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SLIDES (rock slide and slump)
FLOWS (rock avalanche, debris flow, earth flow, and creep)
This type of mass wasting can involve a single rock or thousands of rocks. For a mass-wasting event to be classified as a fall, it must travel at a high rate of speed down a very steep slope. If the slope is vertical or overhung, then the rock(s) will drop straight downward, fragmenting when they hit the base of the slope. Over time, this forms a body of angular rubble called talus, a distinctive transition from the steep slope to flatter ground. Below are descriptions, diagrams and photographs of the two basic types of falls.
1) rock fall - A rock fall consists of one or maybe a few rocks that detach from the high part of a steep slope, dropping and perhaps bouncing a few times as they move very rapidly down slope. Rock falls are very dangerous because they can occur without warning, and because the rocks are traveling at high velocity. You can usually tell where rock falls are common by identifying talus at the base of steep slopes. If you are out hiking or camping in mountains or canyons, avoid talus slopes and the rocks that fall onto them!
Diagram 1 illustrates a very steep slope with the potential for a rock-fall event. The actual cause of such an event might be an earthquake, the movement or weight of an animal, or the freezing and thawing of water. Obviously, hanging out at the base of such a slope is not a good idea.
Diagram 2 shows the rock in the process of falling. This usually occurs without warning, and is rarely witnessed. Sometimes a hiker may hear the fall off in the distance, but upon closer inspection will see only a pile of rock rubble at the base of the slope.
Diagram 3 illustrates the rock debris, or talus, that forms at the base of a steep slope as rock fall and break apart on contact with the base of a slope. The more rocks that fall, the greater will be the buildup of talus.
Below are photographs of slopes prone to rock falls, and the talus that gathers at the bases of such slopes.
The pictures below show where rock falls have occurred from the faces of very steep slopes. The first three are from the Escalante region of southern Utah, where each rock fall travelled only a short distance downward, and the buildup of talus is only minor. The fourth photograph was taken outside of Las Vegas, Nevada. Here, the rock fall shattered on impact, with large boulders rolling up to 100 feet from the base of the slope. The fifth photograph was taken in the Sierra Nevada Mountains of California. Here the scale is much grander than the preceding pictures, with the vertical cliff face that generates hundreds of rock falls each year rising 2000 feet straight up! Needless to say, this is a very dangerous location to stand!
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Both of these photographs are from the Goosenecks area of the San Juan River in southern Utah. The first shows the layer of limestone that forms the rim rock along the canyon, with large blocks that have separated from the rock outcrop. The second picture shows one of these blocks that fell and bounced all the way down to the bottom of the canyon, into the river, a journey of over 800 vertical feet!
The first two pictures below were taken in Colorado. The first is from Colorado National Monument, near the Utah border. The second is from the Front Range region, near Pikes Peak. Both show the classic buildup of talus at the base of very steep slopes. Picture 3 shows talus that is gradually covering a roadway in the San Bernardino Mountains of southern California.
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These photographs were taken along a mountain highway in central Colorado. Here, steep slopes composed of fractured sedimentary rocks are prone to rock falls which can shut down the highway for days at a time. The retaining wall next to the highway provides only minimal protection to mass wasting here.
These two pictures both show steep slopes that have been undercut by wave action along the Pacific Ocean coast of southern California. So, both slopes are extremely prone to rock fall!
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2) rock avalanche - This type of fall usually forms when a massive rock fall explodes apart on contact with a slope. As this occurs, thousands of rocks continue their flying trajectories down slope, colliding with each other and the slope itself, overwhelming anything in their paths. A rock avalanche is a transitional sort of mass wasting event, changing from a pure rock fall to something more like a rapid flow of material as the material moves further from the base of a slope. Therefore, some geologists classify rock avalanches as flows. Whatever the classification, rock avalanches are extremely dangerous, and you should be wary of locations where they occur with frequency in mountainous regions. The diagrams and photographs below illustrate some of the nature and danger of rock avalanches.
Diagram 1 shows a small town a short distance from a tall, steep-faced mountain. Such a setting is fairly common in mountain ranges on all continents, with the mountain providing a spectacular backdrop for the town. Unfortunately, this situation can also be a recipe for a natural disaster.
Diagram 2 shows what can happen if an earthquake vigorously shakes a tall, steep-faced mountain. Here, several massive blocks of the mountainside have peeled and fallen away, traveling at high speed toward the base of the mountain. Anything or anyone directly in the path of such a huge rock fall will be obliterated.
Diagram 3 paints a very bleak picture for the town. As the massive rock fall contacts the base of the mountain, it breaks into thousands of fragments that continue tumbling down slope at high velocity. The great energy of such a large mass can enable the rock avalanche to travel much further from the base of the mountain than one would expect, in this case destroying and burying the town. Fortunately, such an extreme rock avalanche is a rare event.
Below are photographs of features associated with rock avalanches.
The photograph to the left shows the highest peak in the Peruvian Andes, Nevado Huascaran.
In 1970, and earthquake triggered a massive rock fall event that originated from the barren area near the top of the peak.
It quickly became a rock avalanche that flowed across a broad valley, filling it with rock and debris to depths of 300 feet.
Some of the rock and debris from this high-energy mass-wasting event continued moving away from the mountain, becoming a debris flow that traveled through a narrow stream valley as well as up and over a 1000 foot-high ridge (crossing the middle of the photograph). As the flow spread through the lowlands, it buried two villages in its path, killing more than 20,000 people! The villages were in the wide, muddy area in the foreground of the photograph.
This mountain ridge in the San Gabriel Mountains of California shows the pathways for rock avalanches, both past and future. For mountain climbers these straight, barren areas provide quick access to higher elevation, but they are exceptionally dangerous during an earthquake, or when water is freezing or melting within rock fractures.
Below are photographs that further illustrate aspects of rock avalanches. Photographs 1 through 3 show the deposits from previous rock avalanches.
1 A geology class inspects a small avalanche deposit in southern California.
2 Dan Hooker atop an avalanche deposit near Las Vegas, Nevada.
3 A large rock avalanche pathway and deposit in southern Alaska.
4 Rock avalanche pathways along a ridge in the San Gabriel Mountains of southern California.
5 A view down one of the avalanche pathways from the previous picture.
6 A small rock fall and rock avalanche site, where the avalanche moved out onto a tennis court in Palos Verdes Peninsula, California.
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