Ancient Megatsunami Reached Height of Manhattan Skyscrapers

The volcanic island of Fogo. The eastern flank collapse of the ancient crater is clearly visible. Earth Observatory/NASA.
Robin Andrews 05 Oct 2015, 16:33

Volcanic eruptions often produce a plethora of hazards that humans have to deal with, from the devastating, searing pyroclastic flows of volcanoes like Mount St Helens to the near-unstoppable lava flows of Mount Etna. Some are more unpredictable than others, and a new study published in Science Advances describes perhaps the least understood and most unpredictable of them all: a volcanically-induced megatsunami.

Working in the Cape Verde Islands, geologists have found striking evidence indicating that the collapse of a flank of the oceanic volcano Fogo into the sea 73,000 years ago triggered a so-called megatsunami that at one point exceeded heights of 270 meters (886 feet) – approximately the same height as the Chrysler Building in New York City. Crashing into the nearby island of Santiago at speeds of up to 120 meters (395 feet) per second, it flung tons of dense, large blocks of rock onto the island. 

This tsunami would have been nothing like humans have ever observed. For comparison, the 2011 earthquake-induced tsunami that impacted the eastern seaboard of Japan, which killed nearly 16,000 people, was a mere 39 meters (127 feet) in height. A Fogo-style megatsunami impacting a population-dense region would create an untold number of casualties dwarfing that of the 2011 Japanese tsunami.

The study, led by volcanologist Dr Ricardo Ramalho, describes a series of unusual boulders on mainland Santiago. Geologically unlike the young volcanic terrain they were spotted on, up to 600 meters (2,000 feet) inland and up to 200 meters (650 feet) above sea level, these more marine-type rocks resembled those found on the shoreline. Sometimes weighing up to 700,000 kilograms (1.5 million pounds), the most plausible explanation is that a colossal wave ripped them up from the shoreline and threw them farther inland. The dimensions of this wave were calculated by determining the energy required to deposit these boulders in the first place.

Unlike normal tsunamis, which are caused by underwater earthquakes moving a large volume of water, megatsunamis are generated when an extremely large volume of material suddenly collapses into a lake, sea or ocean. Their peak wave height is far higher than that of ordinary tsunamis, and they are often caused by asteroid impacts or volcanic flank collapses triggered by eruptions or associated seismic activity.

As it turns out, a nearby volcano has a rather sizeable scar: the eastern flank of the island of Fogo, residing 55 kilometers (34 miles) to the west of Santiago, is missing. Radiogenic dating techniques placed the deposition of the boulders on Santiago at around 73,000 years years ago, approximately matching that of the flank collapse, dated by a separate study to have occurred between 123,000 and 62,000 years ago. Up to 160 cubic kilometers (38 cubic miles) of rock, trigged by violent volcanic activity at Fogo, collapsed into the sea all at once, forcing a vast amount of oceanic water to rush eastwards towards Santiago at incredible speeds.

Megatsunamis are not just natural phenomena consigned to ancient history. A far more recent historical megatsunami event occurred in 1792, when magma forcing its way up towards the vent of Mount Unzen in Japan generated violent earthquakes that triggered the collapse of the southern flank of the volcano. The resulting megatsunami killed more than 15,000 people.

As for how frequent such events are, volcanologists are uncertain. "Based on the geological record from Hawaii and the Canary islands, people have estimated that large flank collapses happen there at least once every 100,000 years – but I don’t know how solid this estimate is," Dr Ramalho told IFLScience. "We still don’t have solid constraints on the frequency of such events."

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