How does weathering work?
Weathering
Weathering is the break down of rocks by natural forces. There are three different types of weathering:
Weathering is the break down of rocks by natural forces. There are three different types of weathering:
- Physical
- Chemical
- Biological
Physical Weathering - mainly occurs in deserts and high mountains
1. Change in temperature can break up rocks. When rocks are subjected to continuous hot and cold spells, they expand and shrink. Most rocks are made from mixtures of minerals. These expand and shrink at different rates. This causes stress to build up in rocks. Eventually, they crack and crumble away. This is known as stratification. To demonstrate this effect, we can heat a glass rod and drop it into cold water. See animation below:
1. Change in temperature can break up rocks. When rocks are subjected to continuous hot and cold spells, they expand and shrink. Most rocks are made from mixtures of minerals. These expand and shrink at different rates. This causes stress to build up in rocks. Eventually, they crack and crumble away. This is known as stratification. To demonstrate this effect, we can heat a glass rod and drop it into cold water. See animation below:
2. In regions where the temperature drops below 0°C , ice can weather rock away.
When it rains, water collects into the cracks in rocks. If the temperature drops below freezing, it turns into ice and expands. The expansion effect of ice pushes the rocks apart. This is repeated as water thaws and freezes. Eventually the pieces of rock break off by fracturing or shattering and fall to the lowest point on the ground. This process is called Freeze thaw. The broken off pieces of rock are called Scree.
When it rains, water collects into the cracks in rocks. If the temperature drops below freezing, it turns into ice and expands. The expansion effect of ice pushes the rocks apart. This is repeated as water thaws and freezes. Eventually the pieces of rock break off by fracturing or shattering and fall to the lowest point on the ground. This process is called Freeze thaw. The broken off pieces of rock are called Scree.
Chemical Weathering - mainly occurs in warm and wet climates.
Water, oxygen and acid can all weather rocks away. They breakdown rocks by dissolving or reacting with some of the minerals in the rock.
Water attack
Water itself can cause chemical reactions with some rocks. Water attacks one of the minerals in granite, for example. The granite break down into tiny particles of clay. These then get washed away.
Oxygen attack
Oxygen gas in the air can attack some rocks, particularly those containing iron. The rusty brown streaks in some rocks are due to iron reacting with oxygen. This leads to the formation of iron compounds which causes weakness in the rocks, hence the rocks wear away.
Water, oxygen and acid can all weather rocks away. They breakdown rocks by dissolving or reacting with some of the minerals in the rock.
Water attack
Water itself can cause chemical reactions with some rocks. Water attacks one of the minerals in granite, for example. The granite break down into tiny particles of clay. These then get washed away.
Oxygen attack
Oxygen gas in the air can attack some rocks, particularly those containing iron. The rusty brown streaks in some rocks are due to iron reacting with oxygen. This leads to the formation of iron compounds which causes weakness in the rocks, hence the rocks wear away.
Chemical Weathering image courtesy of science-resources.co.uk
Chemical weathering acid rain
Chemical weathering is not just due to "acid rain" caused by pollution, it is also instigated by ordinary rain. You probably think that rain-water is pure. Actually it is slightly acidic. This is because carbon dioxide gas dissolves in rain as it falls. The resulting weak acid (carbonic acid) attacks rocks, mainly those containing calcium carbonate.
Calcium carbonate, in limestone, reacts with carbonic acid:
Calcium carbonate + carbonic acid ---> calcium hydrogencarbonate
CaCo3(s) + H2CO3(aq) Ca(HCO3)2(aq)
Soluble calcium hydrogencarbonate is formed. So the limestone is slowly worn away.
Chemical weathering is not just due to "acid rain" caused by pollution, it is also instigated by ordinary rain. You probably think that rain-water is pure. Actually it is slightly acidic. This is because carbon dioxide gas dissolves in rain as it falls. The resulting weak acid (carbonic acid) attacks rocks, mainly those containing calcium carbonate.
Calcium carbonate, in limestone, reacts with carbonic acid:
Calcium carbonate + carbonic acid ---> calcium hydrogencarbonate
CaCo3(s) + H2CO3(aq) Ca(HCO3)2(aq)
Soluble calcium hydrogencarbonate is formed. So the limestone is slowly worn away.
Biological weathering - caused by plants and animals
Plants and animals are also responsible for breaking up rocks. Soil can gather in the cracks of rocks and if seeds land there, plants can grow. The roots of the plants push their way through the cracks of rocks and as the roots grow they gradually push the rocks apart. See animation opposite.
Burrowing animals and humans also wear away rocks as they dig.
Plants and animals are also responsible for breaking up rocks. Soil can gather in the cracks of rocks and if seeds land there, plants can grow. The roots of the plants push their way through the cracks of rocks and as the roots grow they gradually push the rocks apart. See animation opposite.
Burrowing animals and humans also wear away rocks as they dig.
Biological Weathering image courtesy of science-resources.co.uk
Tags: How does weathering work; physical weathering; chemical weathering; biological weathering; weathering, How do weathering and erosion work together, what does weathering mean, chemical weathering acid rain.
