Who watches the island of Fogo from a satellite photo, you can see a huge bite throughout the east side of the large boiler Cape Verde Island. The strange shape is the result of the cone of collapse, about 73,000 years. At one time, 160 cubic kilometers material have fallen to the sea. The result was the creation of a surge with 170 meters that hit 7.5 minutes after the island of Santiago, to 55 kilometers away. The wave energy was so great that pushed rock blocks with hundreds of tons for hundreds of meters above the island.
Today, in the highlands of Santiago, are still those Cyclopean blocks, jumping in sight because of its size. Were these objects that allowed an international team of researchers, with Portuguese, relate the geological features of the two islands and trace the catastrophic scenario of the mega-tsunami, shows the article published yesterday in the journal Science Advances . Nothing prevents that in future events like this from happening again.
Joseph Wood, a geologist at the Faculty of Science, University of Lisbon (FCUL) and one of the article’s authors, tells us what will last. “The rock mass [that collapses the island to the sea] occupies space that was occupied by water and displaces water mass producing the wave that then spreads,” explains the scientist. “The wave is generated on the east coast of the island of Fogo and will move towards the east, reaches the entire west coast of Santiago and then is refracted around the island.”
The waves will come to the other islands of the archipelago of Cape Verde, but the effect was not the same in all of them. “In the low-lying islands and in May, in Boavista and Sal, there will have been a fairly extensive flooding of the coastline. In the higher islands, the wave may have risen to very high quotas, but did not go so deeply, “argues José Madeira.
It is very possible that the tsunami has reached the African coast, weakened. But since it is unknown if also hit Madeira, the Azores or mainland Portugal, being further. To know this, it must continue to do the same studies that led to this discovery.
It all started a few years ago, in May, a smaller and lower island which is just east of Santiago . “At one point I found sedimentary deposits with strange features that I associated to a large Tsunami ,” says José Madeira. “At the time, I thought that the most likely source for this event could have been the collapse of the island of Fogo.”
According to an earlier dating to this study, the collapse of the cone of the island of Fogo (which was news in 2014 due to a volcanic eruption in November that lasted until February 2015) occurred sometime between 124,000 and 65,000 years ago. However, there was a big discussion on the phenomenon. It was not known if it had occurred in a progressive manner, with several slips material over time, or whether it was a single event.
These two scenarios have different consequences on the formation of a tsunami. Successive rock slide means that less material between the sea at a time, creating smaller waves. While a single collapse results in a much higher wave. This discussion does not end if the island of Fogo, Cape Verde, and its importance goes beyond scientific curiosity. In several islands around the world, there are similar landslides marks, and investigate what happened helps to assess the risk of future landslides and their impact.
To try to understand what happened in the case of Cable Green, Joseph Wood went to the island east of Fire. “I went to the island of Santiago looking for equivalent deposits and found them,” he says. This was about five years. This is where his story meets the observations made by another geologist formed in FCUL, Ricardo Ramalho, who finished his doctorate in 2010 at the University of Bristol, UK, on the Cape Verde geology.
“Ricardo has gone through Lisbon and showed him the deposits he had found. At the time, he told me he had found a strange situation [in Santiago] did not understand and it was a set of large blocks scattered throughout the plateau surface, whose origin was unclear. Then we related the two aspects and realized that they corresponded to the same event, “says José Madeira.
The deposits found by José Madeira and the blocks that caught the attention of Ricardo Ramalho are in different locations on the coast extreme north of Santiago. The coastal strand that area rises progressively with increasing slope and ends in a vertical cliff called cornice. From there begins the plateau. The deposits are closer to the sea, before the mantel, and result of sea sand and shells brought by the tsunami, which mingled with the aftermaths material. Blocks, 600 meters or more from the coast, are on the plateau, 200 meters above sea level.
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