This process of weathering is very common and we see it around us. A good example is an animal that can burrow into a crack in a rock. There are many insects, rodents, and larger animals that live in holes in the ground or rocks. Constantly they burrow and widen cracks and end up breaking rocks apart.

Algae, lichens, bacteria, and mosses often grow on rock surfaces, especially in humid regions. They produce weak acids, which convert some of the minerals to clay. The algae growth can weaken many rock types and make it more open to weathering.

Humans are no exception. Walking and construction activities like road building, mining, and the like involve people ripping rocks apart. They cause weathering too.

Also importantly, weeds and plant roots can get into cracks in rocks and grow from there. As the plant grows bigger the roots grow bigger and deeper, widening cracks and splitting them apart. Decaying roots also add organic acids to the joint, speeding up the weathering process.

Sometimes it is tricky to tell if a particular form of weathering is physical, chemical, or biological. For example, if the pressure exerted by a root breaks up a piece of rock, what kind of weathering would it be? The pressure is physical weathering. At the same time, if chemicals released by roots enhance the microbiological activity, then that can also cause chemical weathering. The same is true for biological weathering too because weathering caused by plants and animals falls under that.