External Geological Processes and Risks: Weathering and Erosion

Posted on Aug 15, 2024 in Geology

External Geological Processes and Risks

Introduction

This document explores the phenomena occurring in Earth’s crust due to the interaction of rocks with the atmosphere, hydrosphere, and living things. These processes include weathering, erosion, transportation, and sedimentation.

Weathering

Weathering is the alteration of rocks on Earth’s surface without the transportation of the resulting materials. These materials remain near the weathered rock. There are three main types of weathering:

  • Mechanical or Physical Weathering: Primarily caused by temperature changes.
  • Chemical Weathering: Caused by water and atmospheric gases.
  • Biological Weathering: Caused by living organisms.

Physical or Mechanical Weathering

This process involves the fragmentation of rocks, increasing their surface area exposed to external geological agents. However, it doesn’t change the rock’s chemical composition. It’s mainly due to temperature changes and occurs in the following ways:

  1. Gelifraction (Frost Wedging): The wedge effect of ice, primarily in cold temperate areas. Water freezes in rock cracks, expands, and exerts pressure, eventually breaking the rock.
  2. Salt Crystallization: Salt-laden water enters rock cracks. Salt crystallization creates pressure, fragmenting the rock.
  3. Thermal Expansion and Contraction: Significant temperature fluctuations, especially in deserts, cause rocks to expand and contract repeatedly, leading to fracturing.
  4. Differential Expansion: Rocks closer to the surface undergo decompression as overlying materials erode. This reduced pressure causes cracks and fragmentation.

Chemical Weathering

This process alters rocks due to chemical agents like water, oxygen, carbon dioxide, and acids. Unlike physical weathering, it changes the rocks’ chemical composition. The main chemical weathering processes are:

  1. Dissolution: Chemicals are removed from rocks and dissociate as ions in water. This dissolves minerals like halite and gypsum.
  2. Carbonation: A type of dissolution where carbon dioxide in water reacts with and dissolves limestone.
  3. Oxidation: Minerals, especially those containing iron, react with oxygen, changing their chemical state and often destabilizing their structure.
  4. Hydration: Water molecules are incorporated into the crystal lattice of minerals, often increasing their volume.
  5. Hydrolysis: The disintegration of the crystal lattice of minerals due to the action of hydrogen ions in acidic waters. This process is particularly significant for silicates, like feldspars, which are transformed into clay.

Biological Weathering

Living organisms contribute to weathering through both mechanical and chemical processes:

  1. Wedge Effect: Plant roots grow into rock cracks, exerting pressure and causing fragmentation.
  2. Mechanical Mixing: Animals like worms and moles create burrows and disturb the soil, promoting rock disintegration.
  3. Chemical Effect: Organisms, particularly bacteria, fungi, and lichens, produce acidic substances that chemically alter rocks. Plant roots also chemically interact with rocks.

Erosion

Erosion is the process of weathering and removal of rock fragments and soil particles by forces like gravity, water, wind, and ice, often exacerbated by human activities.

Wind Erosion

There are two main types of wind erosion:

  1. Deflation: Wind carries away loose, fine particles, leaving behind larger fragments, creating desert pavements.
  2. Abrasion: Wind-blown sand particles act like sandpaper, wearing down rocks and creating features like mushroom rocks.

Water Erosion

Water erosion is driven by the force of flowing water, influenced by factors like flow rate, speed, and slope. The erosive action occurs through:

  1. Dissolution: Water dissolves soluble rock components, such as salts.
  2. Abrasion: The force of water, often carrying sediment, wears down rocks.

Glacial Erosion

The movement of glaciers, along with the rocks and debris they carry, causes significant erosion, leaving behind polished surfaces, grooves, and rounded rock formations.