Weathering: Definition & Explanation

Instructor: Maria Howard

Maria is a teacher and a learning specialist and has master's degrees in literature and education.

Discover how rocks and minerals are broken down by chemical and mechanical forms of weathering. Learn about some of the many geological formations created by weathering, from large caves to beautiful arches made of rock.

From Boulders to Arches

Arches National Park in the U.S. state of Utah is home to a set of geological marvels called arches. Arches are naturally-occurring rock formations, like the upside down letter U. The Landscape Arch, at 93 meters across (306 feet), is the largest in the world and, like other arches, seems to defy gravity.

The Landscape Arch is one of over 2,000 arches in Arches National Park in Utah.
Photo of the Landscape Arch

Obviously, large rocks don't just immediately spring up as arches, but, like sculptures, are chipped away and refined over a long period of time until something unique and beautiful is formed.

What Is Weathering?

Instead of using a chisel like a sculptor, rocks and minerals are broken down in many different ways through a process called weathering. From water and wind to plants and animals, the earth puts all types of rock through natural wear and tear. The pieces of broken rock are then carried away by erosion and are deposited in rivers, lakes, and oceans.

Weathering can be both mechanical and chemical, and they both often work together at the same time. Mechanical weathering causes physical changes (the beautiful arches of Arches National Park), while chemical weathering changes the chemical makeup of the material being weathered (the rusting of iron through oxidation).

Mechanical Weathering: Let's Get Physical


Mechanical weathering wears away at rock through physical forces, causing it to crumble and break apart. The Grand Canyon was created by mechanical weathering (and its pal erosion), as water from the Colorado River pushed past the rocky surface of the canyon for millions of years, making a deeper and deeper V-shape. At its deepest, the canyon is 1.6 kilometers deep (1 mile), where you can stack three Empire State Buildings on top of each other and still have room left over.

Many forces created the Grand Canyon, but the Grand Canyon was primarily formed by the water of the Colorado River.
Photo of the Grand Canyon

Water is powerful stuff. It drips down between the small cracks in rocks, and freezing temperatures cause it to expand and deepen the cracks. Saltwater does double duty, with both the water and the salt crystals working to break up rocks. Some types of rock, like clay, absorb water, causing them to crumble from the moisture (think of the way coffee can soften even the hardest of cookies).


Over time, changes in temperature cause rock to repeatedly expand (grow bigger) and contract (grow smaller), which weakens them and leads to breakage. Deserts, in particular, have very high temperatures during the day and very low temperatures at night. Sculpted by other elements like wind, deserts are the site of the world's most spectacular geological formations, like the Landscape Arch in Utah.


In addition to being in a desert, Arches National Park is also located on an underground bed of salt, left over from when the ocean reached the region 300 million years before. Salt is a less stable foundation than other rocks and is more easily weathered by wind, water, and changing temperatures. Besides its famous arches, the U.S. state of Utah is also known for spires of rock called hoodoos.

Hoodoos at Bryce National Park, also in Utah
Photo of hoodoos

Plants and Animals

As animals and plants are hard at work living and growing, they also break and weaken the rocks around them. Moles, rabbits, and other animals burrow holes and tread heavily over the ground, and if you've ever seen tree roots grow around large boulders, you know that plant roots are powerful enough to break through even very large rocks. So, too, can smaller plants like moss weather rock on a smaller scale.

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