The dense vegetation of Isla de Mona, an uninhabited speck in the Caribbean, hides car-sized boulders studded with corals. Scientists first reported spotting them in the early 1990s, but the strange rocks slipped back into obscurity before anyone investigated their origin. Now researchers have revisited these behemoths and concluded that massive tsunami waves, launched by an underwater landslide, heaved them from the sea.
Many of the boulders are visible from the air, but most are difficult to reach from the ground, says Bruce Jaffe, an oceanographer at the Pacific Coastal and Marine Science Center in Santa Cruz, Calif. He recalls picking his way around the island's poisonous plants, including one that can cause blistering and temporary blindness. “We've talked about going back with hazmat suits,” he says.
During two trips, the researchers visited more than 50 boulders. The largest was more than eight meters long, and the rocks were strewn over a wide area up to 800 meters inland. A storm probably would not have deposited them so far from the shoreline, Jaffe says; a powerful tsunami must have been involved. Ricardo Ramalho, a geologist at the University of Lisbon, who was not involved in the research, agrees: “I would find it surprising if it was the work of a storm,” he says.
Team member Pedro Israel Matos Llavona, a geoscientist at the University of Massachusetts Amherst, pored over maps of the nearby seafloor and found evidence of something that could have triggered such a tsunami: a jagged depression nearly four kilometers wide, the relic of a long-ago underwater landslide. Matos Llavona simulated the landslide and found it would have sent 10-meter-high tsunami waves crashing onto Isla de Mona—with enough force to heave many-ton boulders far inland. The researchers presented these results at the 2020 American Geophysical Union Fall Meeting.
Next the team plans to pinpoint more of these boulders by flying a drone to detect the heat that they emit after a day in the sun. The researchers suggest that tracing how the rocks are scattered may help scientists detect tsunami signatures elsewhere.