Weathering describes the breaking down or dissolving of rocks and minerals on the surface of the Earth. Water, ice, acids, salts, plants, animals, and changes in temperature are all agents of weathering.
Mechanical weathering is the process through which large rocks are broken into increasingly smaller pieces. Sometimes referred to as physical weathering, the process normally happens near the Earth’s surface. Can you believe that the tiny sand grains you see at the beach were once part of massive rocks?
It involves mechanical processes that disintegrate a rock like tree roots growing in cracks in a rock and eventually breaking it up. Mechanical weathering doesn’t change the chemical nature of the rocks.
What Is Mechanical Weathering?
Rocks break due to various reasons like wind, ice, weather, water, acids, and chemical reactions. Even if an external force like the growing of plants takes place on rocks, the roots lead to weathering. Let’s see how mechanical weathering takes place.
Mechanical weathering is also known as physical weathering. In this type of weathering, a large rock is disintegrated into smaller pieces of rock. When rocks disintegrate or break up without experiencing any change in their chemical composition, it is known as mechanical weathering.
Thermal expansion and contraction happen due to the increase or decrease in temperature. This process causes the rock to break into fragments.
Some examples of mechanical weathering are exfoliation, water and salt crystal expansion, thermal expansion, abrasion by wind and water erosion, and even some types of actions by living things (like plant roots or a burrowing mole).
Examples Of Mechanical Weathering
There are two main types of mechanical weathering:
- Freeze-thaw weathering or Frost Wedging
- Exfoliation weathering or Unloading
- Thermal Expansion
- Abrasion and Impact
- Salt weathering or Haloclasty
Let us see in detail about each type of weathering.
Freeze-thaw weathering or Frost Wedging
Frost Wedging occurs when liquid water can seep into cracks and crevices in rock. If temperatures drop low enough, the water will freeze. When water freezes, it expands. The ice then works as a wedge. It slowly widens the cracks and splits the rock.
When ice melts, liquid water performs the act of erosion by carrying away the tiny rock fragments lost in the split.
The freeze-thaw cycle happens repeatedly and finally breaks the rock, and hence it is called Freeze-thaw weathering.
Exfoliation Weathering or Unloading
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.
Unloading occurs when the overlying weight releases. 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.
Thermal Expansion
Minerals normally expand and contract due to temperature fluctuations. This process is called thermal expansion. Rocks are composed of various minerals, which expand and contract at different rates when subjected to rapid temperature changes.
The fluctuations cause stress and small cracks in the rocks, gradually breaking down the rock. Grus is a classic example of the thermal expansion process. It is the coarse-grained and loose fragments deposit that remains behind after weathering. So, Grus is the direct result of the physical weakening and disintegration of rock over time.
Abrasion and Impact
Rocks can be broken up by friction and continuous impact with other rock pieces during transportation. Namely, a rock fragment carried along in the raging currents of a river continuously rubs itself against other fragments and the river bed.
In the end, the fragment disintegrates into small pieces. This type of mechanical weathering also occurs during wind and glacial ice transportation.
Salt weathering or Haloclasty
When saltwater seeps into rocks and then evaporates on a hot sunny day, salt crystals grow within cracks and pores in the rock. The growth of these crystals exerts pressure on the rock and can push grains apart, causing the rock to weaken and break.
There are many examples of this on the rocky shorelines of Vancouver Island and the Gulf Islands, where sandstone outcrops are common and salty seawater is readily available. Salt weathering can also occur away from the coast because most environments have some salt in them.
Interactions With Organisms
Interactions with organisms also cause physical weathering. If you’ve ever seen a sidewalk that has buckled because of a tree root, you’ve seen this process in action. Roots grow into small spaces and cracks in rock; when they expand, they exert pressure on the rock around them and widen the cracks.
On a smaller scale, lichens send tiny tendrils into the spaces between rock minerals, loosening and eventually separating the particles from the main body of the rock.
Animals also contribute to mechanical weathering. Digging animals such as moles 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.