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FALLS (rock fall and rock avalanche)
FLOWS (rock avalanche, debris flow, earth flow, and creep)
There are two versions of slides, but what all slides have in common is that the mass of sediment/rock sticks together as a coherent block as it travels down slope along a tilted plane or surface of weakness. Typically, this surface of weakness coincides with the tilt angle of the slope that mass wastes. Ultimately, as the moving slide mass comes to a sudden stop, it may break apart and continue down slope as a type of flow. Below are the two basic types of slides.
1) rock slide - This type of slide occurs where there is a tilted, pre-existing plane of weakness within a slope which serves as a slide surface for overlying sediment/rock to move downward. Such planes of weakness are either flat sedimentary surfaces (usually where one layer of sediment or sedimentary rock is in contact with another layer), planes of cleavage (determined by mineral foliation) within metamorphic rocks, or a fracture (fault or joint) within a body of rock. Rock slides can be massive, occasionally involving an entire mountainside, making them a real hazard in areas where a surface of weakness tilts in the same direction as the surface of the slope. Rock slides can be triggered by earthquakes or by the saturation of a slope with water. The addition of water to a slope increases its mass, and therefore increases the pull of gravity on the slope. In addition, water can lubricate a layer of clay or shale within a slope, which then serves as a slide surface for the rock above it.
Diagram 1 shows layers of rock tilted downward to the right. The topmost rock layer is prone to sliding because it lacks support at the base of the slope.
Here, in diagram 2, gravity finally overcame the friction between the topmost rock layer and the rock beneath it. Once this occurs, the topmost rock layer slides downward as a coherent block. As it comes to a sudden stop the slide block may break apart and continue moving for some distance as a rock avalanche or debris flow.
The two pictures below were taken in the Sierra Nevada Mountains of California, near Mt. Whitney. Over a period of millions of years, the Sierras have been uplifted along the Sierra Nevada Fault. As a result, the sedimentary and metamorphic rocks that used to overly the igneous rocks of the Sierras have been stripped away. As a result, the igneous rocks have expanded, forming cracks (called joints) along which huge slabs of rock peel away (exfoliate) and slide down slope. The first picture shows numerous joints tilting downward to the left, and freshly exposed surfaces along which rocks have slid. In the second picture, a large slab of rock that was part of a recent rock slide event rests on top of talus generated by past rock fall events.
In addition to the joints formed due to the unloading of igneous (and other) rocks, rock slides also occur along pre-existing weaknesses in metamorphic rocks where flat minerals are aligned parallel to each other. Such planes of foliation are responsible for the small rock slides that have occurred along the Alaskan road in this picture, where Diane Kawahata provides a scale for the viewer.
Rock slides also result from slippage of tilted sedimentary layers along the contacts between layers, called bedding planes. This is especially true if a clay-rich layer becomes wet and slippery. The picture to the left shows where tilted sedimentary rocks have slid into the Pacific Ocean near Gaviota, California, exposing the planes along which they slid.
Pictured and described below are two examples of major rock slide events.
This is a photograph of the Vaiont River Valley in northern Italy, taken by Ed Bromhead. In 1963, a major rock slide resulted in the deaths of approximately 2600 people. The slide block, labeled on the photograph, moved suddenly into the newly filled Vaiont Reservoir, flushing lake water up and over the dam. The wall of water was over 200 feet high as it swept into nearby villages, wiping out everything in its path.
The rock slide and the ensuing flood could have been readily forseen if better geological consulting had been done before construction of the dam and reservoir. The sedimentary rocks of the Vaoint River Valley include layers of shale, a clay-rich rock. And, the rocks comprising Mt. Toc (pictured), tilt steeply toward the reservoir. After the dam was finished in 1960, filling of the reservoir introduced groundwater into the shale layers, causing them to swell and become slippery. At first, the mountainside began slowly creeping down slope at a rate of half an inch per week. As filling continued and more groundwater seeped into the mountain, the rate of slippage increased to eight inches per day, and ultimately to 30 inches per day just before the 1963 disaster.
Lituya Bay was the site of a massive rock slide in 1958. This slide was generated by a powerful earthquake along the Fairweather Fault which cuts through the St. Elias Mountains just north of the bay. Although this is an uninhabited region of southern Alaska' Pacific coast, sailors do occasionally take advantage of the bay's protected waters for anchoring overnight. The night of the earthquake and rock slide, three different vessels were harbored in Lituya Bay. As the rock slide moved into the bay, it displaced millions of gallons of water, forming a wave estimated at 160 feet high. The wave lifted one boat up and over the right side of the island in the bay, causing little damage to the boat and crew. A boat near the left side of the island was carried out into the ocean by the wave, but the crew survived despite losing their vessel. No trace of the third boat was ever seen. (Note that the source of this photograph is uncertain; probably the USGS.)
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2) slump - Slumps are fairly small when compared to rock slides. Slumps form where the base of a slope is removed by natural processes (stream or wave erosion) or by human efforts (road or building construction). Removal of the lower part of a slope effectively removes physical support for the upper part of a slope, causing the formation of a new fracture in the sediment/rock comprising the slope. Soon thereafter, the slope will begin sliding downward, often rotating along the curved surface of rupture. The development of a slump is illustrated in the three diagrams below.
In diagram 1, the residents of the house have a fine ocean view, with stable rocks below them.
In diagram 2, ocean waves have removed the base of the slope beneath the house. Once this support is gone, a fracture will form, angling from the new base of the slope to the cliff top above. Residents of the house will see a widening crack cutting across their lawn, a warning of bad things to come. Usually, people will abandon their homes at this stage, removing as many belongings as possible.
Diagram 3 shows that the slump block has slid downward along the surface of rupture, what was originally the new fracture formed due to the erosion of the base of the slope. Since the fracture's geometry was curved, so too is the surface of rupture, which causes the slump block to rotate outward as it moves downward. Buildings tend to collapse as this occurs. Note that the end of the slump block often breaks apart forming an earthflow which continues to move slowly outward and away from the block.
Below are a series of photographs that illustrate some of the variety and features of slumps. Although most of these pictures are from locations in California, slumps can be found most anywhere there is a slope.
This picture, taken in El Moro Canyon near Laguna Beach, California, shows a new slump. The key features of this slump are labeled, and the outline of the slump block is highlighted with dashed lines. Slumping, along with the natural processes of weathering and erosion by water, causes mountains to become flattened over a period of time.
Point Sal, California The three pictures below were taken at Point Sal, along the coast of central California. Here, a small slump is narrowing the roadway. Picture 1 shows the surface of rupture and slight tilting of the top of the slump block. Pictures 2 and 3 are reverse angle views of the same slump, and the small fissure that can develop as the slump block separates and slides downward along the surface of rupture.
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Portuguese Bend, Palos Verdes Peninsula, California The following pictures were taken at Palos Verdes Peninsula, southern California, at the end of the Portuguese Bend Landslide. This complex area of mass wasting is characterized by earth flow movement within the main body of the mass, and slumping from the point of origin all the way to the end of the mass where it meets the Pacific Ocean. Picture 1 shows the upper end of the slide complex. Pictures 2 through 5 present different views of the end of Portuguese Bend Landslide, showing the relationship of this large landslide to the Pacific Ocean waves which continue to remove the end of the slide mass. As a result, this unstable portion of the California coastline crumbles and slumps into the ocean almost continuously. You should be able to recognize slumps in all four of these photographs. Picture 6 show where a new slump is forming next to the main road in this area, Palos Verdes Drive North.
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Point Fermin, Palos Verdes Peninsula, California Just down the coast from Portuguese Bend Landslide is Point Fermin, famous for its slumps. Until 1929, this area was an active, desirable neighborhood. Unfortunately, ocean waves eroded the base of the cliffs of Point Fermin, causing slumping of the cliffs, forcing abandonment of the neighborhood. What is left, called Sunken City by locals, are broken streets and sidewalks as well as jade plants and palm trees from people's front yards in different stages of movement downward toward the ocean on the tops of several large slump blocks. Pictures 1 and 2 below are views of the Sunken City from the ocean. Picture 1 shows that many people continue to live on the hillside just up from the Sunken City, and one wonders how long it will be until they have to abandon their homes. Pictures 3 and 4 give closer views of the slump blocks and fissures in this area, and pictures 5 and 6 show some Geology students experiencing the Sunken City for themselves.
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La Conchita, California In 1995, the small coastal California community of La Conchita began experiencing a combination of mass-wasting phenomena. It began with a large slump, shown in picture 1 (credit to R.L. Schuster, USGS). As impressive as this slide is from the air, it is even more imposing when viewed from the ground, in picture 2. From this perspective, it appears that the entire mountain is moving into La Conchita. The perspective of picture 3 is from the slump block looking downward into town and the Pacific Ocean. The slump and related earth flow were relatively slow-moving, and no one was killed by this event, although nine homes were destroyed. The earth flow was stopped by a strong retaining wall (pictures 4 and 5), giving La Conchita residents a false sense of security.
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In January of 2005, a series of storms soaked the slopes above La Conchita, adding mass to the unstable slopes and lubricating the sediment grains. The result was a catastrophic failure of a small portion of the slope above La Conchita. This event, labelled in news reports as a mudslide, began as a sudden rupture occurred within the slope, releasing a slump block which instantly crumbled into a fast-moving, fluid-rich debris flow which roared into the eastern part of La Conchita. Geologists estimate that a total of about 400,000 tons of earth moved during this event, which destroyed 13 homes and killed 10 people. The future of the entire community of La Conchita is now very much in doubt because of the ongoing threat of further mass-wasting events, and the prohibitive cost, estimated at $100 million dollars, to stablize the slope above La Conchita.
Below are series of photos taken soon after the 2005 La Conchita disaster. Pictures 1 through 4 are aerial photos of La Conchita, taken by Mr. Allen Krivanek, showing the path of the debris flow and the resulting destruction. Pictures 3 and especially 4 enable you to see what's left of the retaining wall constructed after the 1995 slump-earth flow event. It may have helped to shield some of La Conchita from the 2005 debris flow, most of which was deflected toward the right (east). A careful look at picture 2 suggests that the road that cuts a diagonal across the slope from left to right is the likely culprit for recent slumps above La Conchita: cutting a road into a slope over-steepens the slope and removes support from the slope mass above the road. See the enlarged "Features of a Slump" picture above for clarification of this simple but important concept.
Pictures 5 through 7 below are Associated Press photographs which provide close-up, ground views of the La Conchita disaster. Picture 8, also an AP photograph, gives a different aerial view than the aerial photographs above, showing the perspective of the debris flow from the source.
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The pictures below reinforce the idea that mass wasting can be dangerous and unpredictable in mountainous environments. In each photograph, slumping has eliminated part of a highway in the western United States. In both cases, traffic was interrupted for over a week while expensive repairs were made. And, in each situation, there is a strong possibility that slumping will occur again in the same area.
McClure Pass, Colorado. (Photograph by Terry Taylor, Colorado State Patrol.)
Zion National Park, Utah. (Photograph by R.L. Schuster, USGS.)
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