Wave Erosion and Deposition
Senior 6 · Geography
Wave Erosion and Deposition
Lesson 4 of 5
25 min
Fishing boats working the Dogger Bank, in the North Sea between Britain and Denmark, sometimes bring up bones of mammoths and tools made by people in their nets. During the last ice age, so much water was locked up in ice sheets that sea level stood roughly 120 m lower than today, and the floor of the southern North Sea was dry land, now called Doggerland. As the ice melted, the sea rose and flooded it, between about 10,000 and 7,000 years ago. Sea level has never been fixed. This lesson explains why it changes, the landforms that record those changes, and what today's rise means for coasts and the people who live on them.
Definition
Eustatic change: a worldwide change in sea level caused by a change in the volume of water in the oceans or in the capacity of the ocean basins. Example: the rise of about 120 m as the ice sheets melted after the last ice age.
Isostatic change: a local change in the level of the land relative to the sea, caused by the crust sinking under a load or rising when the load is removed. Example: Scandinavia is still rising after losing its ice sheet.
Relative sea level: the level of the sea measured against the land at a particular place; it is the combined result of eustatic and isostatic change.
| Cause | Effect on sea level |
|---|---|
| Glacio-eustasy: water stored as ice on land during an ice age, released when the ice melts | Fall of about 120 m at the height of the last ice age; rise as the ice melted |
| Thermal expansion: sea water expands as it warms | Rise, at present one of the two largest causes along with melting ice |
| Tectono-eustasy: changes in the volume of the ocean basins, for example when fast sea-floor spreading builds large, high mid-ocean ridges | Rise or fall over millions of years |
| Sedimentation: sediment filling the ocean basins | Very slow rise |
Worked example 1: if Greenland melted
Problem. The Greenland ice sheet holds about of ice, with density . The oceans cover about . Estimate the eustatic rise if it all melted (ignore the small change in ocean area).
Solution. Volume of melt water .
This matches published estimates of about 7.4 m. Only a fraction of the ice would melt in the coming centuries, but even a few per cent of it means tens of centimetres of extra rise.
The crust floats on the denser, slowly flowing mantle beneath it, rather as a block of wood floats in water. Load the crust and it sinks; unload it and it rises. This balance is isostasy.
Figure 10: isostatic depression under an ice sheet and rebound after it melts.
During an ice age the weight of an ice sheet presses the crust down by hundreds of metres. When the ice melts, the sea floods the depressed land at first; then, over thousands of years, the crust rises again (isostatic rebound), lifting beaches that formed at the old shoreline high above the sea. Rebound continues today around the Gulf of Bothnia in Scandinavia and Hudson Bay in Canada, typically at about 1 cm a year or less. Isostatic sinking also happens under other loads: the weight of sediment in a large delta slowly lowers it, which adds to the flood risk there. Tectonic movement can also raise or lower a coast, gradually or suddenly in an earthquake.
Worked example 2: how far does ice press the crust down?
Problem. An ice sheet 3,000 m thick (density ) rests on crust over mantle of density . When isostatic balance is reached, how far is the crust pressed down?
Solution. At balance, the weight of the ice equals the weight of mantle pushed aside, so :
The crust sinks by about 830 m, more than a quarter of the ice thickness. That is why rebound can lift old shorelines tens to hundreds of metres once the ice has gone.
Worked example 3: relative sea level
Problem. At a northern coast (illustrative figures) the land is rising isostatically at 8 mm a year while the sea is rising eustatically at 3.5 mm a year. At a delta coast the land is sinking at 5 mm a year under its sediment and the sea is rising at the same 3.5 mm a year. Find the change in relative sea level at each, over a century.
Solution. Northern coast: , so relative sea level falls by per century: the coast is emerging. Delta: , a rise of 0.85 m per century: the coast is being submerged more than twice as fast as the global rise alone would cause.
Definition
Submergent coast (coast of submergence): a coast where relative sea level has risen and drowned the former land. Emergent coast: a coast where relative sea level has fallen, so that former shore features are now above the reach of the sea.
Ria: a river valley drowned by a rise in sea level. Fjord: a glacial trough drowned by the sea. Raised beach: an old beach left above the present shore by a fall in relative sea level.
Figure 11: cross-sections of a ria and a fjord.
| Feature | How it forms | Characteristics and example |
|---|---|---|
| Ria | Sea level rises and floods a river valley in an unglaciated area | V-shaped in cross-section, deepening gradually seawards, with a winding plan and branching creeks. The term comes from the rías of Galicia in north-west Spain, such as the Ría de Vigo |
| Fjord | Sea level rises and floods a glacial trough | U-shaped, steep-walled and very deep, often with a shallower threshold (rock bar or moraine) near the mouth where the glacier thinned. The Sognefjord in Norway runs 205 km inland and reaches 1,308 m deep |
| Dalmatian coast | Sea level rises along a coast where ridges and valleys run parallel to the shore | Long, narrow islands parallel to the mainland, separated by sounds, as along the coast of Croatia |
| Estuary | The mouth of a river drowned by the post-glacial rise | A wide, funnel-shaped inlet where fresh and salt water mix and mudflats form |
Figure 12: an emergent coast with a raised beach and a relict cliff.
Where the land has risen faster than the sea, the old shoreline is left high and dry. A raised beach is a flat terrace of old beach sand and shingle, often with shells, standing above the present beach. Behind it a relict cliff (abandoned cliff) may still show old caves, notches and arches, now covered in vegetation and weathering instead of being eroded by waves. Raised beaches are common on the west coast of Scotland, which rose as its ice sheet melted. On coasts lifted by tectonic movement, a staircase of marine terraces can record several old sea levels.
The measurements show that global mean sea level rose by about 0.20 m between 1901 and 2018, and that the rate has been rising: about 1.3 mm a year from 1901 to 1971, but 3.7 mm a year from 2006 to 2018 (IPCC, 2021).
| Effect | Why it happens |
|---|---|
| More frequent and deeper coastal flooding | Storm surges and high tides start from a higher level |
| Faster coastal erosion | Waves break closer to the cliff or dune and beaches lose sediment |
| Salt-water intrusion | Sea water moves into coastal aquifers and river mouths, spoiling drinking and irrigation water |
| Loss of wetlands and mangroves | They drown where they cannot build upwards or move inland ("coastal squeeze" against sea walls) |
| Threat to low-lying states and deltas | Atoll nations such as Tuvalu and the Maldives, and densely farmed deltas such as the Nile Delta, lie only a few metres above sea level |
Rwanda has no sea coast, but a rising sea still matters to it: its overseas trade passes through ports on the Indian Ocean coast, and the countries it trades with face the costs of protecting their coasts.
Figure 13: the main features of the ocean floor.
Warning
Common mistakes
Tip
Exam technique. Begin any sea-level question by asking which moved, the sea or the land, and in which direction; then name the resulting coast (submergent or emergent) and give two landforms with an example each. In calculations, add the eustatic and isostatic rates with their signs.
Launchpad Learning: eustatic and isostatic sea-level change at A-level. As you watch, sort each cause into "the sea moves" or "the land moves".
Practice
Summary
Sources and credits
Photos: Domaio - Ría de Vigo (Gabriel González, CC BY 2.0); Sognefjord-Norway-April-2011 (Acediscovery, CC BY 4.0); Raised beach, Jura (Tom Richardson, CC BY-SA 2.0); all via Wikimedia Commons.
Figures 10–13, the worked examples and the exercises were drawn and written by Ganzaa.
Video: "Coasts: Eustatic and Isostatic Sea Level Change | A-level Geography", Launchpad Learning, embedded from YouTube.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
