Eustatic and isostatic sea-level change, coastlines of emergence and submergence, and the implications of contemporary sea-level rise.

What this dot point is asking

WJEC wants you to distinguish eustatic (global, sea-driven) from isostatic (local, land-driven) sea-level change, explain how each produces coastlines of emergence and submergence with their characteristic landforms, and assess the implications of contemporary sea-level rise for people. Strong answers keep the two mechanisms clearly separate, link each to named landforms, and ground the present-day discussion in a located Welsh or UK example such as Fairbourne.

The answer

Eustatic and isostatic change

During a glacial, water is locked up in ice sheets, so the eustatic sea level falls worldwide, while the enormous weight of ice pushes the crust down (isostatic depression) beneath the ice. When the ice melts, the eustatic sea level rises quickly as meltwater returns to the oceans, but the crust rebounds only slowly over thousands of years (isostatic rebound or recovery). Because the two processes act at different rates, the relative sea level at any coast is the net result of both. Northern Britain, once buried under thick ice, is still rebounding and so tends toward emergence, while southern Britain, never as heavily loaded, is sinking and tends toward submergence.

Coastlines of emergence

Where the land rises relative to the sea, former coastal landforms are stranded above the modern shore. Raised beaches are old wave-cut platforms and beach deposits now perched above the high-tide line, often backed by a relict (abandoned) cliff with a degraded wave-cut notch, fossil caves and arches no longer reached by the waves. The west coast of Scotland (Islay, the Isle of Arran) shows classic flights of raised beaches at several heights, each marking a former stillstand of relative sea level.

Coastlines of submergence

Where the sea rises relative to the land, valleys and lowlands are drowned. A ria is a drowned former river valley, branching and shallowing inland, with a dendritic plan, as along the rias of south-west England and Milford Haven in Pembrokeshire. A fjord is a drowned former glacial trough: deep, steep-sided and U-shaped, with a shallow threshold of moraine at its mouth, as in western Norway and the sea lochs of north-west Scotland. A Dalmatian coast forms where submergence drowns valleys running parallel to the coast, leaving long offshore islands, as in Croatia.

Examples in context

Example 1. Raised beaches of western Scotland (emergence). The Isle of Arran and the coasts of Islay and Jura display flights of raised beaches at heights of roughly $8$, $15$ and $30$ m, each a former wave-cut platform now lifted above the sea by isostatic rebound following the melting of the Scottish ice. Behind them stand relict cliffs with fossil caves cut when the sea reached that level. These features are the classic UK example of an emergent coastline and show how the land, freed of its ice load, has risen faster than the post-glacial eustatic rise of the sea.

Example 2. Contemporary sea-level rise at Fairbourne, Gwynedd (Wales). Fairbourne is a low-lying village on the Mawddach estuary built largely below the level of spring high tides and protected by a sea wall and embankments. Rising sea levels and increasing storm-surge risk mean the local authority has identified it as a candidate for managed realignment, with defences potentially decommissioned and the village relocated later this century, making its residents among the first in Britain to face planned retreat. Fairbourne is the leading Welsh case study for the human implications of contemporary sea-level rise and the difficult trade-offs between defending, adapting and retreating.

Try this

Q1. Define the term eustatic sea-level change. [2 marks]

• Cue. A worldwide change in sea level caused by a change in the volume of water in the oceans, mainly through the growth or melting of ice sheets and thermal expansion of the water.

Q2. Explain how a fjord forms. [3 marks]

• Cue. A glacier erodes a deep, over-deepened U-shaped trough; after the last glacial, the eustatic sea level rises and drowns the trough, leaving a deep, steep-sided inlet with a shallow moraine threshold at its mouth.