Sea Level Change

Sea level change is one of the most significant factors shaping coastal landscapes over long time periods. It determines whether coastlines advance seaward or retreat landward, and it creates distinctive suites of emergent and submergent landforms. At A-Level, you must understand the causes of sea level change, distinguish between eustatic and isostatic processes, and evaluate the implications of contemporary and future sea level rise.

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Types of Sea Level Change

There are two fundamentally different mechanisms by which relative sea level changes:

Eustatic Change

Key Definition: Eustatic sea level change is a global change in the volume of water in the oceans, causing the sea surface to rise or fall uniformly across the world.

Causes of eustatic change include:

Cause	Mechanism	Effect on Sea Level	Timescale
Glacial/interglacial cycles	During ice ages, water is locked up in ice sheets; during interglacials, ice melts and returns water to the oceans	Fall during glacials (up to -120 m at Last Glacial Maximum ~20,000 years ago); rise during interglacials	Thousands to tens of thousands of years
Thermal expansion	As ocean water warms, it expands in volume (thermosteric effect)	Rise — currently contributing approximately 1.4 mm/year to global sea level rise (IPCC AR6, 2021)	Decades to centuries
Tectonic changes	Changes in the volume of ocean basins due to sea floor spreading rates and volcanic activity on mid-ocean ridges	Rise when spreading rates increase (ridges displace more water); fall when rates decrease	Millions of years
Sedimentation	Accumulation of sediment on the ocean floor reduces basin volume	Very gradual rise	Millions of years

Isostatic Change

Key Definition: Isostatic sea level change is a local or regional change in the level of the land relative to the sea, caused by loading or unloading of the Earth's crust.

Causes of isostatic change include:

Cause	Mechanism	Effect on Relative Sea Level	Location
Post-glacial rebound	After ice sheets melt, the crust slowly rises as it is freed from the weight of ice	Fall (land rises faster than sea)	Scotland, Scandinavia, Canada
Glacial depression	Under the weight of ice sheets, the crust is pushed down into the mantle	Rise (land sinks)	During glacial periods
Peripheral forebulge collapse	Areas adjacent to ice sheets bulged upward during glaciation; they sink as the ice melts and the forebulge collapses	Rise	Southern England, Netherlands
Sediment loading	Thick sediment deposits (e.g., at river deltas) push the crust down	Rise	Nile Delta, Mississippi Delta, Ganges-Brahmaputra Delta
Tectonic activity	Earthquakes and volcanic activity can raise or lower the land	Variable	Tectonically active margins

The Crucial Distinction

The key difference is that eustatic change is global (affects all coastlines equally), while isostatic change is local/regional (affects specific areas differently). The relative sea level experienced at any coastline is the net result of both eustatic and isostatic changes operating simultaneously.

graph TD
    A["Relative Sea Level Change at a Coastline"] --> B["Eustatic Component (global)"]
    A --> C["Isostatic Component (local)"]
    B --> D["Ice volume changes"]
    B --> E["Thermal expansion"]
    B --> F["Tectonic basin changes"]
    C --> G["Post-glacial rebound"]
    C --> H["Sediment loading"]
    C --> I["Tectonic uplift/subsidence"]

Example — UK differential sea level change: Scotland is currently experiencing isostatic uplift (post-glacial rebound) at rates of up to 1-2 mm/year, which partially or fully offsets eustatic sea level rise. By contrast, south-east England is experiencing isostatic subsidence (forebulge collapse) at approximately 1 mm/year, which adds to eustatic rise. The net result is that relative sea level is rising much faster in London (~2.5 mm/year) than in Edinburgh (~0.5 mm/year).

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Emergent Coastlines

Where relative sea level falls (either through land uplift or sea level drop), previously submerged features are exposed above current sea level. These are called emergent (or uplifted) coastlines.

Raised Beaches

A raised beach is a former beach and wave-cut platform now found above the current high tide level. They provide powerful evidence of past sea levels.

• In Scotland, raised beaches are found at heights of 8-15 m above present sea level, dating from the post-glacial period (approximately 6,000-10,000 years ago)
• The Isle of Arran has particularly well-preserved raised beaches, with the old cliff line, wave-cut platform and beach deposits clearly visible above the current shoreline
• The Gower Peninsula, South Wales, has raised beaches at approximately 8 m above sea level, containing shells and rounded pebbles identical to those on the modern beach below
• Raised beaches may have a relic cliff behind them — a former cliff face that has been weathered since it was removed from wave action

Marine Terraces

Marine terraces are broad, flat platforms cut by wave erosion at a former sea level, now raised above the present coast. They are essentially raised wave-cut platforms.

• Found extensively along the west coast of the Americas, where tectonic uplift has raised multiple terraces to different heights
• On the Palos Verdes Peninsula, California, a series of 13 marine terraces at heights up to 400 m above sea level record episodic uplift over the past 1.2 million years (Muhs et al., 2006)

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Submergent Coastlines

Where relative sea level rises, the sea floods low-lying land and coastal valleys, creating distinctive features:

Rias

A ria is a drowned river valley — a former fluvial valley that has been partially submerged by rising sea level. Key characteristics:

• V-shaped cross-section (reflecting the original river valley shape)
• Deepens and widens towards the sea (unlike fjords, which have a shallow bar at the mouth)
• Water depth increases gradually from the head to the mouth
• The river that formed the valley still flows into the ria at its head

Key Definition: A ria is an inlet formed by the partial submergence of a river valley due to a rise in relative sea level. It has a progressively widening and deepening profile towards the sea and a characteristically V-shaped cross-section.

Examples:

• The rias of south-west England — the Fal, Helford, Dart and Kingsbridge estuaries in Devon and Cornwall are classic rias, drowned during the post-glacial sea level rise (the Flandrian transgression, approximately 6,000-10,000 years ago)
• The Rias Baixas of north-west Spain (Galicia) — a series of large rias that give the coastline its distinctive indented appearance
• Sydney Harbour, Australia — a drowned river valley (the Parramatta River) forming one of the world's finest natural harbours

Fjords