Weathering

Introduction to Weathering

Weathering is the process that breaks down rocks into smaller pieces over time. It occurs in place, meaning the rocks are broken down where they are.

Types of Weathering:

There are three main types of weathering:

• Mechanical Weathering: Rocks are physically broken apart without changing their chemical composition.
• Chemical Weathering: Rocks are gradually dissolved or altered due to chemical reactions, often involving water and gases like carbon dioxide.
• Biological Weathering: Rocks are broken down by the actions of plants, animals, or human activities. For example, roots growing into cracks in rocks or the effects of acid rain caused by pollution.

Freeze Thaw Action

Freeze-thaw action is a type of mechanical weathering that occurs when water enters cracks in rocks, freezes, and then thaws, causing the rock to break apart.

• During the day, rainwater collects in the cracks or joints of rocks.
• At night, when temperatures drop below freezing, the water in the cracks freezes and expands by about 10%.
• This expansion puts pressure on the surrounding rock, widening the cracks.
• When the water thaws, it settles deeper into the joints, repeating the process.
• Over time, the repeated freezing and thawing cause pieces of rock to break off. These loose pieces are called scree and often accumulate at the base of slopes, such as those found on the Great Sugarloaf in Co. Wicklow.

Carbonation

Carbonation is a chemical weathering process where rocks, particularly limestone, are dissolved by carbonic acid.

• As rainwater passes through the atmosphere, it absorbs carbon dioxide, forming a weak acid known as carbonic acid.
• When this acid rain falls on limestone, it reacts with the calcium carbonate in the rock, dissolving it. This creates deep cracks called grikes and flat slabs called clints, forming a unique limestone pavement.
• Over time, the acid weakens and erodes the limestone, sometimes creating features like swallow holes where rivers disappear underground, and caves as the river erodes the rock below the surface.
• Because limestone is permeable, water continues to move through the rock, dissolving more material and enlarging the joints and bedding planes. This can lead to the formation of stalactites and stalagmites in caves as dissolved limestone is deposited.

Underground Limestone Features

Limestone landscapes, especially in karst regions, have unique underground features formed primarily by the process of carbonation.

• Passages: When rivers flow underground, the water dissolves limestone through carbonation. This creates tunnels called passages.
• Caves and Caverns: Over time, passages can expand to form larger chambers known as caves or caverns.
• Stalactites: These are slender columns of calcite that hang from the ceiling of a cave. They form as water containing dissolved calcium carbonate drips from the roof, evaporates, and leaves behind calcite.
• Curtains: When stalactites grow along the ceiling, they can form a long sheet of calcite called a curtain.
• Stalagmites: Thick columns of calcite that build up from the floor of a cave, directly beneath stalactites. They form as calcite-rich water drops onto the cave floor and leaves deposits behind.
• Pillars: When a stalactite and stalagmite grow until they meet, they form a pillar. Example: The Burren in Co. Clare is a well-known area where these features are found.