What is Mechanical 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.
In this article, we look at how mechanical weathering works, its types, and some examples. Let’s dive in.
Process of Mechanical Weathering
The main process in mechanical weathering is abrasion, a physical process by which rocks and clasts are reduced in size. Abrasion by ice, water, and wind processes loaded with sediments can have immense cutting power. The world’s greatest gorges, valleys, and ravines are largely a result of abrasion.
In glacial regions, massive masses of moving ice embedded with soil and rock particles grind down rocks in their path, carrying away large volumes of material. Tree roots sometimes penetrate cracks in rocks and wedge them apart, leading to disintegration.
Temperature fluctuations from daytime to nighttime may cause a rock to expand and contract. This weakens the rock, causing it to fracture and eventually disintegrate. To have a deeper understanding of these processes, let’s explore the different types of mechanical weathering.
Types of Mechanical Weathering
There are five major types of mechanical weathering: thermal expansion, frost weathering, exfoliation, abrasion, and salt crystal growth. Shall we have a detailed look at them?
1. 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.
2. 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.
3. Exfoliation or Pressure Release
When the overlying boulders are stripped by abrasion and other erosion mechanisms, they cause the underlying rocks parallel to the surface to crack, fracture, and expand. Consequently, the underlying rocks release the pressure in them.
As time passes, sheets of rock peel away from the stripped rocks and disintegrate into smaller fragments along the fractures. The process is known as exfoliation. It occurs when rocks parallel to the land surface break up as a result of the pressure release during abrasion, rock uplifts, or retreat of an overlying glacier.
4. Frost Weathering
Frost weathering is also known as ice wedging. It is the collective term for various processes that involve ice. The processes include:
- Frost Wedging
- Frost shattering
Frost weathering occurs largely in mountain areas where the temperatures are close to the freezing point of water. Various frost-prone soils freeze and expand as water migrates through capillary action to develop ice lenses near the freezing point.
This process also takes place within pore spaces of rocks. The accumulations of ice increasingly grow as they draw liquid water from the surrounding pores. The continuous growth of ice crystal weakens the rocks, eventually breaking them up.
Frost weathering is common in environments where there is plenty of moisture, and temperatures often fluctuate below and above freezing point. Alpine and periglacial areas are especially prone to this type of weathering. And chalk is one of the rocks that are susceptible to frost weathering.
It has many pore spaces for ice crystals growth. The formation of tors in Dartmoor is a typical example of frost action on chalk. Once the water that has found its way into the joints freezes, the ice created strains the walls of the joints. This results in the deepening and widening of the joints.
Water could flow further into the rock when the ice thaws. Recurring freeze-thaw cycles makes the rock weak over time, eventually breaking it up along the joints into angular fragments. The fragments accumulate at the foot of the slope to form a scree slope or talus slope. The fragments take different shapes depending on the structure of the rock.
5. Salt-crystal Growth
Salt-crystal growth is also known as haloclasty or salt weathering. It is the process by which saline solutions enter the cracks in a rock and evaporates, leaving behind salt crystals.
When environmental temperatures rise, the accumulated crystals are heated up. As a result, they expand and release pressure on the rock, causing it to break up.
Salt-crystal growth may also occur when rocks like limestone form salt solutions such as sodium sulfate or sodium carbonate. These solutions form crystals when the water molecules in them evaporate. The salts can expand over three times. This process is common in dry and high-temperature regions.
6. Plant and Animal Activities
Plants roots are very powerful and can grow into the cracks of existing rocks. As the roots continue to grow in the cracks, they act as a wedge, exerting pressure on the rock until it cracks further and eventually disintegrates into smaller fragments.
Animals such as rabbits, moles, and groundhogs, on the other hand, burrow holes in the ground, which can expose underlying rocks to the elements of weathering. Water and other agents of mechanical weathering find their way into the previously covered rocks, thanks to the holes. It then starts and accelerates the mechanical weathering process.
Examples of Mechanical Weathering
Real world examples of mechanical weathering are easy to spot. They include the following:
- The inclined Talus slope near Lost river in Virginia is a great example of Frost Weathering
- Bornhardts are products of exfoliation. They are tall, domed, isolated rocks normally found in tropic areas. Sugarloaf Mountain in Rio de Janeiro, Brazil, is perhaps the best example of a bornhardt.
- Underlying tree roots strip out whole slabs of a sidewalk
- Old gravestones are normally hard to read since weathering has washed away the letters
- Ice wedges are a leading cause of potholes in roads and streets. When ice forms in the cracks of a road, the water expands, pushing against the surrounding rock. This makes the cracks wider, eventually disintegrating the rock.
- Forest and range fires can result in weathering in rocks that are located along the ground surface.
- Water flowing in a stream into a rock will eventually bore a hole in the rock.
- Thermal stress weathering normally occurs in desert climates, which are hot during the day and cold at night. The daily heating and cooling processes put stress onto rocks in the outer layer, making them start peeling off in thin sheets.
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