Maarg Manthan · Topic 5.7

Marine Landforms and Coastlines

Geography › Rocks, Erosion and Landforms · Topic 5.7

Marine landforms are shaped by the erosion and deposition of waves, currents and tides along coasts. Erosional forms include sea cliffs, wave-cut platforms, caves, arches and stacks, and depositional forms include beaches, spits, bars, tombolos and lagoons. Coastlines are classed as emergent or submergent, with rias and fjords as drowned coasts, and about a third of India's coast is eroding.

6 min read · UPSC Prelims · Geography · Rocks, Erosion and Landforms

At a Glance

Erosional Cliff, wave-cut platform, cave, arch, stack
Depositional Beach, spit, bar, tombolo, lagoon
Submerged coast Ria, fjord, Dalmatian coast
Emerged coast Raised beach, marine terrace
Indian examples Chilika, Pulicat, Kerala backwaters
Exam link UPSC Prelims, Geography: Rocks, Erosion and Landforms

Where Does It Fit?

This is the seventh topic of the category on rocks, erosion and landforms. The previous topic covered karst and the cycle of erosion, and this one describes landforms made by the sea. The next topic covers glacial landforms. Previous topic: Karst Landforms and the Cycle of Erosion. Next topic: Glacial Landforms.

How Do Waves Shape Coasts?

In short: waves cut back rocky coasts and build up sandy ones.

The coast is the zone where land and sea meet, and the features shaped there are called marine landforms. Waves, driven by wind, are the main agent of change, along with tides and currents. Waves erode by hydraulic action, in which the force of water and compressed air in cracks breaks rock, by abrasion, in which rocks and sand carried by the waves scrape the cliff, by attrition, in which carried pebbles wear each other down, and by solution of soluble rock. Waves are of two kinds. Destructive waves are high and steep, with a strong backwash, and they erode the beach. Constructive waves are low and long, with a strong swash, and they build up the beach. The behaviour of the sea is discussed in Ocean Currents – Causes and Types and Tides and Their Types.

What Are the Erosional Landforms of Coasts?

Sea cliffs: steep rock faces cut back by wave action at their base; they retreat inland.
Wave-cut notch and platform: a notch is cut at the base of the cliff and, as the cliff collapses and retreats, a gently sloping rock surface, the wave-cut platform, is left at the foot.
Headlands and bays: where hard and soft rocks alternate along the coast, soft rock erodes into bays and the hard rock stands out as headlands.
Sea caves, arches, stacks and stumps: waves enlarge cracks into caves; if a cave cuts through a headland it forms an arch; when the arch roof falls, a detached rock pillar, a sea stack, is left, and it is eroded down to a stump. The Twelve Apostles of Victoria, Australia, are well-known stacks.
Blowholes: vertical shafts in sea caves through which spray is forced out.

The white cliffs of Dover in England are made of chalk, a sedimentary rock described in Sedimentary Rocks.

What Are the Depositional Landforms of Coasts?

Landform	Description
Beach	Deposit of sand and shingle between low and high water levels
Longshore drift	Waves arriving obliquely move sediment along the shore in a zig-zag, shifting material in one direction
Spit	A ridge of sand or shingle attached to land at one end and extending into open water, as where the coast changes direction
Bar and barrier island	A ridge across a bay mouth; if it seals the bay it forms a lagoon; barrier islands lie parallel to the coast
Tombolo	A bar that links an island to the mainland
Lagoon	Shallow water body separated from the sea by a bar or spit
Sand dunes	Wind-blown sand piled behind beaches

India has famous lagoons: Chilika in Odisha, the largest brackish water lagoon in Asia and a Ramsar site; Pulicat on the border of Andhra Pradesh and Tamil Nadu; and the backwaters (kayals) of Kerala, such as Vembanad, which are shore lagoons connected by canals.

What Are Emergent and Submergent Coasts?

The shape of a coast also depends on changes in the relative level of land and sea. Submergent coasts form when the land sinks or the sea level rises, drowning river valleys and glacial valleys. A ria is a drowned river valley, with a branching, irregular shape, as in southwest Ireland, Sydney Harbour and the Konkan coast, and a fjord is a deep, steep-sided drowned glacial valley, as in Norway, Chile, Greenland and New Zealand. A Dalmatian coast, as in Croatia, has islands parallel to the coast formed from drowned mountain ridges. Emergent coasts form when the land rises or the sea falls, leaving raised beaches, marine terraces and old cliffs above the present sea level, as along parts of the Tamil Nadu and Gujarat coasts. India’s coast is mainly smooth and has few indentations, apart from places such as the Gulf of Khambhat, as in Coastline of India – Emergence and Submergence.

How Serious Is Coastal Erosion in India?

Assessments by the National Centre for Coastal Research, which covered about 6,900 km of the Indian coastline for 1990 to 2018, found that roughly one-third, about 33.6 per cent, of the coast was eroding, about 27 per cent was accreting and about 40 per cent was stable. Causes include rising sea level, stronger storms, sand mining, dams that reduce sediment supply, and building on the shore. Protection includes sea walls, groynes, artificial beach nourishment, restoration of mangroves, shelterbelts and the Coastal Regulation Zone rules, which restrict construction near the shore. As an analytical point, hard structures such as sea walls often shift erosion to neighbouring beaches, which is why shoreline management plans increasingly favour natural buffers such as mangroves and dunes.

Why Is This Topic Important?

Landforms Cliffs to stacks
Lagoons Indian lagoons often asked
Erosion Current coastal management issue

What Came Next?

The next topic describes glacial landforms. Next topic: Glacial Landforms.

Key People and Terms

Wave-cut platform Rock surface left by cliff retreat
Sea stack Isolated rock pillar
Longshore drift Movement of sediment along the coast
Spit Sand ridge attached at one end
Ria Drowned river valley
Fjord Drowned glacial valley

Exam Corner

Points to Remember

Cliff to cave to arch to stack to stump.
Chilika is the largest brackish lagoon in Asia.
A ria is a drowned river valley; a fjord is a drowned glacial valley.
Constructive waves build beaches.
About one-third of India’s coast is eroding.

Do Not Confuse With

Ria and fjord River versus glacial valley
Spit and tombolo Attached at one end versus linking an island
Swash and backwash Wave up the beach versus water down

Memory Hook

Cave, arch, stack, stump: the cliff retreats; sand, spit, bar and lagoon: the beach advances.

Mains Angle

Questions ask for coastal landforms and management. These points add depth.

Processes Erosion by waves, deposition by drift
Coasts Emergent and submergent
Management Natural buffers

A Question You May Face

An original practice question, not a past paper question.

“Describe the erosional and depositional landforms of coastal regions and discuss the problem of coastal erosion in India.”

How to Answer

Processes Waves and drift.
Landforms Erosional and depositional.
India Lagoons and erosion data.
Measures Buffers and CRZ.

GS Relevance

Prelims: coastal landforms, lagoons, rias and fjords, Chilika. GS Paper 1: geomorphology; GS Paper 3: coastal erosion.

Frequently Asked Questions

What are the erosional landforms of the coast?

Coastal erosional landforms include sea cliffs, wave-cut notches and platforms, headlands and bays, sea caves, arches, stacks and stumps, and blowholes. They are formed by hydraulic action, abrasion and attrition by waves, and develop best on rocky and exposed coasts.

How is a sea stack formed?

Waves enlarge cracks in a headland into a cave, and the cave may break through to form an arch. When the roof of the arch collapses, an isolated rock pillar, a sea stack, is left standing, and it is later worn down to a stump.

What is longshore drift?

Longshore drift is the movement of sand and shingle along a coast. Waves arrive at an angle and push the material up the beach in the direction of the wind, and the backwash carries it straight back down, so the sediment moves in a zig-zag along the shore.

What is the difference between a ria and a fjord?

A ria is a drowned river valley, with a branching irregular shape that deepens toward the sea, as in southwest Ireland. A fjord is a drowned glacial valley, deep and steep-sided, as in Norway and New Zealand. Both result from submergence of the coast.

Which is the largest lagoon in India?

Chilika Lake in Odisha is the largest brackish water lagoon in India and in Asia. It is separated from the Bay of Bengal by a sandbar, and is a Ramsar wetland that supports migratory birds and fisheries and the Irrawaddy dolphin.

PYQ Practice — Statement Analysis

1 A sea stack is left when the roof of a sea arch collapses.

True

It is later reduced to a stump.

2 A fjord is a drowned river valley.

False

It is a drowned glacial valley.

3 Chilika is a lagoon in Odisha.