By Trista L. Pollard /
1The rocks that you step on in your backyard and in front of your house today were formed deep inside our planet. That's right; these unimportant pebbles traveled a long way through extreme temperature and pressure before they reached your backyard. Now that they are on the Earth's surface, these uplifted rocks will be exposed to the gases and water in the atmosphere. These environmental agents change their appearance and composition. Weathering is the process where rocks are changed physically or chemically. Mechanical weathering changes only the physical structure of rocks. Chemical weathering, however, changes only the mineral composition of rock.
2Plants, animals, ice, gravity, running water, and wind are responsible for the mechanical weathering of rocks. As rocks are changed physically, this may also affect the process of mechanical weathering on underlying rock. When rocks are weathered mechanically, their overlying layer is slowly removed, decreasing the pressure on the underlying rock that forms deep beneath the surface. Once example of this process occurs with granite. As the pressure decreases on granite, it expands forming long curved cracks called joints. After these joints have developed on the surface of the granite, the rock breaks into curved sheets. These curved sheets exfoliate or peel away from the underlying rock. One area where you can see granite exfoliation at work is Yosemite National Park.
3Mechanical weathering occurs in three major forms. Ice wedging usually occurs in cold climates where water that seeps into rocks freezes. The freezing water increases in volume, by about 10% causing an increase in pressure on the surrounding rock. When the ice thaws and refreezes, the cracks within the rock become wider and deeper. Over time, ice wedging causes the rock to split or break apart. Rocks that are found in higher elevations also experience ice wedging. These are the areas, along with colder climates, where the temperature regularly rises above and falls below freezing. If you visit the northeastern United States, try to look for evidence of ice wedging.
4As rocks and soil are moved by running water, gravity, and wind, they collide with other rock materials. This causes the rocks to break into small pieces. The process known as abrasion happens daily. It occurs as gravity sends loose soil and rocks down the slopes of hills or mountains. The running water in rivers, streams, and oceans drags particles of sand and rocks along causing them to collide with each other and stationary rocks. The moving and stationary rock materials become weathered over time. The last agent of abrasion is wind. Small particles of soil, rock, and sand are constantly lifted and carried by wind. As they are carried, the wind hurls these particles against other rock surfaces. The wind-blown particles' abrasive force is enough to wear away the surfaces of rocks. Although this process occurs over time, abrasion is a powerful weathering force.
5Plants, animals, and humans also cause mechanical weathering. Every time people walk across rock paths and surfaces, the rock is slowly worn away. The roots of plants and trees create pressure on rocks as they grow and expand. This causes small cracks to form in the rock that will increase in size and eventually break the rock apart. Burrowing animals cause weathering as they move soil around when building their homes. Ground squirrels, prairie dogs, ants, and even earthworms expose new rock surfaces as they move soil. These new rock surfaces are now more susceptible to other agents of mechanical weathering. Over long periods of time, these types of organic activity can change the physical appearance of rocks dramatically. It also makes the rocks more vulnerable to chemical weathering.
6During chemical weathering, the mineral composition of rocks is changed due to its chemical interaction with the environment. This is also known as decomposition. As rocks come into contact with water, carbon dioxide, oxygen, and acids, chemical reactions act on the rocks' minerals. The hydronium ions in water or H3O+ are the acids. Hydroxide ions or OH- are the bases in water. Once the minerals react with either the acid or the base, the structure of the minerals changes and new minerals may form. A rock that has gone through chemical weathering not only has its composition changed, but its physical appearance also changes. Rocks that have iron-bearing minerals are usually weathered through oxidation. Oxidation occurs when elements like iron combine rapidly with oxygen, O2, which was dissolved in water. A new material called iron oxide, Fe2O3, or rust is formed. Soil and rocks that have a reddish color contain iron oxide that was produced through oxidation. Reddish soil is usually found in the southwestern United States.
7Water is the key ingredient in the chemical weathering of soil and rocks. One type of chemical weathering called hydrolysis causes a rock's mineral composition to change when it reacts with water. Feldspar is changed into common clay called kaolin when it reacts with water. The potassium and calcium atoms in feldspar crystals react with the hydronium ions in water. The clay is produced once the ions displace the atoms in the crystal. The minerals in rocks and soil that go through hydrolysis usually dissolve in the water. The minerals then go through the process of leaching. Leaching occurs when the dissolved minerals are carried by water to lower layers of rock. Once these minerals settle in layers beneath the Earth's surface, new deposits may form.
8Carbonic acid, H2CO3, a weak acid, is produced when carbon dioxide, CO2, from the air dissolves in water. When this carbonic acid combines with certain rock minerals, new minerals are formed called carbonates. The process that produces these new minerals is called carbonization. The hydrolysis process occurs at a faster rate due to the higher concentration of hydronium ions in the carbonic acid. As a result, the minerals are changed into carbonates that dissolve quickly in water. Calcite, which is a major mineral found in limestone, is converted into calcium bicarbonate when it reacts with carbonic acid. This new substance dissolves easily in water and speeds up the weathering process of limestone.
9Other acids found naturally in living organisms also contribute to chemical weathering. Mosses and lichens produce weak organic acids. As they grow on rock surfaces, these weak acids erode the rock. Cracks in the rock are produced when the acid seeps into the rock. Eventually, the rocks break apart.
10Acid precipitation is another form of chemical weathering. The small amount of carbon dioxide in the air causes rain to be slightly acidic. However, with the increased burning of fossil fuels like coal, higher amounts of nitrogen oxide and sulfur dioxide are sent into the air. As the compounds combine with water in our atmosphere, nitric acid, nitrous acid, or sulfuric acid are produced. Rain combines with these acids and falls as acid precipitation. Rocks, especially those materials used to build historic monuments and sculptures, are weathered more quickly. Years of acid precipitation has caused damage to these monuments and sculptures. In the United States, acid precipitation was a huge problem between 1940 and 1990. As a result, the Acid Rain Control Program was added to the Clean Air Act of 1970 in 1990. This program included regulations that required power plants to decrease their sulfur dioxide emissions within ten years. Power plants responded by installing scrubbers that remove a majority of sulfur dioxide before it is released into the atmosphere. Due to these scrubbers and government regulation, the amount of acid precipitation from our atmosphere has been reduced.
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The Process of Weathering
1. / Effect: The addition of the Acid Rain Control Program to the Clean Air Act of 1970 in 1990. What is the cause?/ 2. / You are studying soil samples from a particular area when you notice a reddish color present in the soil. Explain the weathering process that contributed to the soil's change in color.
3. / There is an increased population of ground squirrels in your town's park. How will this event affect mechanical weathering in the area?
/ 4. / Hydrolysis causes ______when they react with water.
Minerals to dissolve and move into lower layers of soil
Minerals to combine with carbonic acid to form carbonates
Minerals to combine with weak organic acids and slowly wear away rock
None of the above
5. / What are the common agents of mechanical weathering?
/ 6. / True or False: Ice wedging occurs when rocks carried by wind or water collide with other rocks wearing away their surfaces.
7. / How would frequent rock slides affect the weathering rate of surrounding rocks?
/ 8. / What is leaching?