The concepts of eustatic and isostatic sea level changes

Sea levels are influenced by both global and local factors, leading to significant changes in coastal landscapes over time.

Defining eustatic and isostatic changes

• Eustatic change - Refers to a worldwide alteration in sea level, often linked to the growth or melting of ice sheets. During the peak of glacial periods around 18,000 years ago, sea levels were approximately 100-150 metres below the current level.
• Isostatic adjustment (isostasy) - Describes a localised change in land level relative to the sea. This can occur due to tectonic uplift or the removal of ice weight after melting. Regions like parts of Scandinavia and Canada are still rising at rates of up to 20 mm per year.

The sequence and interaction of sea level changes

Sea level changes follow a cyclical pattern influenced by climatic shifts and ice dynamics.

The cycle of sea level change

1. Cooling climate and glacial advance - As temperatures drop, glaciers and ice sheets expand, locking water on land and causing a eustatic fall in sea levels.
2. Increased ice weight - The growing thickness of ice presses down on the land, leading to isostatic lowering in affected areas.
3. Warming climate and ice melt - Rising temperatures cause ice to melt, releasing water back into the oceans and resulting in a eustatic rise in sea levels.
4. Land rebound - As ice melts and its weight is removed, the land rises isostatically due to the release of pressure.

Variations in sea level impact

While eustatic changes affect sea levels globally, the actual rise or fall observed in specific locations varies due to isostatic changes. This means that different places will have different relative rises and falls in sea level. Global warming (the enhanced greenhouse effect) is currently causing sea levels to rise, particularly threatening low-lying communities.

Advancing and retreating coastlines due to sea level changes

Coastal landscapes evolve as a result of sea level changes combined with processes like erosion and deposition.

Classification of coastlines (Valentin's classification)

• Advancing coastlines - These include emerged coastlines and coasts where deposition is rapid.
• Retreating coastlines - These include submerged coasts and coasts where the rate of erosion exceeds the rate of emergence/deposition.

Factors influencing coastal change

• Uplift - Leads to advancing coastlines by raising land above sea level. For example, a region experiencing 100 m of uplift may see significant emergence if erosion is minimal.
• Submergence - Results in retreating coastlines as land sinks below sea level. A submergence of 100 m can drown coastal features unless deposition balances the loss.
• Erosion - Reduces the effect of uplift, potentially causing a coastline to retreat even in areas of land rise.
• Deposition - Can counteract submergence, leading to an advancing coastline by building up sediment despite rising sea levels.

Features of emerged and submerged coastlines

Sea level changes leave distinct geological and geographical features on coastlines.

Characteristics of emerged coastlines

Emerged coastlines are formed when land rises above sea level due to isostatic rebound or tectonic uplift.

• Raised beaches - Such as the Portland Raised Beach in the UK.
• Coastal plains
• Relict cliffs - Such as those along the Fall Line in eastern USA.
• Raised mudflats - For example, the Carselands of the River Forth in Scotland.

Characteristics of submerged coastlines

Submerged coastlines occur when rising sea levels or sinking land causes coastal areas to be inundated.

• Rias - Such as the River Fal, drowned river valleys caused by rising sea levels or due to a sinking of the land.
• Fjords - Such as Milford Sound, New Zealand, and the Oslo Fjord, Norway, caused by drowning of U-shaped valleys.
• Fjards - "Drowned glacial lowlands".