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© Boardworks Ltd 2003 1 of 7 How are rocks weathered?

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1 © Boardworks Ltd of 7 How are rocks weathered?

2 © Boardworks Ltd of 7 Most slides contain notes to accompany the presentation. This icon indicates that the notes contain particularly detailed instructions or extension activities. To access these notes go to ‘Notes Page View’ (PowerPoint 97) or ‘Normal View’ (PowerPoint 2000/2002). Normal ViewNotes Page View This icon indicates that a Flash file has been embedded into the PowerPoint slide. These files are not editable. Teacher’s notes and Flash files

3 © Boardworks Ltd of 7 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. freeze–thaw scree As this freeze–thaw process is repeated and cracks spread through the rock. Eventually small pieces of rock (called scree) 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.

4 © Boardworks Ltd of 7 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.

5 © Boardworks Ltd of 7 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.

6 © Boardworks Ltd of 7 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

7 © Boardworks Ltd of 7 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.


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