Sand dunes tell tale of volcanic devastation

University of Leicester scientists have made a unique discovery at an Atlantic island popular with British holidaymakers.

They have uncovered giant sand dunes on Tenerife that tell a tale of terrifying destruction in ancient times, when fiery clouds swept right across the island, leaving very little in their wake.

Volcanic islands – volcanoes whose summits poke out of the oceans – make popular holiday destinations, like Madeira, Hawaii and the Azores. Some, like Hawaii, erupt lava flows, but others explode catastrophically with devastating effect.

Tenerife, one of the Canary Islands, has behaved in both ways in the past million years. Two volcanologists from the University of Leicester investigating ash layers on the island for forensic clues to deduce what happened there long before human colonisation, have unearthed a tale of particularly destructive behaviour.

They discovered giant sand-dunes of volcanic ash, formed when a hurricane-like pyroclastic current, a fiery flow of incandescent hot ash and noxious gases, swept across most of the island, an area about the size of Cornwall.

The two Leicester volcanologists Dr Mike Branney and Dr Richard Brown have deduced that the devastating pyroclastic current that created the dunes must have been sustained for several hours, while part of the island collapsed and its ancient forests were swept away.

In an article this month in GEOLOGY, an international magazine published by the Geological Society of America, they show how despite the extended duration of this catastrophic event, surprisingly little trace remains. This is because the vast tonnage of ash and rocks erupted from the volcano simply bypassed the island entirely and flowed into the ocean.

Geologists, who use ash layers to decipher past behaviour of violent volcanoes, are concerned that past occurrences of such devastating currents are easy to under-estimate, or even overlook, because so little trace of them is left behind. The remarkable dunes, perched on the side of the volcano near the tiny fishing resort of Poris, are the first of their type to be discovered.

“It seems amazing that these devastating currents flowed for such a long time, yet leave such little record of their passage” says Mike Branney who has been working on Tenerife for seven years; “finding the dunes was the key, and we shall be looking out for similar examples elsewhere, so that we can recognise where else this particular type of hazardous volcanic behaviour has occurred.”