By Sid Perkins
The extraterrestrial object suspected to have splashed down near Mexico’s Yucatn peninsula more than 65 million years ago did more than just blast a big hole, cause worldwide climate change, and wipe out the dinosaurs. In the short term, seismic vibrations triggered massive underwater landslides and tsunamis far up the eastern coast of North America (SN: 12/9/00,
p. 373: Chalk reveals greatest underwater landslide). Now, scientists say they’ve identified enormous earth movements from the same era along the Pacific Coast.
When the chunk of asteroid or comet struck, it generated an earthquake that probably would have registered about magnitude 13. Even as far as 7,000 kilometers from ground zero, spreading seismic ripples would have heaved the ground up and down as much as 1 meter, says Grant Yip, a geologist at the University of California, Santa Barbara. He and his colleagues contend that the thick layers of jumbled sediments now exposed by erosion in a canyon in northern Baja California record a series of gigantic landslides that stemmed from the impact.
Those landslides filled in an ancient Manhattan-size valley that was at least
200 m deep. The rocks that formed the walls of that canyon are about 70 million years old and still intact, says Yip. The lowest layer of valley-filling debris includes well-rounded rocks ranging in size from pebbles to boulders. The sediment layers atop that hold sand, disk-shaped stones, and many fragments of shells, all of which suggest that the material had been shaped by breaking surf. Next come sandstone layers that formed from material eroded by floods.
Scientists dated a 20-m-thick layer of volcanic pumice, located above the sandstone, to a nearby eruption that occurred during the extended series of landslides triggered by the Yucatn impact. Overlying, thick layers of landslide debris suggest that a series of slumps occurred over months or years after that eruption, says Yip. The researchers report their analyses in the August Geology.
The landslide debris that Yip and his colleagues studied is clearly of the same age as the Yucatn impact, says John V. Firth, a geologist at Texas A&M University in College Station. However, he notes, strong earthquakes that could trigger immense landslides are common in Baja California. Other examples from the same area and era would strengthen the case that the landslides were triggered by the distant extraterrestrial impact and not by local events.
