Exogenic Processes Overview

Exogenic Processes

• Exogenic processes are geological processes that occur at or near the Earth's surface.
• Uniformitarianism is the theory that the Earth's changes over geological time are the result of continuous, uniform processes.
• Key exogenic processes include: erosion, weathering, and deposition.

Erosion

• Erosion is the breaking down and transportation of rocks and other materials by natural forces.
• Agents of erosion include wind, water (streams and waves), and gravity (mass movements).
• Mass wasting is when gravity pulls weathered sediments down slopes.

Weathering

• Weathering is the breaking down of rocks and other materials at or near the Earth's surface.
• There are two main types of weathering: mechanical and chemical.

Mechanical Weathering

• Mechanical weathering physically breaks down rocks into smaller pieces without changing their chemical composition.
• Common causes include:
  • Animal actions: burrowing animals loosen soil and break apart rocks.
  • Freezing and thawing: water seeps into cracks, freezes, expands and breaks apart rocks (frost wedging).
  • Release of pressure: erosion removes material from a rock mass, reducing pressure and causing the outside to crack and flake off.
  • Plant growth: roots enter cracks in rocks, forcing them apart as the plant grows.
  • Abrasion: rocks hitting each other wear down and break apart.

Chemical Weathering

• Chemical weathering breaks down rocks by changing their chemical composition.
• Common causes include:
  • Water: Dissolves minerals and carries other substances that break down rocks.
  • Oxygen: Combining with iron creates rust, making rocks soft and crumbly.
  • Carbon dioxide: Dissolving in water creates carbonic acid which weathers rocks like marble and limestone.
  • Living organisms: Plant roots produce weak acids that break down rocks.
  • Acid rain: Burning fossil fuels puts sulfur, carbon, and nitrogen into the air, which react with water vapor to create acid rain. Acid rain accelerates chemical weathering.

Deposition

• Deposition is the process in which sediment (eroded material) is laid down in new locations.

Rate of Weathering

• Factors influencing weathering rates include:
  • Type of rock: Soft minerals like those in limestone wear down faster than hard minerals.
  • Surface area: Larger surface areas allow for more contact, accelerating weathering.
  • Climate: More moisture increases both physical and chemical weathering, while cold/moist climates increase physical weathering.

Agents of Erosion

• Agents of erosion are the forces that move sediment from one place to another.
• These include gravity, wind, running water (streams), and waves.

Gravity

• Gravity is the force that pulls weathered sediments downhill.
• Types of mass wasting include:
  • Fast: Landslides and mudslides.
  • Slow: Soil creep and slump.

Running Water

• Running water is the dominant form of erosion.
• The amount of water in a stream is called discharge.
• Erosion occurs where the water flows the fastest, usually near the outside of curves in a river channel (meander).

Wind

• Wind can move sand and small sediments, but rarely moves material more than a meter above the ground.
• Deflation: Wind removes loose sediment, lowering land surfaces.
• Abrasion: Wind-blown sand wears down rocks.

Waves

• Waves erode cliffs and transport sediment.
• The process occurs at the base of cliffs where waves are most powerful.

Landforms

• Oxbow lakes: Created when a meander of a river is cut off.
• River deltas: Sediment deposited at the mouth of a river.
• Other landforms include landslides and rockfalls.

Waves

• Wave size depends on how long the wind blows in one direction.
• Water particles move in circular paths over deep ocean water.
• When a wave reaches shallow water, friction causes the bottom to move more slowly, and the wave "breaks".