Catastrophic floods dramatically altered ancient landscapes on Earth and Mars, and their scars linger today. Violent floods carved deep, distinctive canyons known as amphitheater-headed canyons. Unlike canyons sculpted slowly by rivers, these ones end abruptly in steep headwalls at their upstream limits. Scientists have long puzzled over just how much water and time are needed to form these canyons.
Now Lapotre et al. have built a new theoretical model of amphitheater-headed canyon formation that could help reconstruct ancient megafloods. When applied to real canyons, the model suggests that the floods that formed them released less water per second (or water discharge) over longer periods of time than scientists previously predicted.
The new model is based on erosion mechanics and shows how a new canyon could have formed as floodwaters converged at the rim of a horseshoe-shaped waterfall. Fast-flowing water would have toppled blocks of fractured rock at the brink, catastrophically eroding the brink and moving it farther upstream. This would have carved and extended the canyon at a steady width, forming the gorges seen today.
The team applied the new model to 9 canyons on Mars and 14 on Earth, including Box Canyon in Idaho, which may have been carved by a megaflood 45,000 years ago. They found that the discharge of water that sculpted these canyons was 100 to 1000 times lower than expected, given previous ideas that canyons were brimful with water. However, they also found that the floods lasted longer than previously thought, from less than a day to a few months.
The new results could help clear up a long-standing mystery: Pressurized groundwater is thought to have driven Martian megafloods, but the rate at which water could be released from the ground didn’t match up with the huge estimated discharge rates. The new discharge estimates are more in line with groundwater release estimates.
In addition to carving canyons, ancient floods may have been large enough to affect global hydrology and climate on both Earth and Mars. A better understanding of megaflood dynamics provided by the new model could help answer critical questions about the rocky planets’ histories, as well as the past habitability of Mars. (Journal of Geophysical Research: Planets, doi:10.1002/2016JE005061, 2016)
—Sarah Stanley, Freelance Writer