Coastal Changes Driven by Sea Level Variations
- Sea level variations, caused by both global and local processes, are powerful forces that reshape coastal landscapes.
- These changes influence the position of coastlines, form unique landforms, and provide valuable records of Earth's dynamic history.
Understanding Sea Level Changes: Eustatic vs. Isostatic Processes
- Sea level changes occur due to two primary mechanisms:
- Eustatic changes, which are global,
- Isostatic changes, which are local.
- These processes interact to influence the relative position of land and sea, but they operate through distinct mechanisms.
- Fjords: Narrow, lengthened and steep marine gulf, which results from the invasion by the sea of a U-shaped valley dug by a glacier.
- Ria: Formed by the inundation of river valleys by seawater.
Eustatic Processes: Global Sea Level Changes
Definition
Eustatic changes
Eustatic changes refer to worldwide shifts in sea level caused by changes in the volume of water in the oceans or alterations in the size of the ocean basins.
- Ice Melt and Glacial Cycles. During glacial periods, vast amounts of water are locked up in ice sheets, causing global sea levels to fall.
- Thermal Expansion: Warmer temperatures also cause seawater to expand, increasing its volume. This process, known as thermal expansion, is a significant contributor to modern sea level rise linked to global warming.
Isostatic Processes: Local Land-Level Changes
Definition
Isostatic changes
Isostatic changes involve vertical movements of the Earth's crust relative to the sea.
- These movements are often caused by the addition or removal of weight on the land.
- Glacial Rebound: During glacial periods, the immense weight of ice sheets compresses the Earth's crust, causing it to sink.
- When the ice melts, this pressure is relieved, and the land begins to rise in a process known as isostatic rebound.
Parts of Scandinavia and Canada are still rising at rates of up to 20 mm per year due to the melting of glaciers that covered the regions thousands of years ago (in Pleistocene).
- Subsidence: In some areas, land may sink due to the accumulation of sediment or the extraction of groundwater and fossil fuels, leading to a relative rise in sea level.
Remember: Eustatic changes are global and affect the entire ocean, while isostatic changes are localized and depend on vertical land movements.
The Interaction of Eustatic and Isostatic Changes
- While eustatic changes affect sea levels globally, their local impact varies due to simultaneous isostatic adjustments.
- A region experiencing an isostatic rebound may see its coastline advance even as global sea levels rise.
- This interplay results in a complex and varied picture of coastal change worldwide.
- Consider the south coast of Britain, which has experienced both eustatic and isostatic changes.
- After the last Ice Age, melting ice caused global sea levels to rise (eustatic change), while the land in southern Britain sank under the weight of ice elsewhere (isostatic subsidence).
- This interaction has led to significant coastal submergence in the region.
Landforms Are Associated with Sea Level Changes
- Sea level variations, whether driven by eustatic or isostatic processes, leave behind distinctive coastal landforms.
- These features provide evidence of past environmental conditions and help us understand ongoing changes.
