Rocks contain minerals and because they are exposed to the environment, they are subjected to a process called chemical weathering. In this process, the minerals chemically react with water, carbon dioxide or oxygen which causes the rock to change in appearance in terms of its color or even its size. For example, when iron in a rock reacts with oxygen, it forms iron oxide. This compound has the reddish rusty appearance that is seen in many rocks. The mineral iron chemically combines with oxygen in a process known as oxidation. Rocks will look different from each other physically depending on the degree to which chemical weathering has taken place.
Oxygen is an electronegative atom and it can form chemical bonds with many other elements in nature, forming compounds known as oxides. Another example of an oxide is silica dioxide which is the element silicon reacting with oxygen. This compound can be found in sand grains and is a component of the earth's crust in the type of rock called granite. Many minerals in rocks are found as oxides rather than as a free metal.
Weathering is an important process because it causes rocks to break down over time. This is an important step in the soil-building process. Parent material, such as bedrock, slowly weathers and breaks down into smaller and smaller particles which contributes to the slow formation of soil once humus is combined with it and organisms add organic materials to the developing soil.