Megafloods May Have Formed Strange Rock Formations on Earth and Mars

Megafloods likely triggered the creation of unusual rock formations seen both here and on Mars, a new study published in the Proceedings of the National Academy of Sciences reveals.

Canyons shaped like an amphitheater can be seen near Snake River in Idaho as well as satellite images taken of the Red Planet. In the case of Idaho, the canyons, located in Malad Gorge State Park, are carved into a largely flat plain made up of basalt derived from a hotspot that has been active for the last million years.

Known as Woody's Cove and Stubby Canyon, the formations' U-shaped walls soar 150 feet into the air. Geologists are firm in their understanding of how the rocks came to be shaped like columns - lava flows hardening and contracting as it cooled, creating vertical cracks. What's not clear, and what researchers from California Institute of Technology wanted to know, is how these columns of basalt came to be exposed or take on the form of a horseshoe.

The most popular explanation is called groundwater sapping. This idea states that springs at the bottom of the canyon carve tunnels at the base of the rock wall over time until the structure is completely destabilized, causing blocks or columns to fall away and an amphitheater shape to form.

Michael P. Lamb, a professor of geology and co-author of the study, has his doubts about the theory.

"These blocks are too big to move by spring flow, and there's not enough time for the groundwater to have dissolved them away," Lamb said, "which means that large floods are needed to move them out. To make a canyon, you have to erode the canyon headwall, and you also have to evacuate the material that collapses in."

Instead, he and former postdoctoral fellow Benjamin Mackey offer a new explanation.

Their evidence, Lamb said, indicates "amphitheater shapes might be diagnostic of very large-scale floods, which would imply much larger water discharges and much shorter flow durations than predicted by the previous groundwater theory."

A megaflood would not only have the strength to topple the columns, but carry them downstream. Their hypothesis is further supported by evidence of scour marks on surface rocks on the plateau above the canyons that occur when water containing sediments sweeps across land.

The discovery isn't only relevant for south-central Idaho, the researchers note.

"A very popular interpretation for the amphitheater-headed canyons on Mars is that groundwater seeps out of cracks at the base of the canyon headwalls and that no water ever went over the top," Lamb said.

If the new hypothesis is correct, however, it could mean Mars was once a very wet planet.