1. Education

A Superfault Superlandslide


The world's largest landslide has been explained with a novel, even bizarre theory involving hotspots, superfaults and supercritical fluids.

The rocks in and around the Absaroka Mountains of northwestern Wyoming, in the Yellowstone country, have puzzled geologists for more than a century. There lie the remains of an enormous sheet of shattered rock, covering some 3400 square kilometers, lying above rocks that are undisturbed. The detachment surface that forms the base of this sheet is unbelievably flat and nearly level, having a slope of less than 2 degrees. The bulk of the rock sheet ends at the side of Dead Indian Hill, but more pieces are found as far as 50 kilometers beyond. One of these pieces is a few miles north of Cody, a fault-bottomed fragment—a klippe—called Heart Mountain that has given its name to the detachment.

Geologic fieldwork has established that the rocks above the Heart Mountain detachment were once a much thicker and less extensive body. Carbonate rocks of Paleozoic age (as old as Ordovician or 400 million years) were overlain by younger ones, topped with a thick pile of volcanic rocks dating from the Eocene, about 50 million years old.

Then something made this pile collapse catastrophically and spread southeast to nearly three times its former extent, like one of those broken Antarctic ice shelves. When it was all over, ancient limestones had slid sideways to sit atop much younger rocks in the Bighorn Basin. Erosion later removed much of the slide but a few klippen remain, including Heart Mountain, to testify how far it once spread.

Geologist Edward C. Beutner has called upon two unexpected things, hotspots and superfaults, to explain this unique case.

The hotspot is not the same as the hotspots of plate tectonics, but it's close. The plate-tectonic hotspot is best shown in the Hawaiian Island chain, a string of volcanoes that is young at one end and grows steadily older with distance from it. The lava comes from the mantle and penetrates the crust. In Wyoming, Beutner traces a trail of igneous injections 28 kilometers long leading to the Crandall intrusive complex. In his model the "hotspot" is a magma chamber in the lower crust that squirted lava through the Heart Mountain slide as it moved overhead. (A later paper theorized that injected hot water did the trick instead.)

Beutner's hypothesis is that the Crandall intrusion leaked volcanic gases that were trapped by a thick, level layer of dolomite, a carbonate rock that was impervious to the gas. At some point about 48 million years ago, probably during an eruption, the rock gave way and began to slip along a plane 2 meters above the base of the dolomite.

What happened next was very peculiar, to judge from the rocks. The Heart Mountain detachment is a layer of whitish powdered stone a couple of meters thick. It has a swirling texture suggestive of turbulent flow, and it includes little balls of minerals and volcanic glass shards that look exactly like airfall lapilli. Lapilli are pellets of volcanic ash that form in the ash clouds above an erupting volcano, but these clearly formed several kilometers underground.

Beutner's explanation, published in the May 2005 GSA Bulletin with coauthor Greg Gerbi, is that friction on the detachment quickly raised a tremendous amount of heat. When this happens along superfaults, the rock within the fault melts, creating glassy layers called pseudotachylytes. But dolomite can't take the heat, and instead part of it vaporized—more than that, it broke down liberating carbon dioxide gas.

In fact, Beutner proposed that within seconds the detachment was turned into a frictionless plane as the cushion of gas reached the supercritical stage, a high-pressure hot form of matter that is neither gas nor liquid. This lubrication allowed the slide to reach speeds of hundreds of meters per second, other research suggests, even on a very gentle slope.

Supercritical fluids, both carbon dioxide and water, have been cited to explain doings in the mantle, and they are used in modern industry as well. But their appearance in lubricating the Heart Mountain slide sets a new precedent for the Earth's crust.

PS: Creationists typically grasp at straws to cast doubt on the consensus of geologists, and one of their favorite straws is the Heart Mountain slide. It clearly shows, they argue, that geology once operated very differently than today—that the present is NOT the key to the past. If you recognize monstrous landslides of unknown mechanism, how can you deny the Flood? But models like Beutner's put the slide securely into known geophysics and shows where we might look for other examples. They're also based on evidence, not scripture.

©2014 About.com. All rights reserved.