The three Storegga Slides (Norwegian: Storeggaraset) are among the largest known underwater landslides. They happened near the edge of Norway's continental shelf in the Norwegian Sea, around 6225–6170 BCE. The collapse covered an estimated 290 km (180 mi) of coastal shelf, moving about 3,500 km³ (840 cu mi) of debris. This event triggered an ancient tsunami in the North Atlantic Ocean.
Description
Storegga (Norwegian: "Great Edge") is located at the edge of Norway's continental shelf in the Norwegian Sea, 100 km (62 mi) north-west of the Møre coast. Around 6200 BCE, collapses of the shelf caused three underwater slides, which caused huge tsunamis in the North Atlantic Ocean. The collapses involved about 290 km (180 mi) of coastal shelf, with a total volume of 3,500 km (840 cu mi) of debris.
Scientists used carbon dating on plant material found in sediment left by the tsunamis to determine that the most recent event happened around 6225–6170 BCE. In Scotland, evidence of the tsunami was found, with sediment discovered in Montrose Basin and the Firth of Forth up to 29 km (18 mi) inland and 4 meters (13 ft) above today's normal tide levels.
Possible mechanism
The cause is believed to be an earthquake that caused a large release of methane clathrate, a solid made of methane trapped in ice-like structures formed in deep ocean areas under very high pressure. When removed from high pressure and cold conditions, one cubic meter of this solid turns into 164 cubic meters of methane gas. This sudden expansion might have weakened the surrounding rock enough to cause the slide.
Another idea suggests that over time, rivers from melting glaciers carried huge amounts of sediment to the edge of the continental shelf, where it built up in layers. In this case, an earthquake or similar event could have caused a large part of the seafloor to collapse into the deep Norwegian Sea, taking the layers of sediment with it.
Impact on human populations
At or just before the time of the Second Storegga Slide, a land bridge called Doggerland connected Britain, Denmark, and the Netherlands across what is now the southern North Sea. This area likely had a coastline with lagoons, marshes, mudflats, and beaches, and it was a place where early humans hunted, fished, and gathered food.
Although Doggerland was eventually covered by rising sea levels, scientists suggest that a tsunami caused by the Storegga Slide may have temporarily flooded coastal areas of Britain and mainland Europe, including regions now underwater. This event likely caused serious harm to the people living there during the Mesolithic period. It is estimated that up to one-fourth of the Mesolithic population in Britain may have died.
A 2021 study found that about 600 kilometers (370 miles) of Scotland’s northern and eastern coastline were affected, with water reaching 29 kilometers (18 miles) inland. If a similar event happened today, it could cause significant damage to coastal and port areas in places like Arbroath, Stonehaven, Aberdeen, Inverness, Wick, and Montrose.
While the tsunami from the Second Storegga Slide would have been very destructive for those in the affected area, it was not the sole reason for the final flooding of Doggerland.
Future slides
Storegga was carefully studied as part of the planning work for the Ormen Lange gas field near the coast of Norway. The main idea is that the landslide happened because of leftover materials from glaciers that existed before the last ice age. This means another landslide could only happen again if there is a new ice age. After research supporting this idea was published in 2004, it was decided that developing the Ormen Lange gas field would not make a new landslide more likely.