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Rocks and weathering. Physical weathering. Rocks and weathering. Why are rocks all different shapes and sizes?. What is weathering?. Rocks are different shapes and sizes because they are changed by the conditions in their environment.
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Rocks and weathering Why are rocks all different shapes and sizes?
What is weathering? Rocks are different shapes and sizes because they are changed by the conditions in their environment. The breakdown of rocks into smaller fragments is called weathering. These fragments eventually become soil. Rocks are weathered by temperature change, water, frost and the actions of plants and animals. There are three main types of weathering: • physical weathering • chemical weathering • biological weathering.
Different types of physical weathering Physical weathering occurs when rock is broken down into smaller pieces by the effects of temperature and water. • Exfoliationweathering (or ‘onion-skin’ weathering) is caused by very hot weather. • Freeze-thawweatheringis caused by the melting and freezing of water. Exfoliation and freeze-thaw weathering tend to occur in very different types of landscapes. Which type of weathering do you think will primarily occur in the desert, and which will occur in the mountains?
Exfoliation weathering Exfoliation weathering is common in sedimentary rocks in very hot and dry places like Uluru (Ayers Rock) in Australia. In these places the daytime temperature can rise above 40°C. While the inner layers of the rocks stay cool, the outer layers of rocks heat up and expand in the baking heat. At night, when the temperature falls, the outer layers of rocks cool down again and contract. Eventually the repeated expansion and contraction of the rock causes the outer layers to peel like an onion skin.
How can freezing cause damage? What will happen to a glass bottle containing a liquid if it is left in a freezer for too long? The liquid inside the bottle will expand slightly as it freezes. The ice that is formed will create huge forces on the glass, which will then cause the bottle to break! Because water expands as it freezes it can create immense pressure in confined spaces. How does this explain why water pipes often burst in winter?
Freeze-thaw weathering When rain water or melted snow seeps into the cracks in a rock and freezes, it can force the crack to expand. When the ice thaws, the rock contracts and the water moves deeper into the crack. Later when the water re-freezes the crack widens again. Over time the crack widens until the piece of rock breaks apart. This slow cycle is called freeze-thaw weathering.
Where does freeze-thaw take place? Freeze-thaw weathering is common in icy areas where temperatures can rapidlyrise and fall. This means that freeze-thaw weathering is common on high mountains and glaciers, where the water freezes at night, and is melted by the Sun’s heat during the day. For example, the daytime temperature of the base camp on Mount Everest is around 20ºC, but can fall below -10ºC at night.
Biological weathering What has caused these cracks to form?
What is chemical weathering? Slow chemical weathering Rainwater is naturally acidic because carbon dioxide in the air reacts with rainwater to form carbonic acid. This type of acid rain is weakly acidic and reacts slowly with minerals in rock. Rapid chemical weathering The burning of fossil fuels produces oxides of sulphur and nitrogen which make rainwater more acidic. This type of acid rain reacts quickly with minerals and weather rock more rapidly.
Examples of chemical weathering How has chemical weathering affected these rocks?
What is transportation? by wind by water by glacier Transportation is the movement of rock fragments from one place to another. The rock fragments can be transported in different ways:
What is deposition? Deposition occurs when pieces of weathered rock sink to the bottom of the river bed or sea, forming sediment. If animals walk across the sediment, their footprints may be recorded in it, and if dead organisms become trapped in it, they may form fossils.
Examples of sedimentary rocks How can you tell that these are sedimentary rocks?
Glossary • deposition –The settling of rock fragments after transportation. • erosion –The wearing away of rocks by wind, water or ice. • exfoliation –Weathering of rocks caused by repeated heating and cooling, also called onion-skin weathering. • freeze-thaw –Weathering of rocks caused by the repeated freezing and thawing of water in cracks in rocks. • mineral –A solid substance, usually a compound, which is found in rocks. • rock –A mixture of minerals. • transportation –Movement of rock fragments from one place to another. • weathering –The breakdown of rocks into smaller pieces by physical, chemical and biological processes.
