Weathering is the process wherein natural occurrences and reactions break down materials over time. One type of weathering is mechanical, caused by physical action. The other type of weathering is chemical, caused by chemical reactions. Both of these types of weathering can at times fall under another category known as biological weathering. While there are several specific types of biological weathering, in general it is characterised as being caused by organic, or living, substances and organisms.
A number of different organisms produce and secrete acids, including moulds, fungi and lichens. According to jersey.uoregon.edu, these acids can chemically weather rocks and minerals and essentially "etch" irregularities into their surfaces. These irregularities can in turn serve as footholds or anchors for other organisms, which can further biologically weather the rocks or minerals.
In addition to organisms producing and secreting acid directly, the decomposition of organic debris such as animal excrement, animal carcases, and dead leaves and plants can also release this corrosive substance into the environment. According to jersey.uoregon.edu, when organic debris decomposes, carbon escapes into the atmosphere. This carbon then interacts with water molecules and forms acids, which can then fall as rain and weather rocks, minerals and other substances. When rain is especially acidic, it is referred to as "acid rain" and can be harmful to organisms and drinking water supplies. However, the majority of acid rain cases are associated with man-made pollution and are not "natural" occurrences.
One of the most commonly occurring and easy-to-observe types of biological weathering is root damage. If you live in an area that features trees butting up against paved streets or sidewalks, you have likely seen this phenomenon. Root damage occurs when large, sturdy tree roots run out of growing space and break through the surfaces that are restricting them. This can produce cracks and holes in concrete. Root damage is a mechanical, as opposed to chemical, process.
Digging or Boring
Another type of biological/mechanical weathering is the digging or boring of certain organisms into rock and other materials. According to mc2.vicnet.net.au, one example of this weathering can be seen in termites, which are small insects that eat complex carbohydrate cellulose, or plant matter, and commonly tunnel through wood in the process. However, termites also use chemicals to break down the cellulose, so the weathering in not entirely mechanical. According to geolsoc.org.uk, another example of biological/mechanical weathering caused by digging or boring can be observed in the piddock, a bivalve mollusc, which scrapes away bits of rock until it has bored out a suitable home.