Marine Science (9693) | Weathering, erosion and sedimentation

AS Level Marine Science (9693): Earth Processes

Chapter 2.2: Weathering, Erosion, and Sedimentation

Hello Marine Scientists! This chapter is all about understanding how the land bordering the ocean is constantly being shaped. From huge cliffs to gentle muddy flats, the processes of breaking down rock, carrying it away, and dropping it elsewhere are fundamental to creating the diverse habitats we study. Don't worry if these geological terms seem similar; we'll break them down clearly!

1. Distinguishing Weathering from Erosion

The key to understanding coastal formation lies in knowing the difference between breaking something up and moving it away.

1.1 Definitions (LO 2.2.1)

Weathering is the process of breaking down rocks, soils, and minerals through contact with the Earth's atmosphere, water, and biological organisms. The rock is broken down in situ (in its original place).

Erosion is the process by which soil, rock, or dissolved material is removed from the Earth's surface by external forces (like water, wind, or ice) and transported elsewhere.

Memory Aid: Think of a demolition crew.
Weathering: Breaking the building into rubble.
Erosion: Loading the rubble onto trucks and driving it away.

2. The Three Types of Weathering (LO 2.2.2)

Weathering occurs through three main mechanisms: physical force, chemical reactions, and biological activity.

2.1 Physical (Mechanical) Weathering

This involves the breakdown of rock solely by mechanical stress, without any change to the chemical composition of the rock material.

• Process Example: Freeze-thaw action. Water enters cracks in the rock, freezes (expanding its volume by about 9%), and exerts pressure, forcing the crack wider. Over many cycles, the rock shatters. This is common in temperate and polar marine environments.

2.2 Chemical Weathering

This involves chemical reactions that alter the composition of the minerals within the rock, making them unstable and causing them to break down.

• Process Example: Dissolution or Carbonation. Acids (often weak carbonic acid formed when CO₂ dissolves in rainwater) react with minerals like calcium carbonate (found in limestone or chalk cliffs), dissolving the rock material.

2.3 Organic (Biological) Weathering

This is the breakdown of rock caused by living organisms.

• Process Example 1 (Physical): Roots from coastal trees (like mangroves) grow into cracks, expanding them and physically splitting the rock material apart.
• Process Example 2 (Chemical): Lichens or molluscs secrete weak acids to extract nutrients or bore into the rock surface, chemically degrading the substrate.

3. The Agents of Erosion (LO 2.2.3)

Erosion relies on an agent—a force or medium—to move the weathered fragments (sediment).

The four main types of erosion, based on the moving agent, are:

3.1 Erosion by Water

In the marine environment, water is the dominant agent. This includes the powerful forces of waves (abrasion and hydraulic action), coastal currents, and river discharge flowing into the sea. This action carries sediment (sand, silt, pebbles) down coastlines or out to sea.

3.2 Erosion by Ice

In polar or very cold regions, moving glaciers or powerful sea ice can scrape (abrade) and pluck rock material from the coastline, transporting huge quantities of sediment.

3.3 Erosion by Wind

Wind primarily affects coastal areas with loose, dry sediment, such as sandy beaches. It lifts and transports fine sand particles, leading to the formation of dunes behind the beach.

3.4 Erosion by Gravity (Mass Movement)

This is the movement of material downslope due to gravity. Coastal cliffs often suffer from mass movement like rockfalls, landslides, and slumping, especially when the base of the cliff is undercut by wave erosion.

4. Sedimentation: The End of the Journey (LO 2.2.4 & 2.2.5)

After the material has been weathered and eroded, it must eventually stop moving.

4.1 Defining Sedimentation

Sedimentation is defined as the deposition of suspended particles. When the carrying agent (usually water or wind) loses enough energy, it can no longer hold the particles, and they settle out. The resulting material is called sediment (e.g., mud, sand, silt).

4.2 How Water Flow and Particle Size Control Processes

The balance between erosion, transport, and sedimentation is critically determined by two factors: the speed of the water flow (current velocity) and the size of the particles (sediment).

The speed of the water dictates which process is happening:

• High Flow Speed: The water has high kinetic energy. This leads to erosion (it picks up new particles) and transport (it carries all sizes of particles).
• Medium Flow Speed: The water has enough energy to keep existing particles moving (transport) but not enough to erode new, large particles from the seabed.
• Low Flow Speed: The water loses energy, and gravity takes over. Particles are no longer suspended and drop out, leading to sedimentation.

The relationship with particle size is slightly counter-intuitive for fine particles:

1. Large Particles (Gravel, Pebbles): Require high flow speeds for erosion and transport, simply because they are heavy.
2. Medium Particles (Sand): Requires medium flow speeds. These are the easiest to transport once they are moving.
3. Fine Particles (Silt, Clay/Mud): These require surprisingly high flow speeds for initial erosion, because once they are deposited, they clump together (cohesion) and become very sticky, making them hard to lift again. However, once they are suspended, they are the first to be deposited when the flow speed slows down (low speed leads to rapid sedimentation of fine muds).

Did you know? This relationship is often visualized using a graph called the Hjulström Curve, which shows exactly how flow speed determines if a river or current will erode, transport, or deposit different sizes of sediment.

5. The Littoral Zone (LO 2.2.6 & 2.2.7)

These processes of W, E, and S are most visible in the area where the sea meets the land—the littoral zone.

5.1 Defining the Littoral Zone

The littoral zone is the area of a shoreline, or intertidal region, defined geographically as the area between the highest spring tide marks and the lowest spring tide marks. This area is periodically exposed to air and covered by water, making it a challenging and dynamic habitat.

5.2 Examples of Littoral Zones (Shore Types)

The balance between erosion and deposition determines the physical shape, or morphology, of the shore. Examples include:

• Rocky Shores: Highly erosive environments.
• Sandy Shores: Dynamic environments dominated by transport.
• Muddy Shores: Low-energy, depositional environments.
• Estuaries: Semi-enclosed coastal bodies where fresh and saltwater mix.
• Deltas: Landforms created when sediment is deposited by rivers as they enter the ocean.

6. The Morphology of Shores (LO 2.2.8)

The type of shore that forms is a direct result of the long-term interaction between weathering, erosion, and sedimentation.

6.1 Rocky Shores

• Dominant Process: Erosion.
• How they form: These shores exist where the land material is highly resistant to weathering (hard rock, e.g., granite or basalt) and/or where wave action (erosion by water) is very high. Strong wave action quickly removes any loose sediment, leaving behind bare rock, cliffs, and sometimes boulders. Weathering often produces the initial cracks and fragments, but erosion immediately carries them away.

6.2 Sandy Shores

• Dominant Process: Transport (Erosion and Deposition are often balanced).
• How they form: These shores are found in areas with moderate wave action. The water speed is high enough to transport medium-sized particles (sand) but not high enough to remove them entirely or to erode surrounding bedrock quickly. Sand particles shift constantly, making the substrate unstable.

6.3 Muddy Shores, Estuaries, and Deltas

• Dominant Process: Sedimentation (Deposition).
• How they form: These shores develop in low-energy environments, such as sheltered bays, estuaries (where rivers meet the sea), or behind sandbars. Since the water flow speed drops drastically, fine sediments (silt and clay) are able to settle out of suspension. The cohesive nature of these fine particles makes the substrate stable once deposited, forming soft, muddy flats.

Understanding how these three fundamental processes—weathering, erosion, and sedimentation—interact with flow dynamics is vital for predicting coastal changes and studying the marine ecosystems that inhabit these zones! Keep up the great work!