What is chemical and mechanical weathering?
Weathering is the break down of rock in situ (where they are)
Weathering is the break down of rock in situ by the action of rainwater, extremes of temperature, and biological activity.
Mechanical weathering is the breakdown of rock without changing its chemical composition. This means the rock breaks up without its chemical makeup changing. Freeze-thaw weathering is the main type of mechanical weathering that affects coasts.
Freeze-thaw weathering occurs when rocks are porous (contain holes) or permeable (allow water to pass through). Water enters the rock and freezes. The ice expands by around 9%. This causes pressure on the rock until it cracks. Repeated freeze-thaw can cause the rock to break up.
Recently weathered rock can be seen at the foot of chalk and limestone cliffs and is easily identified because it is angular. Over time it will become smoother, forming peddles and then eventually sand.
Salt weathering is when salt spray from the sea gets into a crack in a rock. It may evaporate and crystallise, putting pressure on the surrounding rock and weakening the structure.
Chemical weathering is the breakdown of rock through changing its chemical composition. When rainwater hits rock it decomposes it or eats it away. This is known as carbonation. This occurs when slightly acidic (carbonic) rain or sea water comes into contact with sedimentary rock, such as limestone or chalk, it causes it to dissolve. A chemical reaction occurs between the acidic water and the calcium carbonate and forms calcium bicarbonate. This is soluble and is carried away in solution. Carbonation weathering occurs in warm, wet conditions.
Hydrolysis is when acidic rainwater breaks down the rock, causing it to rot.
Oxidation is when rocks are broken down by oxygen and water.
Weathering weakens cliffs and this then speeds up rates of erosion.
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