How do rocks turn red?
In the desert Southwest, red rocks are common. Tourists flock to Sedona, Arizona to see the beautiful red rocks, which are set off very nicely by the snow in this photo. What makes the rocks red? The same process that makes rust red!
Chemical weathering is the other important type of weathering. Chemical weathering may change the size of pieces of rock materials, but definitely changes the composition. So one type of mineral changes into a different mineral. Chemical weathering works through chemical reactions that cause changes in the minerals.
No Longer Stable
Most minerals form at high pressure or high temperatures deep in the crust, or sometimes in the mantle. When these rocks are uplifed onto Earth’s surface, they are at very low temperatures and pressures. This is a very different environment from the one in which they formed and the minerals are no longer stable. In chemical weathering, minerals that were stable inside the crust must change to minerals that are stable at Earth’s surface.
Remember that the most common minerals in Earth’s crust are the silicate minerals. Many silicate minerals form in igneous or metamorphic rocks. The minerals that form at the highest temperatures and pressures are the least stable at the surface. Clay is stable at the surface and chemical weathering converts many minerals to clay (Figure below).
Deforestation in Brazil reveals the underlying clay-rich soil.
There are many types of chemical weathering because there are many agents of chemical weathering.
Chemical Weathering by Water
A water molecule has a very simple chemical formula, H2O, two hydrogen atoms bonded to one oxygen atom. But water is pretty remarkable in terms of all the things it can do. Remember that water is a polar molecule. The positive side of the molecule attracts negative ions and the negative side attracts positive ions. So water molecules separate the ions from their compounds and surround them. Water can completely dissolve some minerals, such as salt.
Weathered rock in Walnut Canyon near Flagstaff, Arizona.
Hydrolysis is the name of the chemical reaction between a chemical compound and water. When this reaction takes place, water dissolves ions from the mineral and carries them away. These elements have been leached. Through hydrolysis, a mineral such as potassium feldspar is leached of potassium and changed into a clay mineral. Clay minerals are more stable at the Earth’s surface.
Chemical Weathering by Carbonic Acid
Carbon dioxide (CO2) combines with water as raindrops fall through the atmosphere. This makes a weak acid, called carbonic acid. Carbonic acid is a very common in nature, where it works to dissolve rock. Pollutants, such as sulfur and nitrogen from fossil fuel burning, create sulfuric and nitric acid. Sulfuric and nitric acids are the two main components of acid rain, which accelerates chemical weathering (Figure below). Acid rain is discussed in the chapter Human Impacts on Earth's Systems.
This statue at Washington Square Arch in New York City exhibits damage from acid rain.
Chemical Weathering by Oxygen
Oxidation is a chemical reaction that takes place when oxygen reacts with another element. Oxygen is very strongly chemically reactive. The most familiar type of oxidation is when iron reacts with oxygen to create rust (Figure below). Minerals that are rich in iron break down as the iron oxidizes and forms new compounds. Iron oxide produces the red color in soils.
When iron-rich minerals oxidize, they produce the familiar red color found in rust.
Plants and Animals
Now that you know what chemical weathering is, can you think of some other ways chemical weathering might occur? Chemical weathering can also be contributed to by plants and animals. As plant roots take in soluble ions as nutrients, certain elements are exchanged. Plant roots and bacterial decay use carbon dioxide in the process of respiration.
Mechanical and Chemical Weathering
Mechanical weathering increases the rate of chemical weathering. As rock breaks into smaller pieces, the surface area of the pieces increases Figure below. With more surfaces exposed, there are more surfaces on which chemical weathering can occur.
Mechanical weathering may increase the rate of chemical weathering.
- Chemical weathering changes the composition of a mineral to break it down.
- The agents of chemical weathering include water, carbon dioxide, and oxygen.
- Living organisms and humans can contribute to chemical weathering.
- How does the structure of the water molecule lead to chemical weathering?
- How does carbon dioxide cause chemical weathering?
- How does oxygen cause chemical weathering?
- How does mechanical weathering increase the effectiveness of chemical weathering processes?
Use this resource to answer the questions that follow.