Landslides, Mudslides and Serpentinite Rock
How Serpentine Minerals May Contribute to Mass Wasting
Sep 25, 2009
Landslides and mudslides have been a difficulty in Oregon and California for many decades. They tend to be more frequent during the winter months when the hillsides are saturated with water. This increases the weight of the underlying soil, particularly on steep terrain, and intensifies the sheer force from gravity to cause a destructive flow of rock and debris to move rapidly down slope. This geomorphic process, called mass wasting, is mostly dependent on the amount of water in the soil and the effect of gravity upon it. Another contributing factor to this type of erosion may be due in part to the large amount of serpentinite rock found in those regions.
Petrology of Serpentinite
Serpentinite is a metamorphic rock composed of the mineral serpentine, a group of ultramafic minerals that were formed beneath the ocean floor. Serpentine comes primarily from peridotite, an ultramafic igneous rock. It is the main element of the Earth’s upper mantle and its elastic quality is what allows for the movement of the continental and oceanic plates. Serpentine is formed when peridotite, seawater and sediments high in iron and magnesium undergo metamorphosis. Its crystals develop into flat, leafy sheets, which when bonded together creates a weakly structured mineral.
There are four major varieties of serpentine. Each is easily weathered, which may be a contributing factor in landslides where serpentinite is prevalent. As far as rocks go, serpentinite is fairly soft and malleable. Serpentine minerals are relatively low on the hardness scale (3.5 – 5.5). It is because of this attribute that serpentinite is easily squeezed to the surface, finding the paths of least resistance among harder and denser rock. Exposures can be found from Alaska to central California. Serpentinite is so prolific in California, that it’s the official state rock.
Serpentine Soil Composition
Serpentinite begins to break down rather quickly once it’s exposed to weathering. The soil is loose, granular, and orange in color from iron oxide. There are two properties of serpentinite that may be contributing factors in landslides where serpentine soil is present. First, it is not a good aquifer and in areas of moderate to high rainfall, excess water in the ground can decrease its sheer strength by reducing the surface tension that holds the soil together. Second, high concentrations of iron, nickel, chromium and magnesium make it a poor soil for vegetation. Sparse plant life, especially a lack of grasses, leaves the soil deficient of root structures needed to make it stable. These were the conditions in January of 1974 at Canyonville, Oregon when a mudslide triggered by high rainfall killed nine men.
Serpentinite and Landslides
Along the southern Oregon coast, where weathering from rain, wind and salt spray occurs, rock and debris slides happen annually along the coastal highway. The areas around Gold Beach, Oregon and the Pistol River are particularly susceptible where serpentinite outcrops extend up the hillsides. Farther inland, serpentinite formations run the length of highway along the Smith River canyon, producing two or three rockslides a year. California’s coastal highway, from the Oregon border to the Santa Barbara area, is one of the most hazardous slide regions in the state. While much of the rock is a mix of sandstone, mudstone and shale, large deposits of serpentinite weave their way through the Franciscan Complex in Northern California and the Great Valley Group in the central coastal area.
Landslide Prevention Techniques
The Oregon and California Departments of Transportation (ODOT & CALTRANS), continue to proactively work toward solutions for reducing and containing landslides. Drainage pipes bored into hillsides and roadcuts can relieve the soil of excess water. Retention walls can help to stabilize hillsides and block slide debris, while cliff screens block and trap falling rocks. Introduction of calcium and phosphorous into ultramafic soil has been known to increase plant production, providing more stability from root structures. While none of these are permanent solutions, they do help to lessen the possibility of a slide and help decrease the amount of damage and injury if one occurs.
Sources:
McGeary, et al. Physical Geology, Earth Revealed, New York: McGraw Hill, 2001. Print.
Bonewitz, et al. Rock and Gem, The Definitive Guide to Rocks, Minerals, Gems and Fossils, New York: DK Publishing, 2005. Print.
Alt, David D, and Hyndman, Donald W. Roadside Geology of Oregon, Missoula: Mountain Press, 1978. Print.
McKinley, Mason. Serpentine: Expressions of Earth’s Deep and Dark Inner Workings, University of Washington, (June 2001), PDF.
Qwest, “Preserving Phone Company Heritage”, Qwest.com, 2009. Web. Jan. 1999
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