Mechanical weathering is the breakdown of rocks into sediments through physical means. For example, weathering can carve out potholes in our streets, break down rocks to form soils and even tear down mountains.
The major types of mechanical weathering processes are as follows:
- Frost wedging
- Biological activity
Unlike chemical weathering, mechanical weathering does not alter the chemical composition of the rock. Typically, both chemical and mechanical weathering simultaneously play a role in breaking rocks down into smaller sediments.
1 Frost wedging and freeze-thaw cycles
Why do streets in colder climates have more potholes than warmer climates? This is due to the unique property of water contracting and expanding from frost.
As water turns into a liquid, it freely enter existing cracks within the rock itself. When temperature freezes, water stretches out and disintegrates the rock.
This is why some streets in northern climates looks like someone laid the hammer down. Not only does frost wedging tear apart our streets, but it affects mountains too.
The repeated freezing and thawing of water provides enough mechanical force to disintegrate mountain rocks. But this frost wedging is much more common for mountains that experience varying temperatures and significant moisture.
2 Temperature change and exfoliation
Thermal expansion can cause mechanical fracturing in rocks. Because of repeated expansion and contraction, this type of mechanical weathering can peel away at rocks.
Frequent temperature change from day-night cycles causes rock exfoliation. But if you have rapid temperature change say from a forest fire, this can break apart rock more abruptly.
Fractures from exfoliation can range in millimeters or meters in size. From the initial stress of the rock to the gradual crack, this mechanical weathering varies based on the composition of rocks.
For example, igneous rocks like granite are stronger and more resistant to mechanical weathering than sedimentary rocks. Basalt weathers quickly with water. And one of fastest weathering rocks is limestone.
3 Biological weathering and terrain abrasion
There are several ways how biological activity can exert mechanical weathering. Although they can exhibit chemical weathering, they can be mechanical in nature too.
For example, trees anchor their roots into the ground for stability. The main purpose is to absorb water. But at the same time, these roots alter the soil and puts pressure on rocks.
It’s not only plants that cause biological weathering, but burrowing animals can disturb soils and rocks. They can expose and pry away at rocks to the surface physically altering the environment.
Lastly, in steep areas, abrasion, landslides and falling rock are another form of mechanical weathering. Because the existing landscape, instabilities in terrain can pick away at rocks.
Mechanical weathering is common in deserts and tundras
Rates of mechanical weathering varies geographically. For example, deserts and tundra climates tend to experience more mechanical weathering then drier areas.
Weathering is also affected by temperature, water and composition. Out of all factors, the presence of water is the most important.
Not only is water key for chemical weathering, but it’s important for mechanical weathering too. This is why moist regions weather quicker.
Temperature causes rock to expand and contract. But with the presence of water, extreme day and night temperatures can cause rock to shatter quicker.