The sea bed can fail when physical conditions change, just as on land, creating an underwater landslide. These mass movements of vast amounts of sediment are called submarine landslides or just slides.
The sea covers over 70 per cent of the planet and so there are probably many more offshore landslides than there are on land. Submarine landslides can be found even on very low slope angles — as low as 1°, and mapping them underwater is difficult and expensive.
Over 8000 years ago a massive submarine landslide occurred off the coast of Norway. It is known as the Storegga Slide, which also resulted in a tsunami reaching the north-east coast of Britain.
The area of the Storegga Slide is about the size of Scotland.
About 3300 km3 of sediment slid down often as large blocks, up to a kilometre across. Studies have shown that similar events have happened about every 100 000 years in this area.
Sediment deposited during glacial periods can fail when large earthquakes occur. These earthquakes are caused by the crust responding to unloading from the melting of the ice cap over Scandinavia — sometimes called post-glacial rebound or continental rebound.
The Afen Slide, located north-west of Shetland, is about 4 km wide and 13 km in length and displaced about 200 million m3 of sediments.
The area of the Afen Slide, which took place about 2800 years ago, is about the size of a large city such as Edinburgh.
As climate changes, variations in ocean currents can lead to changes in sedimentation.
Likewise, changes in sedmentation on the continental slopes between periods of ice advance and retreat during glacial periods can lead to unstable slope conditions.
These sediments may fail with even a relatively small earthquake.
Several small slides have been found in the soft sediments of Scottish sea lochs.
Holy Loch Slide, also known as the Strone Slide, is in the Firth of Clyde.
These landslides have displaced the soft sediments which were deposited as the glaciers of the last ice age retreated back onshore.
The sediments have become unstable because they were deposited very rapidly, trapping a lot of water. As sea levels have moved up and down and also organic matter has broken down to produce gas, the local pressure conditions have increased leading to instability in the slopes.
Learn more about mapping the sea bed at Surveying Loch Lomond