The Great Edge Collapse
Off the coast of Norway, hidden beneath the Norwegian Sea, lies the immense scar of a catastrophic collapse. Known as the Storegga Slide ("Storegga" is Old Norse for "Great Edge"), this event was one of the largest known submarine landslides in history. Around 6225–6170 BCE, a section of the continental shelf approximately 290 kilometers (180 miles) long gave way. An estimated 3,500 cubic kilometers (840 cubic miles) of sediment and rock plunged from the shelf edge into the deep ocean. The resulting debris field covers an area of 95,000 square kilometers (about 36,700 square miles), a region roughly the size of Scotland.
The displacement of such a colossal volume of material generated a massive tsunami in the North Atlantic. Evidence for this ancient wave is found in sediment deposits across the region. In Scotland, traces of the tsunami have been found up to 29 kilometers (18 miles) inland and 4 meters (13 feet) above current normal tide levels. Models estimate that the tsunami waves could have exceeded 20–25 meters (65–80 feet) in height along the coast of the Shetland Islands and 10–12 meters (33–39 feet) along the Norwegian coast. These deposits provide a geological marker, helping scientists date coastal archaeological sites and understand the ancient coastline.
Causes and Future Risks
Scientists believe the Storegga Slide was preconditioned by the geology of the region. During past ice ages, glaciers deposited vast amounts of sediment on the continental shelf. These sediments layered over weaker marine clays. The leading hypothesis for the trigger is an earthquake that destabilized this already precarious arrangement. Another contributing factor may have been the dissociation of methane hydrates, ice-like substances that trap methane gas under high pressure. A change in pressure or temperature could have caused this gas to expand rapidly, weakening the sediment structure.
The main collapse 8,200 years ago was the last in a series of slides that occurred in this region at intervals of about 100,000 years, often coinciding with the end of glacial periods. Geological analysis of the slide scar and surrounding area, conducted in preparation for the Ormen Lange offshore gas field, suggests that the unstable sediments were removed by the last major slide. The prevailing conclusion is that a new period of glaciation and subsequent deposition would be required to create the conditions for another slide of this magnitude. While the immediate threat of a repeat event is considered low, the Storegga Slide shows the dynamic nature of submarine environments and the potential hazards they hold.