Rocks found on the surface of the Earth undergo a process over time call weathering. Weathering is the breaking down of rock material.
What is a Rock?
A rock is a solid aggregate of mineral materials. Rocks are categorized into three main groups based on chemical composition and how they are formed: igneous rocks, metamorphic rocks, and sedimentary rocks.
Some examples of common rocks are granite, basalt, limestone, and sandstone.
What are the Two Main Types of Rock Weathering?
There are two main types of weathering: physical and chemical.
Physical, or mechanical, weathering happens when rock is broken through the force of another substance on the rock such as ice, running water, wind, rapid heating/cooling, or plant growth.
Chemical weathering occurs when reactions between rock and another substance dissolve the rock, causing parts of it to fall away.
Here are some examples of physical and chemical weathering of rocks.
Physical Weather Examples
Physical weathering occurs when rock is broken down through mechanical processes such as wind, water, gravity, freeze-thaw cycles, or the growth of roots into rock.
Water movement is a major force in physical weathering. The persistent crash of waves against rocks causes physical weathering.
When water seeps into rocks and freezes, it expands and causes the rock to crack. When water transforms from a liquid state to a frozen state, it expands. Liquid water seeps into existing cracks in the rock, freezes and then expands those cracks.
This type of physical weathering is called freeze-thaw.
These mushroom-shaped rock pinnacles in Bryce Canyon, known as hoodoos, are formed by wind weathering the sandstone.
As rocks heat up (and expand) and then cool (and contract) they can weaken over time and break up into smaller pieces. This temperature related weathering is known as thermal stress.
For example, hot days can trigger rockfalls on Yosemite’s granite cliffs.
Roots from plants grow into rocks, cracking the rocks and causing weathering. Roots typically will expand into existing cracks and cause them to widen.
Chemical Weathering Examples
Chemical weathering can been seen in this photo of Blue Basin located in the John Day National Monument in Oregon.
The green color of the claystone is from by chemical weathering of a mineral called celadonite.
Chemical and Physical Weathering
Honeycomb weathering is a type of weathering that is believed to have both physical and chemical weathering components.
Salt weathering is where expanding salt crystals break fragments of rock that create an increasingly larger hole over time. The pattern that results is known as honeycomb weathering.
This rock in Puget Sound, Washington is an example of honeycomb weathering of sandstone.
The honeycomb weathering seen in this photo from Utah.