Marine Cycle of Erosion

Imagine you are standing on a rocky coastline, with waves crashing against the cliffs. The sea seems restless, continuously shaping the land. This dramatic interplay of sea and land is what we call the Marine Cycle of Erosion, a natural process that transforms coastlines over time. Just like rivers shape land through their erosion cycles, the sea also follows a similar cycle — Youth, Maturity, and Old Age.

This concept was popularized by William Morris Davis, who believed that coastal landscapes evolve in a cyclic manner, influenced by sea waves, currents, tectonic activities, and the nature of coastal rocks. The marine cycle of erosion is often interrupted by tectonic movements, causing the cycle to reset or shift. But generally, the cycle progresses from rugged, irregular coastlines to smooth, straight coastlines.

Let’s now visualize this process in two different scenarios — Shorelines of Submergence (where the land sinks or sea level rises) and Shorelines of Emergence (where the land rises or sea level falls).

Marine Cycle of Erosion Along Shorelines of Submergence

Initial Stage: The Land Sinks, the Sea Advances

Picture yourself walking along a coastal village. One day, a major climatic event or tectonic shift causes the sea level to rise or the land to sink. As a result, the coastline is submerged under seawater. This can happen in two ways:

• Sea level rise due to melting glaciers, or
• Tectonic subsidence where land physically sinks.

As the sea advances inland, low-lying coastal areas get flooded, and the coastline becomes highly irregular.

Youth Stage: The Sea’s Aggression Begins

Now imagine powerful sea waves, especially breaker waves, relentlessly attacking the rocky coast. These waves act like natural bulldozers, using two main forces:

• Hydraulic Action: The force of water striking the rock, creating pressure.
• Corrasion (Abrasion): The waves carry sand, pebbles, and debris, which continuously scrape and erode the coastal rock.

As a result:

• Sea caves start forming in weaker rock areas.
• Headlands (protruding landmasses) and bays (indented coastlines) develop.
• Sea waves carve notches at the base of cliffs, which eventually leads to the formation of sea cliffs.
• The broken rock debris gets deposited in shallow water, forming a wave-cut platform.

The coastline is now jagged, irregular, and dramatic — like the rugged coastlines of Norway (Fjords) or Southwest Ireland (Ria Coastline).

Mature Stage: The Battle Between Erosion and Deposition

After years of relentless wave action, the coastline starts to stabilize. The sea waves still erode the coast, but the rate of erosion and deposition becomes balanced.

What happens now?

• Most of the caves collapse, leaving behind arches and stacks.
• The coastline straightens as headlands get eroded and sediment is deposited in bays.
• The wave-cut platform extends further into the sea, reducing the cliff height.

The coastline now starts resembling a straight, regular coast.

Old Stage: A Tamed Coastline

After centuries, the once-mighty coastline has now been reduced to a flat, low-lying coastal plain. The sea has done its job — reducing high cliffs, filling bays with sediments, and straightening the shoreline.

Key Features:

• Sea cliffs are almost gone.
• The coastline becomes a low, flat plain.
• The sea finally achieves equilibrium with the land.

But nature is unpredictable — a tectonic event or a change in sea level can reset the cycle anytime.

Marine Cycle of Erosion Along Shorelines of Emergence

Now, imagine a different scenario — instead of the sea advancing, the land itself rises or the sea level falls. This leads to the Shoreline of Emergence.

Initial Stage: The Land Rises, the Sea Retreats

Suppose there’s a sudden tectonic uplift or a sea level fall. The coastline, which was once underwater, now emerges above the sea. You can literally see old sea floor sediments and marine shells on the newly exposed land. This coastline initially looks straight and regular.

Youth Stage: Nature Starts Building New Landforms

As the sea retreats, it leaves behind a few bars of sand and sediments called submarine bars. These bars soon unite and form offshore bars parallel to the coast.

Here’s what happens next:

• The area between the offshore bar and the coast forms a lagoon (a water body separated from the sea).
• Gradually, rivers and winds fill the lagoon with sediment, and vegetation starts growing, turning it into marshes or swamps.
• Sometimes, the sea breaks the offshore bar, allowing water to flow in and out. This opening is called a Tidal Inlet.

A classic example is the Chilika Lake in Odisha.

Mature Stage: Coastal Features Disappear

As time passes:

• The offshore bars get eroded or merged with the coast.
• The lagoons dry up or get filled with sediments.
• The coastline becomes steep, and water depth increases.
• Eventually, most of the coastal irregularities disappear.

The coastline now starts resembling a simple, straight, and steep coastline.

Old Stage: Theoretical End Point

Theoretically, if the coastline remains undisturbed for thousands of years, the land and sea would reach complete equilibrium. However, in real life, tectonic movements, climatic changes, and sea-level fluctuations prevent this stage from happening.

In theory, the old stage would look like:

• Completely flat coastal plains.
• No lagoons, offshore bars, or tidal inlets.
• The sea quietly lapping against the land without dramatic erosions.

Key Insights

• The Marine Cycle of Erosion is nature’s way of shaping coastlines over time.
• Submergence creates rugged, irregular coastlines (like Fjords in Norway).
• Emergence creates flat, straight coastlines (like the Chilika Lake coast in Odisha).
• However, tectonic activities, climatic changes, and sea-level fluctuations constantly reset the cycle, making the old stage almost impossible to achieve.

✅ Bonus Thought: Why is this Important for India?

• India’s western coast (Konkan, Malabar) shows evidence of Submergence — marked by deep harbors and drowned valleys.
• The eastern coast (Coromandel, Andhra) shows evidence of Emergence — marked by large deltas, lagoons (like Chilika), and wide beaches.

Understanding the Marine Cycle of Erosion helps in coastal management, disaster mitigation, and urban planning in coastal area