IB Geography B.2.3 Advancing and Retreating Coastlines Notes

Relative Sea Level Determines Coastal Advance or Retreat

1. Relative sea level describes the height of the sea compared with the adjacent land.
2. A coastline advances seaward when relative sea level falls or when deposition builds land faster than water level rises.
3. A coastline retreats landward when relative sea level rises or when erosion removes land faster than deposition replaces it.
4. The position of the shoreline therefore depends on changes in both water level and land elevation.
5. A global rise can coincide with local stability or fall where land uplift is sufficiently rapid.

Emergent and submergent coasts record opposite changes

1. An emergent coast develops where relative sea level falls and former shoreline features are raised above present wave action.
2. A submergent coast develops where relative sea level rises and the sea floods a former terrestrial landscape.

1. Emergent features include raised beaches and relict cliffs, while submergent features include drowned glacial valleys such as fjords.

Eustatic Change Alters Ocean Water Level Globally

Water volume can rise or fall

1. Growth of land-based ice stores water outside the oceans and lowers global sea level.
2. Melting land ice returns water to the oceans and raises global sea level.
3. Warming seawater expands through thermal expansion, increasing ocean volume without adding water mass.
4. Cooling seawater contracts and can contribute to a fall in global mean sea level.

Ocean-basin capacity can also change

1. Tectonic processes alter the volume and shape of ocean basins over geological timescales.
2. Rapid seafloor spreading produces warmer, more buoyant mid-ocean ridges that occupy more basin space.
3. A smaller basin capacity displaces water upward and raises eustatic sea level.
4. Slower spreading and cooling can increase basin capacity and lower sea level.

Isostatic Adjustment Moves the Land

Ice loading depresses the crust

1. A thick ice sheet adds mass and pushes the lithosphere downward into the more ductile mantle.
2. Mantle material is displaced away from the loaded area, and peripheral regions may rise as a forebulge.
3. Relative sea level rises where the land subsides beneath the ice load.

Unloading produces delayed rebound

1. When an ice sheet melts, the load on the crust decreases.
2. Mantle material flows back and the depressed crust rises towards equilibrium.
3. This post-glacial rebound can continue for thousands of years because mantle flow is slow.
4. Relative sea level falls where uplift outpaces eustatic sea-level rise.
5. Former beaches and cliffs can therefore be stranded above the modern shoreline.

Raised Beaches Preserve an Abandoned Shoreline

Falling relative sea level isolates beach material

1. Waves first deposit sand, shingle or shells within the active shoreline zone.
2. Land uplift or a fall in ocean level moves the former beach beyond the reach of ordinary waves.
3. A new active shore develops at a lower elevation while the old deposit remains inland.
4. The raised deposit may retain rounded clasts, shells, berms or other evidence of marine action.

Relict cliffs lose direct marine attack

1. A relict cliff is a former sea cliff left inland or above present wave action.
2. It may retain an abandoned notch, cave or platform at the elevation of the former shoreline.
3. Marine erosion becomes inactive after emergence, but weathering and mass movement can continue to modify the face.
4. A modern beach or low cliff may form seaward of the relict feature.

Fjords Form When the Sea Floods Glacial Valleys

Glacial erosion creates the valley before submergence

1. A valley glacier deepens and widens a pre-existing river valley through abrasion and plucking.
2. The glacier cuts a steep-sided U-shaped trough that may extend below present sea level.
3. Erosion is often greatest in the main valley, while tributary glaciers leave shallower hanging valleys.
4. A rock threshold or moraine may remain near the mouth where erosion was weaker or deposition occurred.

Deglaciation opens the valley to the sea

1. Warming causes the glacier to retreat and releases meltwater into the oceans.
2. Eustatic sea-level rise and local land motion determine when seawater enters the valley.
3. The resulting fjord is narrow, deep and steep-sided, often with a shallower sill near its mouth.
4. Hanging valleys can create waterfalls down the fjord walls.

Advancing and Retreating Coasts Have Different Profiles

An advancing coast may expose new land

1. Relative sea-level fall shifts the shoreline seaward across a gently sloping surface.
2. Raised beaches and relict cliffs form a stepped profile above the present shore.
3. New terrestrial habitats may colonize land that is no longer flooded.
4. Rivers can incise into the newly exposed surface as their base level falls.

A retreating coast floods or erodes landward

1. Relative sea-level rise shifts the shoreline inland across low-lying ground.
2. Former valleys, river mouths and glacial troughs may become estuaries, rias or fjords.
3. Wave attack can reach higher on a cliff and accelerate erosion where sediment cover is limited.
4. Coastal habitats may migrate inland where relief and human structures allow space.

Evidence Separates the Processes

1. Similar shoreline movement can result from different combinations of eustatic change, isostatic adjustment, erosion and deposition.
2. Raised marine deposits provide evidence of former sea level relative to the land.
3. Submerged U-shaped valleys indicate glacial erosion followed by marine flooding.
4. Tide-gauge measurements record sea level relative to the land at one location.
5. Satellite altimetry measures changes in sea-surface height, while geodetic measurements detect vertical land motion.

Scale and Rate Control the Coastal Outcome

1. Eustatic change operates across connected oceans, while isostatic adjustment varies spatially with former loading and mantle response.
2. A steep coast may show little horizontal movement for a given vertical rise, while a low-gradient coast can retreat far inland.
3. Abundant sediment can build beaches, barriers or deltas that temporarily offset inundation.
4. Storms can produce rapid shoreline change superimposed on much slower sea-level trends.
5. The present coast therefore records processes acting over event, historical and geological timescales.