Weathering

Fundamental Concepts of Denudation

  • Denudation: The breaking down and moving away of rock and surface material over time by weathering and erosion.

  • Weathering: The breaking down or dissolving of rock in place (in situ) on Earth's surface.

  • Erosion: The wearing down and moving of rock material by natural forces:

    • Moving water (rivers, waves, rain)

    • Wind

    • Moving ice (glaciers)

  • The Three Types of Weathering:

    • Mechanical Weathering: Physical breaking of rock into pieces without chemical changes.

    • Chemical Weathering: Breakdown of rock through chemical reactions.

    • Biological Weathering: Breakdown of rock caused by plants and animals.


Cliffs of Moher showing coastal cliff erosion
Mechanical Weathering: Freeze-Thaw Action

  • Where it happens: High, mountainous areas where the temperature regularly rises above and drops below 0 ∘C0\,^\circ\text{C}.

  • How it works:

    1. Water collects: During the day, rain or melted snow gets into cracks in the rock.

    2. Water freezes: At night, the temperature drops below 0 ∘C0\,^\circ\text{C}, turning the water into ice.

    3. Ice expands: Ice takes up 9%9\% more space than water (ΔV=+9%\Delta V = +9\%). This outward push widens the crack.

    4. Ice melts: The next day, the ice thaws back into water, letting even more water seep deeper into the wider crack.

    5. Rock breaks: Over repeated freezing and thawing cycles, chunks of rock break off and tumble down the slope due to gravity.

  • Scree: The pile of broken, jagged rock pieces that gathers at the foot of a mountain or slope.


Later stage of freeze-thaw action showing rock breaking off to form scree
Chemical Weathering: Carbonation

  • Carbonation: A chemical process where acidic rainwater dissolves limestone rock.

  • The Chemical Process:

    1. Rain absorbs carbon dioxide (CO2\text{CO}_2) from the air to form weak carbonic acid:      H2O+CO2→H2CO3\text{H}_2\text{O} + \text{CO}_2 \rightarrow \text{H}_2\text{CO}_3

    2. This acidic rain falls onto exposed limestone, which contains calcium carbonate (CaCO3\text{CaCO}_3).

    3. The acid converts the solid calcium carbonate into soluble calcium bicarbonate:      CaCO3+H2CO3→Ca(HCO3)2\text{CaCO}_3 + \text{H}_2\text{CO}_3 \rightarrow \text{Ca(HCO}_3)_2

    4. Water dissolves and washes the calcium bicarbonate away, hollowing out the rock.

Karst Landscapes and Landforms

  • Karst Landscape: An area of exposed limestone shaped mainly by carbonation (such as the Burren in Co. Clare).

  • Surface Features:

    • Limestone Pavement: A wide, flat area of exposed limestone rock.

    • Grikes: Deep vertical grooves or cracks carved out by acidic rainwater.

    • Clints: Flat blocks of limestone left standing between grikes.

    • Karren: Small circular puddles or dips dissolved into the surface of clints.

    • Swallow Hole: A large hole in the ground where a river or stream vanishes underground.


Surface karst landforms including clints, grikes, karren, and a swallow hole

  • Underground Features:

    • Caves: Large hollow spaces carved underground by flowing acidic water.

    • Stalactites: Slender rock shapes hanging down from the cave ceiling. They form when dripping water evaporates and leaves behind tiny bits of calcite.

    • Stalagmites: Short, thick mounds of calcite growing upward from the cave floor where droplets land.

    • Pillars: Formed when a stalactite and a stalagmite meet and join together.

    • Emerging Stream: An underground stream that returns to the surface lower down the slope.


Underground limestone cave features showing stalactites, stalagmites, and a pillar
Biological Weathering

  • Definition: The breakdown of rock caused by plants and animals.

  • Plant Action (Root Wedging):

    • Plant seeds or tree roots take hold in small cracks.

    • As the roots grow thicker, they push against the rock walls and split the rock apart.

  • Animal Action (Burrowing):

    • Animals like rabbits dig tunnels, loosening and moving rock pieces.

    • Burrowing clears away topsoil, leaving the bare rock exposed to rain and frost.


Biological weathering showing tree roots breaking through rock
Case Study: The Burren, Co. Clare

  • Overview:

    • Covers 360 km2360\,\text{km}^2 in western Ireland.

    • Recognized as a UNESCO Global Geopark and an EU Special Area of Conservation.

    • Home to 75%75\% of Ireland's native plant species.

  • Benefits of Tourism:

    • Attracts more than 1 million1\text{ million} visitors each year.

    • Creates local jobs in guiding, hospitality, and outdoor education.

    • Leads to improved roads, transport, and walking trails.

  • Environmental Challenges:

    • Building car parks and visitor centres can destroy sensitive habitats.

    • Walking off designated paths tramples rare plants and erodes thin soils.

    • Ecotourism encourages the rule: "Take only photographs, leave only footprints."