Tsunamis have always been big news – huge unstoppable waves crashing against vulnerable coastlines are an ever-present danger.
Whilst earthquakes are a significant cause of tsunamis, they’re not the only source. Tsunamis may also be caused by submarine landslides and flanks of volcanoes collapsing into the sea on volcanic islands.
Scientists are continually investigating how tsunamis are caused and how far they my travel. When scientists modelled a collapse in the Hawaiian islands which took place 1.5 million years ago, they discovered that the subsequent tsunami would have struck the west coast of North America – over 1000 miles away, with waves tens of metres high.
They discovered this by looking at sediment on the island of Bermuda in the western Atlantic. Bermuda is 700 miles away from the nearest land, so the objective was to see if the sediments had been deposited by a tsunami, and if so from where. The sediment is graded, becoming finer-grained towards the top, a distinctive feature of tsunami sediments, because the suspended sediment in the tsunami wave is laid down when the water stops moving. If the sediment was an old beach deposit, the internal structure and shells within the sample would be similar to those in the modern tropical beaches; they are not.
Based on the shell remains of small creatures (foraminifera) it turns out that the sediments were from four different sources: the sea, the beach, the reef and the lagoon. Within the shells are tiny calcium carbonate crystals. The shapes of these crystals provides a fascinating history of the environments in which they were formed and preserved.
Some of the carbonate crystals within the shell chambers were created in marine conditions. However, careful examination showed that these shells are in rock fragments that contained other crystals on the outside which definitely formed in freshwater conditions.
This compromises the beach origin theory. Some rock fragments were indeed formed on the coast, but were subsequently moved by an energetic process and deposited high above their original location. Most likely a tsunami struck Bermuda. Dating of the sediment indicates that this event was earlier than between 310,000 and 360,000 years ago.
The tsunami source is uncertain, although there are a number of candidates. Earthquakes along the active Caribbean subduction zones are a possibility, as are collapses of volcanic edifices in the Caribbean island arc. Further afield we know that there have been giant submarine landslides along the margins of the eastern Atlantic. But there is very little information on the age of any of these features.
Perhaps significantly, the age of the sediments correlates with the giant lateral collapses of volcanoes in the eastern Atlantic such as those of the Canary Islands, El Julan on the island of El Hierro and Orotava on Tenerife. There has been a fierce controversy about whether the collapse of these volcanoes can in fact create destructive tsunamis.
Recent work indicates that, like Hawaii, they can locally, but could an extremely large wave really cross an ocean? Modelling of the large Hawaiian collapses indicates that they can, but the modelling assumes a single catastrophic event. Volcanic collapses in Hawaii and the Canary Islands are infrequent, perhaps taking place every 100,000 years, so the hazard is small. Future research on sediments both in the Canary Islands and elsewhere in the Atlantic may help us to clarify just how small this is.