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.
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.
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). TemperatureOver 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. SaltIn 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.
Plants and AnimalsAs 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. Chemical WeatheringCarbon DioxideChemical weathering differs from physical weathering because it doesn’t just weaken and break apart rock, it also changes the chemical makeup of rocks. For example, carbon dioxide and water combine to create carbonic acid, which can dissolve rocks like limestone. Carbonic acid is responsible for the incredible networks of caves in the Carlsbad Caverns National Park in the American Southwest. The largest cave, the Big Room, would be called ‘big’ if it was the size of one football field, but it is the size of six football fields. KarstAnother type of chemical weathering occurs when limestone dissolves to create formations called karst. Water slowly erodes the chalky limestone, creating elaborately weathered columns, caves, sinkholes, and even underground rivers.
Rust and Acid RainTwo other agents of chemical weathering are rust and acid rain. Rust acts on rocks that contain iron through the process of oxidation. Rust weakens rocks and eventually they break. Acid rain occurs when chemicals released into the air from the burning of fossil fuels, like coal, oil, and natural gas, and blends with precipitation and sunlight. Acid rain weathers all kinds of stone, including important older buildings, monuments, and even gravestones. Learning OutcomesFollowing this lesson, you should have the ability to:
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