Welcome to the Changing Coastline!

Have you ever stood on a beach and wondered if the spot where you're standing was once deep underwater? Or perhaps it was miles inland? Coastal landscapes are never "finished"—they are constantly being reshaped by the rising and falling of the sea. In this chapter, we will explore why sea levels change, the different timescales this happens on, and the incredible landforms left behind when the ocean moves.

1. The Two Big Players: Eustatic vs. Isostatic Change

Understanding sea level change can be tricky because sometimes the water is moving, and sometimes the land itself is moving! Geographers split these into two main types:

Eustatic Change (The Global Scale)

Eustatic change is a global change in sea level. This happens when the actual volume of water in the ocean changes, or when the shape of the ocean basins themselves changes.
Analogy: Imagine a bathtub. If you turn on the tap, the water level rises everywhere in the tub at once. That is eustatic change.

Main causes include:

  • Climate Change: When the world warms up, ice sheets on land melt, pouring more water into the sea.
  • Thermal Expansion: As water gets warmer, the molecules move more and take up more space, causing the sea level to rise.

Isostatic Change (The Local Scale)

Isostatic change is a local change in the level of the land. The land can actually sink or "bounce back" upwards.
Analogy: Imagine sitting on a foam sofa cushion. When you sit down, the cushion sinks (compression). When you stand up, it slowly pops back up to its original height (rebound).

Main causes include:

  • Post-glacial Rebound: During an ice age, the weight of massive ice sheets pushes the land down into the Earth's mantle. When the ice melts, the land slowly starts to rise back up.
  • Tectonic Activity: Earthquakes or volcanic activity can suddenly lift or drop sections of the coast.
Quick Review: Which is which?

Eustatic = Everywhere (Global water change).
Isostatic = Isolated (Local land change).

2. Timescales of Change

Sea level change doesn't just happen at one speed. It occurs across different timescales:

  • Long-term: Over thousands of years, such as the transition from a glacial period (ice age) to an interglacial period (warmer times like today).
  • Short-term: Coastal flooding and changes can happen rapidly due to storm surges or seasonal variations in wind and atmospheric pressure (this is explored further in chapter 2B.9).

3. Emergent Coastlines (When the Sea "Leaves")

An emergent coastline is formed when the sea level falls relative to the land. This can happen because of a eustatic fall in sea level or an isostatic rise in the land. As the water retreats, features that were once underwater are now "high and dry."

Key Emergent Landforms:

1. Raised Beaches: These are old beaches that are now located well above the current high-tide mark. You might see rounded pebbles and sand far inland from the modern shore.

2. Fossil Cliffs: These are ancient cliffs found behind a raised beach. They often show signs of past marine erosion (like wave-cut notches or sea caves) but are no longer reached by the sea. They are "relict" features.

4. Submergent Coastlines (When the Sea "Arrives")

A submergent coastline is formed when the sea level rises relative to the land. This "drowns" the existing landscape, creating very distinctive shapes.

Key Submergent Landforms:

1. Rias: These are drowned river valleys. When sea levels rise, the lower course of a river and its floodplains are flooded. From above, they look like a long, winding estuary with many "fingers" of water branching off. (Example: Many found in South Devon and Cornwall, UK).

2. Fjords: These are drowned glacial valleys. Unlike rias, fjords are very deep and have incredibly steep, U-shaped sides. They are often deeper than the sea outside them! (Example: Famous in Norway and New Zealand).

3. Dalmatian Coasts: These occur where the geological structure (folding) runs parallel to the coast. When the sea level rises, the valleys between the ridges are flooded, leaving a chain of long, narrow islands that run parallel to the mainland.

5. Why Does This Matter Today? (The Risk Factor)

Understanding these past changes helps geographers predict the future. Today, we are seeing a rapid eustatic rise in sea level due to human-induced climate change. This increases the risk of coastal flooding and coastal retreat.

Note: For more on how these changes affect people and how we manage them, check out the chapters on "Rapid coastal recession and coastal flooding risk" (2B.8/9) and "Coastal management" (2B.11/12).

Common Mistake to Avoid:

Don't confuse Rias with Fjords! Just remember: Rias are Rivers (winding and V-shaped origin); Fjords are Frozen (carved by ice, steep U-shaped sides).

Summary Key Takeaways

  • Eustatic change is a global rise or fall in the water level itself.
  • Isostatic change is a local rise or fall of the land level.
  • Emergent landforms (raised beaches, fossil cliffs) appear when the sea level drops relative to the land.
  • Submergent landforms (rias, fjords, Dalmatian coasts) appear when the sea level rises relative to the land.
  • Sea level change is a major driver of coastal risk in the 21st century.