Megatsunami

An 800-foot wave?

tsunami picture
A megatsunami, rising to a height of some 800 feet, deposited this huge boulder of marine origin, scientists say (for scale, note the scientist sitting on top). Image: Ricardo Ramalho

Location of the Cape Verde Islands
Location of the Cape Verde Islands

megatsunami
Satellite view of Fogo, the island volcano that produced the prehistoric mega wave (region of collapse visible at right). Fogo is the Portuguese word for fire. Image: NASA

megatsunami
These researchers think that a huge wave washed over the island of Santiago about 73,000 years ago, including the place where they are shown standing. Image: Kim Martineau/Lamont-Doherty Earth Observatory

10/5/2015 — The apparent collapse of a volcano in the Cape Verde Islands 73,000 years ago, and the resulting massive wave, have scientists wondering if there are relevant implications for today’s coastal communities.

In a paper published in the journal Science Advances, author Ricardo Ramalho and other

researchers say that the sudden collapse of the eastern flank of the island of Fogo produced an 800-foot-high ocean wave, a megatsunami,

which covered even the uplands of Santiago Island 34 miles (55 km) to the east (see map below). Fogo, which is one of the most prominent and active oceanic volcanoes on earth, lost an estimated 40 cubic miles (160 km³) of rock all at once, and this generated the wave, they say. The resulting tsunami was "higher than the Eiffel Tower," says Ramalho. Other scientists dispute this hypothesis.

During the voyage of the Beagle, Charles Darwin also researched the geology of Santiago. If this wave had struck during his sojourn on the island, there would have been no Origin of Species.

Ramalho did graduate work on Santiago in the Cape Verdes before joining Columbia University's Lamont-Doherty Earth Observatory. He found giant boulders, weighing as much as an estimated 770 tons, some 2,000 feet inland, on a volcanic plateau (see photo at right above). Their composition—limestones, conglomerates and submarine basalts—wasn’t like the surrounding volcanic rocks. He believes a mega-wave grabbed the big boulders from the shoreline and tossed them up onto the plateau.

In order to date the boulders, Ramalho and Gisela Winckler, a colleague at the Earth Observatory, measured helium isotopes near the boulders’ surfaces. The isotopes change if there has been exposure to cosmic rays. The analysis determined the rocks were 73,000 years old, providing “the link between the collapse and impact, which you can make only if you have both dates," said Winckler.

Cape Verde Islands map
Location of Fogo and Santiago within the Cape Verde Islands. Image: Wikimedia, Dr. Brains

Bill McGuire, a tsunami expert from University College London, said the study provides evidence that when volcanoes collapse, they can do so really quickly. And though megatsunamis happen only once every 10,000 years, they are “a clear and serious hazard in ocean basins that host active volcanoes."

Ramalho says that his hypothesis doesn’t mean there’s going to be a sudden trend of island volcanoes producing these huge waves. But he does say that maybe they aren’t as rare as was once thought.

More news >>

Related >>

More about Darwin on Santiago >>


This story was based on information obtained from the AAAS. Source >>

Fogo
Aerial view of Fogo, the source of the massive tsunami. Region of collapse is visible at right. Image: Wikimedia, Aldo Bien
What its like during an ordinary tidal wave


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