Coastal Landscape Development - Changes in Relative Sea Levels

The relative position of land to sea is rarely consistent. The land can rise or fall over:

• very short time scales (tectonic uplift or downthrust)
• over the medium term measured in thousands of years (isostatic change) and
• over millions of years (continental separation and collision)

Similarly, sea level can rise or fall:

• relatively rapidly (thermal expansion)
• over the medium term (global climate change and melting ice) and
• over the very long term of millions of years (tectonic plate movement)

As different regions of coastline may be subject to different processes the consequence is that while some coastlines may be ‘emerging’ from the sea, others may be ‘submerging’ while others are changing in balance with any sea-level change so that there is no apparent alteration in the level of the sea.

Tectonic factors (short term)

A coastal region experiencing seismicity may experience land being shifted upwards (uplift) or downwards (downthrust) as geological pressures are released and adjacent rocks adjust at fault lines. As a result, there can be a rapid change in relative sea-level depending on whether land has been shifted up or down. Some notable land/sea level changes are:

• The Great Alaskan earthquake of 1964 with a magnitude of 9.2 (the second most powerful earthquake ever recorded) had a tilting effect such that the coastline 200 miles south-west of Anchorage was left 9.1 m. higher, while south-east of the city the coastline dropped by 2.4 m.
• Over 200 miles of the eastern coastline of Honshu, northern Japan, dropped by 0.6 m. as a result of the Tohoku earthquake of 2011, known for its destructive tsunami.

Isostatic changes (medium term)

Isostasy is the state of crustal material floating on top of more pliable mantle material beneath. The natural buoyancy of crustal rocks may see them depressed by the weight of accumulating ice on top, or released if long-existing surface ice subsequently melts. Isostatic sea level change most commonly arises from movement of the land relative to the sea.

• Submerging land: during periods of ice advance land masses in higher latitudes have been weighted down, sinking deeper into the mantle, a process called compression. (While this might be expected to raise sea level relative to the land, so much water was locked in ice sheets that sea level dropped at a faster rate than the land was depressed during the glacial maximum).
• Emerging land: at the beginning of the current warmer interglacial ice sheets started to rapidly melt after 18, 000 BCE and land masses began rising in a process known as decompression. This is thought to take place in two phases:
  • Increased buoyancy: the early stages of isostatic uplift occur as the weight of ice is suddenly released and, like a cork that has been pushed down, it ‘bounces back up’.
  • Mantle readjustment: a more extended phase of uplift (that is still going on in northern Scotland as it continues to rise after the last ice advance) as viscous mantle that had been forced away from beneath the depressed crust very slowly flows back to its former location.

Key point: isostatic factors are specific to particular regions of coastline while eustatic factors operate globally across the world’s oceans. As a consequence, some coastlines may be emerging while at the same time some are submerging, and others achieving a state of dynamic equilibrium with a changing sea level.

Historic sea level changes

• 18 000 BCE the global sea level was around 120 m below current levels as so much water was locked up in ice sheets and glaciers.
• Sea levels rose very rapidly as ice sheets melted with as much as a 10 m rise in just 500 years after 17,000 BCE. (equivalent to 20 mm per year on average)
• A slower general rise was punctuated by pulses of much faster melting and sea level rise. The pulse of 6 000 BCE is thought to have added 6.5 m in less than 140 years and finally separated the British Isles from the European continent.
• Sea level stabilised around 1000 BCE until the 19^th century. Since then it has been rising, with an average rate of 1.8 mm per year during the 20^th century.
• Projections of likely sea level rise by 2100 range from a low estimate of 0.28 m to a high of 0.98 m on 1990 baseline level, building on the current 3.4 mm./yr rise