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How are rocks weathered? These icons indicate that teacher’s notes or useful web addresses are available in the Notes Page. This icon indicates the slide contains activities created in Flash. These activities are not editable. For more detailed instructions, see the Getting Started presentation. 1 of 19 © Boardworks Ltd 2005 Learning objectives What is weathering? What is freeze-thaw weathering? What is onion-skin weathering? What is biological weathering? What is chemical weathering? 2 of 19 © Boardworks Ltd 2005 What is weathering? weathering erosion and transportation deposition Weathering is the break-up of rocks due to the exposure of the atmosphere. There are two main types of weathering – physical (or mechanical) and chemical. 3 of 19 © Boardworks Ltd 2005 What happens to weathered rock? 4 of 19 © Boardworks Ltd 2005 Learning objectives What is weathering? What is freeze-thaw weathering? What is onion-skin weathering? What is biological weathering? What is chemical weathering? 5 of 19 © Boardworks Ltd 2005 Weathering – freeze-thaw You may have heard of the saying, “hard as rocks”. Most rocks are hard, but despite this they can be broken by just a small amount of water getting into cracks in the rock. This is because water freezes as it expands. This creates powerful forces that can enlarge the cracks. As this freeze–thaw process is repeated and cracks spread through the rock. Eventually small pieces of rock (called scree) break off altogether. scree 6 of 19 © Boardworks Ltd 2005 Freeze-thaw 7 of 19 © Boardworks Ltd 2005 Learning objectives What is weathering? What is freeze-thaw weathering? What is onion-skin weathering? What is biological weathering? What is chemical weathering? 8 of 19 © Boardworks Ltd 2005 Exfoliation or onion-skin weathering 9 of 19 © Boardworks Ltd 2005 Exfoliation or onion-skin weathering During the day the sun heats up the surface of the rock, causing the rock to expand. During the night the As the rock expands rock cools down and and contracts over contracts. and over again, small pieces of surface rock begin to flake and fall off. Onion-skin weathering is a form of physical weathering. 10 of 19 © Boardworks Ltd 2005 Learning objectives What is weathering? What is freeze-thaw weathering? What is onion-skin weathering? What is biological weathering? What is chemical weathering? 11 of 19 © Boardworks Ltd 2005 Biological weathering 12 of 19 © Boardworks Ltd 2005 Biological weathering 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 What type of weathering is biological weathering? 13 of 19 © Boardworks Ltd 2005 Biological weathering What has caused the cracks to appear? 14 of 19 © Boardworks Ltd 2005 Learning objectives What is weathering? What is freeze-thaw weathering? What is onion-skin weathering? What is biological weathering? What is chemical weathering? 15 of 19 © Boardworks Ltd 2005 Chemical weathering How do you think these rocks have become weathered? 16 of 19 © Boardworks Ltd 2005 Chemical weathering 17 of 19 © Boardworks Ltd 2005 Weathering – chemicals 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). Firstly, there is carbon dioxide gas which dissolves in rain to form weak carbonic acid. This very slowly eats away at certain rocks. Secondly, there are nitrogen and sulphur oxides which produce much more acidic rain that can rapidly chemically dissolve the rocks. 18 of 19 © Boardworks Ltd 2005 Can you explain… 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. 19 of 19 © Boardworks Ltd 2005