One of the basic principles that you will learn in any geology class is the types of weathering agents that affect your environment. Weathering agents are substances that break down and change the character of rocks that are exposed to weather and the environment. These agents can be mechanical, chemical or biological.
Erosion is a form of mechanical weathering, and occurs when the movement of a force like rain water or moving water sources like a river wear down the surface of rocks and carry the worn away parts with the flow of the water to other areas. Erosion can also occur with forces like wind, glacier movement, tides or waves.
Freeze and thawing is another form of mechanical weathering. This can easily be observed in colder cities that experience temperatures that regularly drop below freezing during the evening, and then warm up during the day. As the water freezes on the road, it expands the affected surface. As the road freezes and thaws repeatedly over the course of fall or winter, the expanding process creates instability, resulting in potholes.
Another agent of weathering is chemical weathering such as oxidation or hydrolysis. Chemical weathering occurs when heat and moisture react with chemicals in the rock to change the properties of the rock. Oxidation occurs when oxygen creates a chemical reaction, with a common example being rust, which makes the rock softer over time. Hydrolysis occurs when water reacts with compounds in the rock to create other compounds. You can usually tell when chemical weathering is occurring because the rock will be a different color, such as orange, red or yellow.
Biological weathering occurs due to the interference of animals or other living creatures on the rock. For example, algae and fungus called lichens that live on rocks can cause biological weathering by eating away at the rock material, slowly breaking it down into smaller pieces. Plant roots can also act as a form of biological weathering as plants roots grow into small cracks in the rock and grow and expand, breaking the rock apart.