Eustatic, isostatic and tectonic sea level change

In this section you will learn about:

• eustatic, isostatic and tectonic sea level change
• how sea levels have changed over the last 10,000 years
• the causes of long-term and short-term sea level change
• how sea level change varies between locations
• evidence for past and present sea level change

What is sea level change?

Sea level change refers to changes in the height of the sea relative to the land.

Sea levels are constantly changing over different timescales:

• daily changes caused by tides
• short-term changes caused by storms
• long-term changes linked to climate and tectonic activity
• Over the last 10,000 years, sea levels have generally risen as the Earth has warmed since the end of the last Ice Age.

Evidence for past sea level change

Evidence for former sea levels includes:

• raised beaches
• submerged forests
• marine sediments found inland
• former coastal features located above current sea level
• historical and archaeological evidence

Scientists also use:

• ice cores
• sediment cores
• coral records
• radiocarbon dating

to reconstruct past sea levels.

Sea levels during the last Ice Age

During the last glacial period:

• huge ice sheets covered large areas of the Northern Hemisphere
• large quantities of water were stored on land as ice

As a result:

• global sea levels were much lower than today
• some estimates suggest sea levels were around 120 metres lower than present levels

At this time, Britain was connected to mainland Europe the North Sea did not fully exist in its present form

Sea level rise after the Ice Age

As global temperatures increased:

• glaciers and ice sheets melted
• meltwater flowed into the oceans
• sea levels rose rapidly

Most of this sea level rise occurred between about 15,000 and 8,000 years ago. The rate of rise slowed as the major ice sheets disappeared.

Eustatic sea level change

Eustatic change occurs when the volume of water in the oceans changes globally. This means sea levels rise or fall worldwide.

Causes of eustatic sea level rise

Melting ice sheets and glaciers

When global temperatures rise:

• glaciers melt
• ice sheets shrink
• water is transferred into the oceans

This increases the global sea level.

Thermal expansion

As seawater warms:

• it expands
• ocean volume increases
• sea levels rise further

This is an important modern cause of sea level rise linked to global warming.

Causes of eustatic sea level fall

Sea levels fall when:

• global temperatures decrease
• more water becomes stored as ice
• evaporation and snowfall increase ice accumulation over long periods

This occurred during glacial periods.

Isostatic sea level change

Isostatic change occurs when land rises or falls relative to the sea. It is caused by changes in pressure on the Earth’s crust.

Glacial loading and unloading

During the last Ice Age:

• thick ice sheets placed enormous pressure on the crust
• land beneath the ice sank under the weight

This process is known as isostatic subsidence.

When the ice melted:

• pressure was removed
• the crust slowly began to rise again

This process is called isostatic rebound or isostatic recovery.

Isostatic change in the UK

In the UK:

• northern and western areas are still rising slowly because they were heavily glaciated
• southern and eastern areas are sinking slightly

This creates a “pivot” effect across Britain. As a result:

• relative sea level rise is greater in southern and eastern England
• flood risks are increasing in low-lying areas such as East Anglia

Tectonic sea level change

Tectonic sea level change occurs when movements of the Earth’s crust cause land to rise or fall.

This may happen because of:

• earthquakes
• volcanic activity
• plate movement

Unlike eustatic change, tectonic change is usually local or regional.

Tectonic uplift

In some areas:

• tectonic activity raises coastal land
• relative sea level falls

This may happen at:

• destructive plate margins
• collision zones
• areas experiencing crustal uplift

Tectonic subsidence

In other locations:

• the land sinks due to tectonic movement
• relative sea level rises

This increases the risk of:

• flooding
• coastal erosion
• permanent inundation

The 2004 Indian Ocean earthquake and tsunami

The 2004 Indian Ocean earthquake caused significant tectonic sea level change.

The earthquake:

• occurred along a destructive plate boundary
• caused sections of the seabed to rise and others to sink
• displaced enormous volumes of water

Some coastal areas near Banda Aceh in Indonesia:

• were permanently lowered
• experienced increased flooding after the event

This demonstrates how tectonic activity can rapidly alter relative sea level.

Sea level change beyond the UK

Scandinavia

Parts of Scandinavia are still rising because of post-glacial isostatic rebound following the melting of large ice sheets.

In some areas:

• land is rising faster than global sea level
• relative sea level is falling

The Maldives

The Maldives are highly vulnerable to rising sea levels because:

• most islands are only 1–2 metres above sea level
• even small rises increase flood risk

Eustatic sea level rise linked to climate change threatens settlements and tourism.

Bangladesh

Bangladesh experiences:

• rising sea levels
• delta subsidence
• increased storm surge risk

Large areas of low-lying coastline are vulnerable to flooding and salinisation.

Modern sea level rise

Scientists now believe global sea levels are rising because of:

• climate change
• melting ice sheets
• thermal expansion of seawater

Modern sea level rise is occurring faster than during much of the last several thousand years.

Impacts of sea level rise

Rising sea levels may lead to:

• increased coastal flooding
• greater erosion rates
• more frequent storm surge impacts
• displacement of people
• loss of ecosystems such as saltmarshes and mangroves

Low-lying coastal regions are most at risk.

Why sea level change is important

Sea level change influences:

• coastal processes
• erosion and deposition
• flood risk
• coastal management decisions

Understanding past sea level changes helps scientists predict how coastlines may change in the future.