Weathering Part 2: Chemical Weathering

In my previous article (Weathering Part 1: Physical Weathering), we explored how physical weathering works to break down rocks through abrasion, extremes of temperature, changes in pressure, and the actions of animals and plants. Over time, physical weathering wears away a rock, breaking them into smaller and smaller particles, but the rock always remains the same. For example, granite is exfoliated and then broken by the action of frost then eroded into a stream where other rocks hit it, abrading it further. Throughout the process of physical weathering, the granite remains granite. But other weathering processes are in action, working to change the chemical nature of the granite. Remember our definition of weathering, a process that changes the physical and chemical character of rocks at or near the surface. We will now look at some of the weathering agents that chemically affect rocks.

The rocks we see at the surface may seem to be permanent fixtures of our world, but in reality the rocks around us are always changing, trying to reach a balance with the conditions around them. Since surface conditions often change over time, the rocks tend to be constantly changing as well, trying to reach equilibrium. The two main agents for chemical weathering are oxygen and acids.

Oxygen is an important weathering agent, often combining with minerals in exposed rocks to form new minerals. This process is called oxidation. You can see examples of oxygen's role in weathering if you have ever seen a car that has rust on it. The rust is the result of oxygen reacting with the iron in the metal forming iron oxide. When examining rocks in the field that contain iron you can see the evidence of oxygen's role in chemical weathering by the color of the rocks. They are usually brown, yellow, or red in color and result from alteration of the ferromagnesian group of minerals (pyroxenes, amphiboles, biotite, and olivine) into the mineral hematite. These rocks, located at Devil's Tower National Monument, are a good example of rocks affected by oxidation.

Acids probably play the most important role in the chemical weathering of rocks. When you think of acids, you might think about acids used in chemistry such as hydrochloric acid, or sulfuric acid. Such acids do exist in nature, usually in remote locations near volcanoes. Here, the acids are emitted during volcanic eruptions, or from hydrothermal vents and react with the surrounding rocks. As the acid reacts with the rock, new minerals are formed, or sometimes the rocks (and other items, like this storm grate) are dissolved by the action of the acids.

Go To Page: 1 2 3

Articles in this Topic    Discussions in this Topic