Weathering

Weathering refers to the breakdown of rocks at or near the surface “in situ” (in their original place) by agents such as temperature changes, water and ice. There are three main types of weathering. They are mechanical, chemical and biotic weathering. Let us look at each in turn.

Mechanical Weathering

This refers to the physical breakdown of rock without a change in the mineral composition of the rock. Mechanical weathering results in rocks being broken down into smaller pieces without any chemical changes taking place within the rock. Some mechanical weathering processes include:

Frost Action: This form of weathering is also known as freeze- thaw action. It occurs in areas where the temperature fluctuates around zero degrees Celsius for at least part of the year. Water may find its way into cracks and joints in the rock when the temperature rises above freezing. When the temperature dips below zero degrees Celsius, the water freezes and expands, exerting pressure on the rock and widening the cracks. This process is repeated over and over until the pressure exerted by the ice eventually shatters the rock.

Click here to see a photo of a shattered rock!

Pressure Release: This occurs when the overlying the material is removed by erosion, exposing the underlying rock. The release of pressure causes the rock to expand. This expansion causes fractures to form parallel to the rock surface. This causes the rock to develop several layers which are similar to the layers of an onion. Over time, these sheets of rock will break away from the rock mass. This is called exfoliation.

Click here to see the effects of pressure release!

Temperature Changes: This occurs in places which have a large daily temperature range, such as the Sahara Desert.  A little known fact about deserts is that they are very cold at night. During the day, the high temperatures cause rocks to expand slightly. During the night, the low temperatures cause rocks to contract slightly. Repeated expansion and contraction causes cracks to form within the rocks and eventually they will begin to fall apart.

Chemical Weathering

Chemical weathering refers to the breakdown of rocks due to chemical changes in the minerals which they are made up of. Forms of chemical weathering include:

Hydrolysis: This is the result of chemical reactions between water and minerals in the rock. These reactions change the chemical composition of the minerals. For instance the process of hydrolysis changes feldspar, olivine and pyroxene into clay minerals.

Oxidation: This occurs when minerals in the rock react with oxygen. Iron, for example, may combine with oxygen forming ferrous iron oxide and ferric iron oxide. This weakens the structure of the rock. This process is also known as rusting. The effects of oxidation can be seen in the photo below.

The effects of oxidation are clearly evident in this rock

Rock showing the effects of oxidation

Solution: Some rocks contain minerals which are soluble in water. Water can dissolve these minerals, making these rocks structurally weaker. Examples of such minerals are calcium carbonate and rock salt.

Carbonation: Rainwater or groundwater may absorb carbon dioxide, becoming a weak acidic solution. This solution is very effective at dissolving calcium based rocks such as limestone. Carbonation is often responsible for the formation of caves in limestone areas.

Carbonation has removed much of the rock and formed a cave

Harrison’s Cave in Barbados

Biotic Weathering

Biotic weathering refers to the role plants and animals play in breaking down rock. Burrowing animals often aid in the breakdown of rock. Birds called sand martins often burrow into sandstone cliffs to build their nests. Also, the roots of plants may force their way into cracks in rock, thereby expanding them and eventually breaking the rock apart. The photo below shows a plant growing on rock just on the outskirts of the town of Soufriere in St. Lucia.

Biotic Weathering - plant growing on rock

Plant growing on rock

A closer look reveals that the roots of the plant are growing into the rock and prying it apart. This is clearly shown in the photo below.

These roots are slowly breaking this rock apart

Roots growing into rock

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