Physical and Chemical Weathering of Rocks

| |

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.

An array of rocks with vegetation interspersed. Photo: John J. Mosesso, USGS. Public domain.
An array of rocks with vegetation interspersed. Photo: John J. Mosesso, USGS. Public domain.

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 Weathering

Water movement is a major force in physical weathering. The persistent crash of waves against rocks causes physical weathering.

Physical weathering by waves at Arcadia National Park in Maine.  Photo: John J. Mosesso, USGS. Public domain.
Physical weathering by waves at Arcadia National Park in Maine. Photo: John J. Mosesso, USGS. Public domain.

Freeze-thaw 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.

Horsetail Falls, Columbia River Gorge.  Photo: USDA, public domain
Horsetail Falls, Columbia River Gorge. Photo: USDA, public domain

Wind Weathering

These mushroom-shaped rock pinnacles in Bryce Canyon, known as hoodoos, are formed by wind weathering the sandstone.

Bryce Canyon is a unique sandstone formation in southern Utah. It is home to a large number of hoodoos, which are oddly shaped pillars of rock that formed due to different erosion rates for the dolomite that caps them and the sandstone that forms their base. Photo: Alex Demas, USGS. Public domain.
Bryce Canyon is a unique sandstone formation in southern Utah. It is home to a large number of hoodoos, which are oddly shaped pillars of rock that formed due to different erosion rates for the dolomite that caps them and the sandstone that forms their base. Photo: Alex Demas, USGS. Public domain.

Thermal Stress

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.

A large pinnacle may be seen adjacent to Yosemite Falls. Pinnacles form as the granite is weathered away via ice, water, or plant growth. Photo: Alex Demas, USGS. Public domain.
A large pinnacle may be seen adjacent to Yosemite Falls. Pinnacles form as the granite is weathered away via ice, water, or plant growth. Photo: Alex Demas, USGS. Public domain.

Root Weathering

Roots from plants grow into rocks, cracking the rocks and causing weathering. Roots typically will expand into existing cracks and cause them to widen.

Precambrian metamorphic rock wall in Lamar Canyon. Yellowstone National Park, Photo: Jim Peaco, NPS, public domain
Precambrian metamorphic rock wall in Lamar Canyon. Yellowstone National Park, Photo: Jim Peaco, NPS, public domain

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.

Blue Basin, Oregon Bonnie Moreland, public domain
Blue Basin, Oregon. Photo: Bonnie Moreland, public domain

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.

Honeycomb weathering of sandstone located on the shores of Puget Sound.  Photo: Collin Smith , U.S. Geological Survey. Public domain.
Honeycomb weathering of sandstone located on the shores of Puget Sound. Photo: Collin Smith , U.S. Geological Survey. Public domain.

The honeycomb weathering seen in this photo from Utah.

Honeycombs, Utah Utah Geological Survey, Public Domain
Honeycombs, Utah. Photo: Utah Geological Survey, Public Domain

Related

Share:


Enter your email to receive the Geography Realm newsletter: