3.1.3.3 - Sea level and emergent or submerged coasts

Ocean change and land movement

Relative sea level is the height of the sea compared with the local land, so it can change even when global ocean volume does not.

Eustatic change concerns global sea level, principally changes in ocean-water volume or basin capacity. Warming seawater expands; melting land ice adds water. Floating sea-ice melt is not equivalent to adding the same volume of land ice. Isostatic change involves vertical adjustment of the crust to changing loads: land depressed by ice can rise after unloading, while formerly raised peripheral areas can subside. Tectonic change involves crustal movement associated with plate processes, including sudden earthquake uplift or subsidence and longer-term deformation.

In an original illustrative ten-year example, the sea surface rises 40 mm while land rises 20 mm. Relative sea level rises 20 mm. If land rose 60 mm instead, relative sea level would fall 20 mm even with ocean rise. A tide-gauge record is local relative change; it is not automatically a global eustatic record.

GeoNet's evidence for the 14 November 2016 Kaikōura earthquake identifies local uplift of about 2.1–2.4 m at Half Moon Bay, New Zealand. That exposed previously marine surfaces, but it is not a uniform uplift height for the approximately 110 km affected coastal stretch. Locate the measurement before extending its meaning.

Change over the last 10,000 years

Holocene coastal change combines post-glacial ocean rise with geographically uneven land adjustment.

At the start of the last 10,000 years, melting remaining continental ice continued to raise ocean level rapidly compared with much of the later Holocene. Many continental-shelf areas flooded, shorelines moved landward and former valleys became marine inlets. As large ice sheets diminished, the rate generally slowed, while regional isostatic and tectonic changes continued. The much larger rise since the Last Glacial Maximum began earlier than this 10,000-year window: do not present that entire change as occurring within it.

The BGS southern North Sea reconstruction published in 1992 gives a dated regional example of progressive Holocene flooding. Its reconstructed levels combine regional evidence and uncertainties; they are not a present global tide-gauge series. Scotland illustrates land rebound after ice loss: uplift can preserve raised beaches even while global ocean level rises. Elsewhere, subsidence or limited uplift produces a different relative history.

When reading a curve, check whether years are calendar years or years before present, whether the vertical axis is global or relative level, the datum, geographic area and uncertainty. A smooth reconstruction does not mean change was steady at every location. Past shorelines and deposits provide evidence with dating and preservation limits; recent measured series have different resolution. Compare rates only over stated, comparable intervals.

Emergent shores and raised features

A coast emerges when relative sea level falls, leaving former shore features above the reach of ordinary waves.

A raised beach is former beach sediment stranded above the modern shoreline. A raised marine platform is a rock surface formerly shaped near sea level and now elevated. The platform may retain abandoned cliffs, notches or caves inland. A present wave-cut platform is still in the current wave/tidal zone; a raised platform records a former relative level. Distinguish rock surfaces from sediment deposits when using them as evidence.