B.2.3 Advancing and Retreating Coastlines Notes

Coastal Changes Driven by Sea Level Variations

1. Sea level variations, caused by both global and local processes, are powerful forces that reshape coastal landscapes.
2. 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

1. Sea level changes occur due to two primary mechanisms:
   1. Eustatic changes, which are global,
   2. Isostatic changes, which are local.
2. These processes interact to influence the relative position of land and sea, but they operate through distinct mechanisms.

Eustatic Processes: Global Sea Level Changes

1. Ice Melt and Glacial Cycles. During glacial periods, vast amounts of water are locked up in ice sheets, causing global sea levels to fall.
2. 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

1. These movements are often caused by the addition or removal of weight on the land.
2. Glacial Rebound: During glacial periods, the immense weight of ice sheets compresses the Earth's crust, causing it to sink.
3. When the ice melts, this pressure is relieved, and the land begins to rise in a process known as isostatic rebound.

1. 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.

The Interaction of Eustatic and Isostatic Changes

1. While eustatic changes affect sea levels globally, their local impact varies due to simultaneous isostatic adjustments.

Landforms Are Associated with Sea Level Changes

1. Sea level variations, whether driven by eustatic or isostatic processes, leave behind distinctive coastal landforms.
2. These features provide evidence of past environmental conditions and help us understand ongoing changes.

1. Relict Cliffs and Raised Beaches

1. When land rises relative to the sea due to isostatic rebound, former coastal features are left stranded inland.
   1. Relict Cliffs: These are steep, rocky cliffs that once faced the ocean but now stand far from the current shoreline.
      1. They often feature wave-cut notches and other erosion marks that hint at their marine origin.
   2. Raised Beaches: These are flat, sandy or pebbly areas that were once at sea level but are now elevated above it.
      1. Raised beaches are common in areas like Scotland, where isostatic rebound has lifted the land.

2. Fjords

1. These U-shaped valleys are typically found in regions with a history of glaciation, such as Norway or New Zealand.
2. Formation: During glacial periods, advancing glaciers eroded deep valleys into the landscape. As the glaciers melted and sea levels rose, these valleys were inundated by seawater, creating fjords.
3. Characteristics: Fjords are characterized by their steep walls, deep waters, and often branching arms.

Implications of Sea Level Changes

Sea level changes have profound implications for coastal environments and human settlements.

1. Ecological Impacts
   1. Rising sea levels can inundate coastal habitats, displacing species and altering ecosystems.
   2. Conversely, land uplift may expose new areas for colonization by terrestrial species.
2. Human Challenges
   1. Coastal communities face risks from both rising sea levels and land subsidence.
   2. Flooding, erosion, and saltwater intrusion into freshwater systems are growing concerns, particularly in low-lying areas.
3. Geological Insights
   1. Studying relict cliffs raised beaches, and fjords help geographers reconstruct past climates and predict future changes.
   2. These landforms serve as natural records of Earth's dynamic history.

Reflection and Broader Implication

1. Understanding coastal changes driven by sea level variations requires a careful examination of both global and local processes.
2. Eustatic changes, such as ice melt and thermal expansion, operate on a global scale, while isostatic adjustments, like glacial rebound, create localized effects.
3. Together, these processes shape the coastline, leaving behind features like relict cliffs, raised beaches, and fjords.