Marine Landforms : Erosional and Depositional

The most powerful agents of marine erosion are waves which originate due to the sweeping of winds over the water surface, setting up a series of undulating swells surging forward.

The main force responsible for shaping coastlines is the action of sea waves. These waves are created when winds blow across the surface of the ocean or sea. As the wind moves over the water, it transfers energy to it, causing the water to rise and fall in a repeated pattern. These movements are known as waves or swells.

 As waves travel across deep water, they move freely. However, when they approach the coast:

• • The water becomes shallower, which slows the waves down.
  • The waves begin to bend or change direction to match the shape of the coastline. This process is called refraction.
  • When the depth of water becomes less than the height of the wave, the wave becomes unstable.
  • The top part of the wave (called the crest) curls forward and breaks, releasing energy onto the shore.

When waves reach the beach, two important movements occur:

• • Swash: This is the water that rushes up onto the shore after a wave breaks. It often carries sand, pebbles, and small rocks with it.
  • Backwash: This is the water that flows back down the beach into the sea due to gravity.
    

    

Another important movement beneath the surface is:

• • Undertow: A current that flows away from the shore near the bottom of the sea.
    
    • It can be dangerous for swimmers because it pulls water and objects away from the beach.
      

Swash and backwash

To understand coastal processes, it is important to clearly distinguish between these two movements:

• Swash:
  • • Moves up the beach
    • Carries materials like sand and stones towards the land
    • Plays a role in building up beaches
• Backwash:
  • • Moves down the beach
    • Pulls materials back into the sea
    • Plays a role in removing sediments
• The balance between swash and backwash determines whether a beach grows or erodes over time.

Types of Marine Erosion

• Marine erosion refers to the process by which coastal land is worn away by natural forces. These forces include:
• • Waves
  • Wind
  • Ocean currents
  • Tides
  • Storms
• Each of these contributes to breaking down rocks and transporting materials. There are four main types of marine erosion, explained below.

Corrasion or Abration

• This type of erosion happens when waves throw rock fragments against the coast, wearing it down over time.
• Waves carry materials such as:
  
  • Sand
  • Pebbles
  • Larger rocks
• These materials act like natural tools or sandpaper, hitting the base of cliffs repeatedly.
• Over time, this causes the rocks to break apart and wear away.
• After erosion, the loosened material is carried away by waves and tides, preventing it from accumulating at the base.
• In simple terms: Rocks carried by waves help grind down the coastline.

Attrition

• Attrition is the process in which rock particles collide with each other and break into smaller pieces.
• As waves move materials around:
  
  • Rocks hit against each other repeatedly
  • Sharp edges become smooth and rounded
• Over time, large rocks:
  
  • Break into smaller stones
  • Eventually turn into fine sand
• This is one of the main reasons why many beaches are covered in soft sand instead of sharp rocks.
  

  

Hydraulic Action

• Hydraulic action is caused by the force of moving water hitting rocks.
• Here is how it works:
  • Waves crash into cracks and openings in rocks.
  • Air gets trapped inside these cracks.
  • The pressure from the water compresses the air.
  • When the wave moves back, the compressed air expands suddenly.
• This repeated compression and expansion:
  • Weakens the rock structure
  • Causes pieces of rock to break off
• Hydraulic Action is a physical (mechanical) process, not a chemical one.

Solvent Action

• Solvent action, also called chemical erosion, occurs when sea water reacts with certain types of rocks.
• It mainly affects soluble rocks, such as:
  
  • Limestone
  • Chalk
• Sea water contains natural chemicals that can dissolve minerals.
• For example, calcium carbonate in limestone reacts with water and slowly breaks down.
• The Result is :
  • Rocks gradually weaken and disintegrate
  • This process is slower but very effective over long periods

Marine transportation

• After rocks are broken down by erosion, the materials are moved from one place to another by the sea. This movement is called marine transportation.
• There are four main ways in which materials are transported:

Marine transportation is important because it determines where sediments will eventually be deposited, forming new landforms.

Marine Landforms – Erosional

Headlands and Bays

• Coastlines are not straight because different rocks erode at different speeds.
• Soft rocks (like clay and sand):
  
  • Erode quickly
  • Form bays, which are curved inward areas of the sea
• Hard rocks (like chalk):
  
  • Resist erosion
  • Remain sticking out into the sea as headlands
• Characteristics:
  • Headlands:
    
    • Face strong wave action
    • Experience intense erosion
  • Bays:
    
    • Are sheltered from strong waves
    • Often accumulate sand and form beaches

Cliffs and Wave-Cut platforms

• Cliffs are steep rock faces formed along coastlines.
• Soft rock cliffs:
  
  • Erode quickly
  • Have a gentle slope
• Hard rock cliffs:
  
  • Erode slowly
  • Are steep and rugged
• A wave-cut platform forms at the base of cliffs through repeated erosion.
• Step-by-step formation:
• • Waves attack the base of the cliff between high and low tide levels.
  • A small hollow called a notch forms.
  • The notch grows larger, making the cliff unstable.
  • The upper part of the cliff collapses.
  • The debris is washed away by the backwash.
  • A flat surface remains → this is the wave-cut platform.
• This process continues, causing the cliff to move backward over time.

Caves, Arches, Stacks, and Stumps

• These landforms show the progressive stages of coastal erosion.
• Formation process:
• • Small cracks in rocks are widened by wave action.
  • Cracks become caves.
  • Caves grow larger and may break through to form an arch.
  • The roof of the arch eventually collapses, forming a stack (a tall rock column).
  • Continued erosion reduces the stack to a stump.

Ria, and Cove

• Ria:
  
  • A river valley that has been flooded by the sea
  • Appears as a long, narrow inlet
• Cove:
  
  • A small, enclosed bay with a narrow entrance
  • Formed when softer rock is eroded faster than surrounding hard rock
    
    

Geos and Gloups (blowholes or marine geyser)

• Blowholes (Gloups):
  
  • Form when air and water are forced through a vertical hole in the rock
  • Caused by pressure from waves inside caves
• Geos:
  
  • Form when the roof of a cave collapses
  • Create long, narrow inlets along the coast

Chasms

• Chasms are:
  • Deep and narrow gaps in rocks
  • Formed when waves erode along lines of weakness
• Over time:
  • The chasm widens
  • Eventually develops into a bay

Creek, and Inlet

• Creek:
  
  • A small, narrow water channel
  • Often found in coastal or marshy regions
• Inlet:
  
  • A long, narrow opening in the coastline
  • May lead to larger water bodies like lagoons or bays

Marine Landforms – Depositional

When waves lose their energy, they drop the materials they were carrying, leading to the formation of new coastal features.

Beaches

• Beaches are formed by the accumulation of sand, pebbles, and other materials.
• They usually form in:
  • Areas with low wave energy
  • Sheltered regions like bays
• Types of beaches:
  • Sandy beaches:
    
    • Smooth and gently sloping
    • Found in calm waters
  • Pebble beaches:
    
    • Steeper
    • Found in high-energy wave areas
• Beach features:
  • Berms: Raised ridges formed by waves
  • Larger materials at the top, finer materials near the water

Spits and Hook

• A spit is a long, narrow strip of sand extending into the sea.
• Formation process:
  • Sediments are moved by longshore drift (movement along the coast).
  • A change in coastline causes deposition.
  • A narrow ridge forms → spit.
  • A change in wind direction may create a hook-shaped end.
• Behind the spit:
  • Water becomes calm
  • Fine sediments settle to form:
    
    • Mudflats
    • Salt marshes

Bars, Lagoons, and Barrier

• Bar:
  
  • Forms when a spit connects two land areas
• Lagoon:
  
  • A shallow water body trapped behind a bar
• Barrier:
  
  • Similar structure formed above sea level

Tombolos & Dumb Ball

• Tombolo:
  
  • A sand bar connecting an island to the mainland
• Dumb Ball:
  
  • A sand bar connecting two islands

Marine Dunes & Dune Belts

• Marine dunes are formed when wind blows sand from the beach inland.
• These dunes can:
  
  • Spread over large areas
  • Cover farmland and settlements
• They can be controlled by applyng following measures:
  • Planting vegetation such as:
    
    • Marram grass
    • Pine trees
• These plants help hold the sand in place and prevent it from spreading further.