or wind. These elements can carry tiny particles of sediment or rock that then collide against the rock's surface. When these particles rub against the rock's surface, they break off tiny pieces of the rock. Over time, abrasion can wear down and smooth extremely large sections of rock.
Impact and Collision
Mechanical weathering can also be caused by more dramatic and sudden physical processes. In a landslide or avalanche, falling matter will dent or shatter rocks below. Falling rocks will break by colliding with rocks below or will be smoothed by rolling against other rocks in a process similar to abrasion.
Interactions With Organisms
Rocks can also be physically weathered by their interactions with organisms. If you've ever seen a sidewalk that has buckled because of a tree root, you've seen this process in action. Roots can grow into small spaces and cracks in rock; when they expand, it exerts pressure on the rock around them and widens the cracks.
Animals also contribute to mechanical weathering. Digging animals such as moles can break apart rocks underground, while the movement of animals on surface rock can scratch the rock's surface or exert pressure that causes the rock to crack.