An 800-Foot Ancient Tsunami Once Heaved Truck-Sized Boulders Onto a High Plateau
"Extreme geology" isn't just a thing of the past. Events like this could happen again.
Geologists have concluded that 73,000 years ago, a tsunami six times taller than the wave that hit Japan in 2011 struck off the coast of West Africa, in the Cape Verde islands.
The tsunami was so powerful that it hurled elephant- and truck-sized boulders more than 600 feet uphill and onto a island plateau.
Scientists had discovered these basalt and limestone rocks a few years ago—but, as is often the case in geology, they weren’t sure how they could’ve have ended up so far from their original formation place. All they knew was that 73,000 years ago, a massive chunk of an ancient volcano on the island of Fogo had collapsed and fallen into the sea. On the nearby Santiago Island, evidence of debris from an ensuing tsunami suggests that the event had wide-reaching geologic effects. But until now, researchers didn’t know how big the tsunami was, nor how many of them occurred.
The giant boulders, though, provide a missing link. The team, comprised of researchers from Columbia University’s Lamont-Doherty Earth Observatory, concluded that the 49 boulders (weighing up to 700 tons) they studied could only have been lifted to their current position by a colossal tsunami. The researchers report their geologic interpretation in the journal Science Advances .
In the 19th century, such extreme explanations for Earth’s features were actually quite commonplace; scientists went so far as to name one geologic period—the Antediluvian (i.e., “before the flood”)—after the Great Flood of Noah fame. Here’s Marcia Bjornerud, writing for The New Yorker:
Within a few decades, however, the maturing field of geology had distanced itself from Biblical accounts of Earth’s evolution, and the incongruous rocks were reinterpreted as glacial hitchhikers, remnants of the Ice Age. Gradualistic explanations for geologic phenomena, informed by processes that could be observed today, became de rigueur; invoking the Noachian flood—or any catastrophic origin for rocks or landscapes—became a geologic taboo. (Even so, a vestige of the old flood paradigm persists in the name of an unglaciated region along the Upper Mississippi Valley: the Driftless Area.) The taboo finally lost some of its power in the nineteen-eighties, when it began to seem probable that a meteorite impact at the end of the Cretaceous Period had wiped out the dinosaurs. This unleashed into the scientific literature a pent-up flood of neocatastrophist papers, which sometimes veered toward the sensationalism of tabloids (with references cited, of course). Geologists were at last free to suggest that calamitous but infrequent events—powerful deluges, colossal landslides, gargantuan volcanic eruptions, supersonic impacts from extraterrestrial objects—did in fact play a role in shaping our world. Bad things sometimes happen to good planets.
Now, of course, geologists need to construct a legitimate narrative if they’re going to claim the possibility of past disasters. Such narratives are important: they explain the story of our Earth so that we might divine its future. In this case, the Columbia scientists argue that submarine landslides have long been overlooked in the study of tsunami hazards. While point-source events like volcanic collapse probably wouldn’t do as much widespread damage as an earthquake-generated tsunami would, they still pose a threat to thousands of Pacific islands.
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