Weathering also occurs when chemical interactions change the chemical structure or composition of minerals within a rock. These changes can often lead to the breakdown of rocks.
This piece of sandstone is gradually oxidizing from the outside in.
Oxidation is a chemical weathering process commonly known as rusting. It occurs when rocks and minerals are exposed to oxygen. Oxygen (O2) reacts with certain minerals, changing their chemical composition and structure. This change can be accelerated in environments of high moisture and temperature.
A common example of oxidation involves rocks containing iron (Fe). Iron will begin to oxidize when exposed to oxygen, altering its chemical structure to iron oxide (Fe2O3), a reddish brown rust. As iron within a rock is oxidized, the rock is weakened, causing it to crumble easily.
Hydrolysis
Hydrolysis occurs when minerals react with ions, such as hydrogen (H+) from water. For example, when rain water comes into contact with silicate minerals, new minerals may form. This reaction can cause large rocks to break into smaller pieces or to change into silts or clays.
Living Organisms
Weak acid secreted by lichens can chemically weather a rock.
A lichen is often thought of as a plant, but it is actually part fungus and part alga. It can survive in some of Earth’s most extreme environments. It has been found growing in the arctic tundra, high mountain tops, and in harsh deserts. It could even survive in space!
When attached to rocks, lichens can se- crete a weak acid that dissolves some of the minerals in the rock. These dissolved minerals provide the lichens with some of the nutrients that help them grow.
Carbonation
Carbonation occurs when carbon dioxide (CO2) dissolves in water (H2O), forming carbonic acid (H2CO3). This chemical reaction occurs naturally in the atmosphere as water and atmospheric carbon dioxide mix. When this happens, it produces the weak acid known as acid rain.
Chemical weathering results when acid rain falls on and seeps into the cracks of rocks rich in minerals that react readily with acid, like calcite (CaCO
3). The acid rain causes a chemical reaction to occur which breaks down the calcite. The effects of acid rain can be seen in many old buildings, statues, and tombstones that were constructed using limestone and marble, which are both rich in calcite.
