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How are rocks weathered?

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Weathering – freeze-thaw

You may have heard of the saying, “hard as rocks”.

This is because as water freezes it expands.

This creates powerful forces that can enlarge the cracks.

As this freeze–thawfreeze–thaw process is repeated and cracks spread through the rock. Eventually small pieces of rock (called screescree) break off altogether.

scree

Most rocks are hard, but despite this they can be broken by just a small amount of water getting into cracks in the rock.

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Freeze-thaw!

Colin forgot to chill the wine so he put it in the freezer to quickly make it cold – but then forgot it was there!

Next time he went to the freezer he found it totally shattered.

Explain what has happened.

The water expanded as it froze creating huge forces.

These shattered the glass bottle.

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Weathering – expansion of rock

Freeze thaw is the not the only cause of weathering.

In places with large daily changes in temperature (e.g. deserts) expansion and contraction of the rock itself occurs. The surface gets the hottest and so expands the most. This may cause it to “peel off.”

Additionally, some rocks contain crystals that expand by very different amounts. This too can cause cracks.

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Weathering – plants and lichens

Plant roots can get into tiny cracks and can physically open them up further. In addition, decaying plant roots also produce acid which can chemically eat away at the rock.

Similarly lichens produce acids which weather the rocks upon which the lichens are growing.

Plant roots can cause cracks in

rocks

pH876543

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Weathering – chemicals

Firstly, there is carbon dioxide gas which dissolves in rain to form weak carbonic acid. This very slowly eats away at certain rocks.

There are also acids in the rain that can chemically eat away at rocks – especially rocks consisting of metal carbonates (such as chalk, limestone and marble).

Secondly, there are nitrogen and sulphur oxides which produce much more acidic rain that can rapidly chemically dissolve the rocks.