What Affects Weathering?

Physical conditions

• Changes in the physical conditions such as temperature and pressure can affect the weathering of a rock. This is especially true when there are extreme changes. Rocks expand and contract with the temperature on a daily basis as well as a seasonal change. Repetition of these changes will eventually cause a rock to fracture. Ice or crystals form in these fissures, causing further force on the rock. Landslips put sudden pressure on a rock, causing it to split or crumble. A forest fire is another example of how a rock can suddenly be put under extreme pressure.

Chemical change

• When a rock buried deep underground comes to the surface it experiences very different conditions. It is exposed to the atmosphere and to chemicals it has not met up with before. These include free oxygen, which causes oxidation of the rock's minerals. Other atmospheric gases such as nitrogen and carbon dioxide will cause nitrates and carbonates to form on the exposed surface. Water in the form of rain will wash away some of these new chemical compounds. Acid rain -- minute traces of sulfur dioxide from industrial plants causes rain to contain sulfuric acid - which also acts on the rock's surface.

Biological conditions

• There are many pictures of large trees that, over the ages, have split open large rocks with their roots, an occurrence that started with a tiny root penetrating a crack or fissure in the rock. Smaller organisms also play their part, especially in damp climates. One can often see lichens or bacteria growing on the surface of a rock, usually the side exposed to the prevailing wind. Different organisms in the soil exude byproducts causing changes in its pH level. This in turn may cause acids to form which react with a rock's surface.

Taking precautions

• The affects of weathering can most easily be seen on the stone from which ancient buildings have been built. Take a look at some of the oldest buildings in your area built from stone. This is especially evident when recent repairs have been carried out to the building using newly quarried stone of the same type. The older stone will show a change in color, its surface will be pitted and it may even have developed cracks. The surface of many newer buildings built from stone has often been treated so as to minimize the affects of possible future weathering.