The surface of the Earth is changing all the time. Mountains are becoming craggier, valleys are becoming wider and deeper, glaciers are moving that extra centimetre each day and coastlines are being altered. There are many factors that contribute to this change, but one that can be seen locally is the factor known as weathering.

What Is Weathering?

Weathering is the breakdown of rocks by water, frost, temperature change, flora and fauna and it is sometimes wrongly twinned with erosion. Weathering is the deterioration of rock, and does not involve the removal of said rock as in erosion. This breakdown of rock allows soils to develop, and land ecosystems in our time were forged by using weathering as one ingredient.

There are three main types of weathering, each with their own methods of breaking down rock.

Mechanical Weathering

This is where the rock is broken into smaller pieces by the effects of temperature and sometimes water.

Frost Shattering Weathering

Frost shattering, or freeze-thaw weathering, is mainly found in mountainous regions such as the Himalayas. The main cause of this is a temperature fluctuation from below freezing point to above it.

Frost shattering is caused by water filling cracks in rock during the day. The water freezes at night and exerts pressure as it expands, causing the crack to break further. The ice melts in the daytime, and the whole process begins again, hence the name freeze-thaw.

When the rock is finally broken into small pieces, it leaves the edges of mountains jagged. Near the foot of the mountains, there may be piles of rock fragments which have been broken by frost shattering weathering. These are known as scree.

Exfoliation

This has nothing to do with human skin. Exfoliation in rocks¹ involves a fluctuation in temperature like frost shattering, but in a different range.

Exfoliation occurs mainly in hot desert areas. This includes places like Arizona in the USA and Alice Springs in Australia. In these places, the temperature can get as high as 40°C. As rock is made up of layers (especially in sedimentary types like sandstone and metamorphic types like marble), in the intense heat, the top layer of rock expands. However, the inner layers remain cool. At night, when temperatures drop, the top layer contracts. Repeated heating and cooling causes the surface of the rock to peel off like an onion skin.

Not all weathering is caused by nature's processes. Weathering can be stimulated by human causes, as the next type of weathering shows.

Chemical Weathering

Chemical weathering is caused by water. Rainwater contains small amounts of carbonic acid², but in many places affected by so-called acid rain, such as Scandinavia, chemical weathering is a real problem. Sulphur dioxide emissions can increase the acidity of water.

When rainwater comes into contact with rock, such as sandstone or limestone, a chemical reaction takes place, causing the rock to crumble. This is visible on stone buildings such as old churches, and in cemeteries, where the features of stone gargoyles and gravestones have been weathered away. Water and heat can speed up this process, so it mainly occurs in warm and wet places, such as Britain.

The main reason for the worn features in sandstone and limestone is that they contain an insoluble substance called calcium carbonate. This reacts on contact with the acid rain, forming hydrogen carbonates, or bicarbonates. This has been happening for millions of years, but modern pollution has accelerated the process. Therefore, chemical weathering produces those intricate cave formations that are used so often by potholers.

To prevent chemical weathering, hewing sculptures from granite would prevent them from being dissolved by acid rain. However, it may not be a stalwart defence against...

Biological Weathering

If there is a space, she will fill it. If there is a crack, she will break it open. Try as you may, there is no stopping Mother Nature. This is biological weathering and it is caused by the actions of plants and animals.

Flower Power

One of the ways a plant spreads its seed is by the wind. Dandelion seeds are an example of this, as are sycamore 'helicopter' seeds. Seeds carried by the wind may drop into cracks in rock. The crack proves to be an ideal place for the seed to germinate as the crack unwittingly provides moisture and shelter.

On growing into a young plantlet, the plant's roots delve deep into the rock. This causes the crack to widen, and eventually the rock falls apart. Unfortunately for the plant, it is now exposed to...

Animal Antics

Burrowing creatures such as badgers, moles and rabbits also cause rock to shatter. By burrowing into a crack, sometimes for food, it causes the rock to split and break.

Can Anything Be Done?

Not really. Short of destroying every plant and animal in the world or destroying all the rock in the world, realistically, the only solution is to cut back on carbon dioxide emissions. However, it may be too late to stop acid rain and rising temperatures.

On the other hand, a moral which could be squeezed from this is that if you want the headstone of your grave to be readable in a hundred years time, have it hewn out of granite and facing west (away from the direction of the Sun and the cold northerly wind) and in a country which has a temperate climate. That would slow the weathering process in the short-term, but it won't help the rest of the world.