Saidmarch, Blackhawk, and Heart Mountain landslides

The Saidmareh landslide in Iran.

Geology.com is a great source of information for this blogger, and the image from the right, originally from NASA is from that site; it was also featured on Dave Petley's landslide site in 2009.  This landslide, which occurred about 10,000 years ago, is believed to be the largest yet identified on the surface of the earth.  About 20 cubic kilometers of limestone slid 1600 meters (a mile) vertically, spread across the Karkheh River and its valley. Some material traveled 14 kilometers. The slide dammed the Karkheh River, causing a landslide lake to form behind the earthen dam.  This lake eventually breached the dam.

Massive landslides are often triggered by earthquakes.  In the U.S., one of the most catastrophic occurred in 1959 in southwestern Montana.  An earthquake, M 7.3-7.5, caused a huge landslide that killed 28 people and cost $11 million 1959 USD in damage.  This slide blocked the Madison River, resulting in the creation of Quake Lake.  The earthquake is known as the Hebgen Lake earthquake.  Fearing that the lake would burst through the dam in a catastrophic flood, the Army Corps of Engineers almost immediately began to cut a channel into the slide, and within a month, water was flowing through this cut.  In contrast, the landslide dam blocking the Karkheh River in Iran lasted long enough that 150 meters of sediment accumulated at the bottom of the lake before the dam failed.

Landslides that travel long distances occur not only on Earth, but also on Venus, Mars, and Io. The conditions that permit such large, heavy masses to travel long distances have been, and are still, subjects of controversy.  The runouts exceed distances calculated from simple models in which friction is a retarding force.  One hypothesis, based on field observations of the base of the Blackhawk Landslide in California, is that there is a cushion of air that lubricates the base of the landslide.  Another suggestion is that internal vibrations could "fluidize" the rock debris, making the effective coefficient of friction much lower than would be characteristic of a sliding solid mass.

Within the U.S., the Heart Mountain landslide in northwestern Wyoming has a runout distance of about 50 km.  How it traveled so far has been a source of scientific controversy for decades.  In a recent paper, Goren et al. have proposed that a feedback between "shear heating, thermal pressurization, and thermal decomposition of carbonates" at the sliding interface accounts for the large runout distance. The model suggests that the sliding velocity was a few tens of meters per second to more than 100 m/s, and that it took only a few tens of minutes for the whole sliding event.