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Ancient tsunami claimed to be almost as high as Chrysler building

Oct. 5, 2005
Courtesy of The Earth Institute at Columbia University
and World Science staff

A tsu­na­mi 73,000 years ago may have gen­er­at­ed waves at least 270 me­ters (890 feet) tall, al­most as high as New York’s Chrys­ler Build­ing, a study claim.

How­ev­er, the find­ing may not be the fi­nal word on a de­bate about an­cient tsu­na­mis, which oth­er sci­en­tists have ar­gued to be much smaller.

The mon­ster wave de­scribed in the new study would make an­y­thing in known hu­man his­to­ry look pu­ny. The big­gest known re­cent tsu­na­mis, which dev­as­tat­ed the In­di­an Ocean’s coasts in 2004 and east­ern Ja­pan in 2011, reached about 100 feet.

The tsunami generated by Fogo's collapse supposedly swept boulders like this one from the shoreline up into the highlands of Santiago island. Here, a researcher chisels out a sample. (Credit: Ricardo Ramalho)


The an­cient dis­as­ter seems to have swept up boul­ders weigh­ing up to 770 tons like toy boats, ac­cord­ing to sci­en­tists. For com­par­i­son, each such boul­der would weigh the equiv­a­lent of about 20 eight­een-wheel­er trucks packed with car­go.

The col­lapse of the side of a vol­ca­no trig­gered the event, ac­cord­ing to re­search­ers.

“Flank col­lapses can hap­pen ex­tremely fast and cat­a­stroph­ic­ally, and the­re­fore are ca­pa­ble of trig­ger­ing gi­ant tsu­na­mis,” said lead au­thor Ri­car­do Ra­mal­ho, who did the re­search as a post­doc­tor­al as­so­ci­ate at Co­lum­bia Un­ivers­ity’s La­mont-Do­herty Earth Ob­serv­a­to­ry, where he is now an ad­junct sci­ent­ist. 

“They probably don’t hap­pen very of­ten. But we need to take this in­to ac­count when we think about the haz­ard po­ten­tial of these kinds of vol­can­ic fea­tures.”

Most of the tsu­na­mis in doc­u­mented his­to­ry, like the two events in Asia, are trig­gered in a dif­fer­ent way, by move­ments of un­der­sea earth­quake faults. 

Ra­malho and col­leagues worked in the Cape Verde Is­lands off West Af­ri­ca. The study ap­peared Oct. 2 in the jour­nal Sci­ence Ad­vanc­es.

The ap­par­ent col­lapse oc­curred at the Fogo vol­ca­no, one of the world’s larg­est and most ac­tive is­land vol­ca­noes. Ac­cord­ing to the re­search­ers, the re­sult­ing tsu­na­mi en­gulfed Santi­ago Is­land 34 miles (55 km) away. It was un­in­hab­it­ed then, but a quar­ter of a mil­lion peo­ple live there to­day.

Fogo Vol­ca­no, tow­er­ing 2,829 me­ters (9,300 feet) above sea lev­el, is still ac­tive and erupts about every 20 years, most re­cently last fall. 

There is no dis­pute that vol­can­ic flanks pre­s­ent a haz­ard, Ra­malho said; at least eight smaller col­lapses have oc­curred in Alas­ka, Ja­pan and else­where in the last sev­er­al cen­turies, and some have gen­er­at­ed deadly tsu­na­mis. 

But many sci­en­tists doubt wheth­er big vol­ca­noes can col­lapse with the sud­den­ness that the new study sug­gests. Rath­er, they en­vi­sion land­slides com­ing in grad­u­al stages, gen­er­at­ing mul­ti­ple, smaller tsu­na­mis. A 2011 French study al­so looked at the Fogo col­lapse, sug­gest­ing that it took place some­where be­tween 124,000-65,000 years ago; but that study said it in­volved more than one land­slide. The French re­search­ers es­ti­mate that the re­sult­ing mul­ti­ple waves would have reached only 45 feet—even at that, enough to do plen­ty of harm to­day.

A hand­ful of pre­vi­ous oth­er stud­ies have pro­posed much larg­er pre­his­tor­ic col­lapses and re­sult­ing mega-tsu­na­mis, in the Ha­wai­ian is­lands, at Italy’s Mt. Et­na, and the In­di­an Ocean’s Re­un­ion Is­land. But crit­ics have said these ex­am­ples are too few and the ev­i­dence too thin. The new study adds a new pos­si­ble ex­am­ple; it says the es­ti­mated 160 cu­bic kilome­ters (40 cu­bic miles) of rock that Fogo lost dur­ing the col­lapse fell all at once.

Sev­er­al years ago, Ra­malho and col­leagues were work­ing on Santiago when they spot­ted un­usu­al boul­ders ly­ing as far as 2,000 feet in­land and nearly 650 feet above sea lev­el. They’re to­tally un­like the young vol­can­ic ter­rain around them, Ra­malho said; rath­er, they match marine-type rocks that ring the is­land’s shore­line. Some weigh up to 770 tons. 

The only real­is­tic ex­plana­t­ion the sci­en­tists could come up with: a gi­gantic wave must have ripped them from the shore­line and lofted them up. They came up with the size of the wave by cal­cu­lat­ing the en­er­gy it would have tak­en to ac­com­plish this feat.

To date the event, in the lab Ra­malho and Lamont-Doherty ge­o­chem­ist Gisela Winck­ler meas­ured iso­topes, or vari­ants, of the el­e­ment he­li­um em­bed­ded near the boul­ders’ sur­faces. Such iso­topes change de­pend­ing on how long a rock has been ly­ing in the open, ex­posed to cos­mic rays. The anal­y­ses cen­tered around 73,000 years—well with­in the ear­li­er French es­ti­mate of a smaller event. The anal­y­sis “pro­vides the link be­tween the col­lapse and im­pact, which you can make only if you have both dates,” said Winck­ler.

Tsu­na­mi ex­pert Bill McGuire, a pro­fes­sor emer­i­tus at Un­ivers­ity Col­lege Lon­don who was not in­volved in the re­search, said the study “pro­vides ro­bust ev­i­dence of mega-tsu­na­mi forma­t­ion [and] con­firms that when vol­ca­noes col­lapse, they can do so ex­tremely rapid­ly.” 

Based on his own work, McGuire said that such mega-tsu­na­mis probably come only once every 10,000 years. “Nonethe­less,” he said, “the scale of such events, as the Fogo study tes­ti­fies, and their po­ten­tially dev­as­tat­ing im­pact, makes them a clear and se­ri­ous haz­ard in ocean basins that host ac­tive vol­ca­noes.”

Ra­malho cau­tions that the study should not be tak­en as a red flag that anoth­er big col­lapse is im­mi­nent here or else­whe­re. “It does­n’t mean every col­lapse hap­pens cat­a­stroph­ic­ally,” he said. “But it’s may­be not as rare as we thought.”

James Hunt, a tsu­na­mi ex­pert at the U.K.’s Na­t­ional Ocean­og­ra­phy Cen­tre who was not in­volved in the stu­dy, said the re­search makes it clear that “even mod­est land­slides could pro­duce high-amplitude anom­a­lous tsu­na­mi waves on op­pos­ing is­land coast­lines.” The ques­tion, he said, “is wheth­er these trans­late in­to haz­ardous events in the far field, which is de­bat­able.”

When Fogo erupted last year, Ra­malho and oth­er ge­ol­o­gists rushed in to ob­serve. La­va flows (since calmed down) dis­placed some 1,200 peo­ple, and de­stroyed build­ings in­clud­ing a new vol­ca­no vis­i­tors’ cen­ter. “Right now, peo­ple in Cape Verde have a lot more to wor­ry about, like re­build­ing their liveli­hoods af­ter the last erup­tion,” said Ra­malho. “But Fogo may col­lapse again one day, so we need to be vig­i­lan­t.”


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A tsunami 73,000 years ago may have generated waves at least 270 meters (890 feet) tall, almost as high as New York’s Chrysler Building, a study claim. However, the finding may not be the final word on a debate about ancient tsunamis, which other scientists have argued to be much smaller. The monster wave described in the new study would make anything in known human history look puny. The biggest known recent tsunamis, which devastated the Indian Ocean’s coasts in 2004 and eastern Japan in 2011, reached about 100 feet. The ancient disaster seems to have tossed boulders weighing up to 770 tons like ping-pong balls, according to scientists. For comparison, each such boulder would weigh the equivalent of about 20 eighteen-wheeler trucks packed with cargo. The collapse of the side of volcano triggered the event, according to researchers. “Flank collapses can happen extremely fast and catastrophically, and therefore are capable of triggering giant tsunamis,” said lead author Ricardo Ramalho, who did the research as a postdoctoral associate at Columbia University’s Lamont-Doherty Earth Observatory, where he is now an adjunct scientist. “They probably don’t happen very often. But we need to take this into account when we think about the hazard potential of these kinds of volcanic features.” Most of the tsunamis in documented history, like the two events in Asia, are triggered in a different way, by movements of undersea earthquake faults. Ramalho and colleagues worked in the Cape Verde Islands off West Africa. The study appeared Oct. 2 in the journal Science Advances. The apparent collapse occurred at the Fogo volcano, one of the world’s largest and most active island volcanoes. According to the researchers, the resulting tsunami engulfed Santiago Island 34 miles (55 km) away. It was uninhabited then, but a quarter of a million people live there today. Fogo Volcano, towering 2,829 meters (9,300 feet) above sea level, is still active and erupts about every 20 years, most recently last fall. There is no dispute that volcanic flanks present a hazard, Ramalho said; at least eight smaller collapses have occurred in Alaska, Japan and elsewhere in the last several centuries, and some have generated deadly tsunamis. But many scientists doubt whether big volcanoes can collapse with the suddenness that the new study suggests. Rather, they envision landslides coming in gradual stages, generating multiple, smaller tsunamis. A 2011 French study also looked at the Fogo collapse, suggesting that it took place somewhere between 124,000-65,000 years ago; but that study said it involved more than one landslide. The French researchers estimate that the resulting multiple waves would have reached only 45 feet—even at that, enough to do plenty of harm today. A handful of previous other studies have proposed much larger prehistoric collapses and resulting mega-tsunamis, in the Hawaiian islands, at Italy’s Mt. Etna, and the Indian Ocean’s Reunion Island. But critics have said these examples are too few and the evidence too thin. The new study adds a new possible example; it said the estimated 160 cubic kilometers (40 cubic miles) of rock that Fogo lost during the collapse fell all at once. Several years ago, Ramalho and colleagues were working on Santiago when they spotted unusual boulders lying as far as 2,000 feet inland and nearly 650 feet above sea level. They’re totally unlike the young volcanic terrain around them, Ramalho said; rather, they match marine-type rocks that ring the island’s shoreline. Some weigh up to 770 tons. The only realistic explanation the scientists could come up with: A gigantic wave must have ripped them from the shoreline and lofted them up. They derived the size of the wave by calculating the energy it would have taken to accomplish this feat. To date the event, in the lab Ramalho and Lamont-Doherty geochemist Gisela Winckler measured isotopes, or variants, of the element helium embedded near the boulders’ surfaces. Such isotopes change depending on how long a rock has been lying in the open, exposed to cosmic rays. The analyses centered around 73,000 years—well within the earlier French estimate of a smaller event. The analysis “provides the link between the collapse and impact, which you can make only if you have both dates,” said Winckler. Tsunami expert Bill McGuire, a professor emeritus at University College London who was not involved in the research, said the study “provides robust evidence of mega-tsunami formation [and] confirms that when volcanoes collapse, they can do so extremely rapidly.” Based on his own work, McGuire said that such mega-tsunamis probably come only once every 10,000 years. “Nonetheless,” he said, “the scale of such events, as the Fogo study testifies, and their potentially devastating impact, makes them a clear and serious hazard in ocean basins that host active volcanoes.” Ramalho cautions that the study should not be taken as a red flag that another big collapse is imminent here or elsewhere. “It doesn’t mean every collapse happens catastrophically,” he said. “But it’s maybe not as rare as we thought.” James Hunt, a tsunami expert at the United Kingdom’s National Oceanography Centre who was not involved in the study, said the research makes it clear that “even modest landslides could produce high-amplitude anomalous tsunami waves on opposing island coastlines.” The question, he said, “is whether these translate into hazardous events in the far field, which is debatable.” When Fogo erupted last year, Ramalho and other geologists rushed in to observe. Lava flows (since calmed down) displaced some 1,200 people, and destroyed buildings including a new volcano visitors’ center. “Right now, people in Cape Verde have a lot more to worry about, like rebuilding their livelihoods after the last eruption,” said Ramalho. “But Fogo may collapse again one day, so we need to be vigilant.”