Mass Wasting (includes landslides, rockfall, etc.)
Geologists refer to the gravity driven down-slope movement of earth materials as “mass wasting.” There are numerous types of mass wasting distinguished by the type of earth material moving down-slope, the speed with which it moves, and sometimes the mechanics involved in its motion. The following are all types of mass wasting; creep, earthflows, landslides, mud and debris flows, rockfalls, rock and debris avalanches, rockslides, rock topple, and slumps. Non-geologists often refer to this entire group of processes and the landforms that they create as landslides.
Causes of Mass Wasting
Geologic, climatic, and human factors may all contribute to the occurrence of mass wasting events.
One of the primary geologic factors that contribute to mass wasting is the steepness of slope (topography,) generally the steeper the slope the more likely it is to fail. Other geologic considerations include the type of rock, rock chemistry, and fractures, joints, crystal growth, or bedding planes in the rock. Earthquakes and volcanic eruptions are geologic processes which often act as triggers for large mass wasting events.
Climate can have a dramatic influence on mass wasting events. Areas with a moist temperate climate are subjected to frequent freeze and thaw cycles. Freeze-thaw cycles affect rocks because when water seeps into cracks in a rock and then freezes it expands putting tremendous pressure (24 tons per ft2) on the rock, forcing the cracks to expand. Over time this type of physical weathering can cause even the strongest rocks to break apart. Sometimes this process is referred to as frost heaving or freeze thaw action. Thermal expansion and contraction of rock bodies also may occur in areas with extreme temperature variations and this likewise affects the stability of rock. Moisture, in addition to its influence through freeze thaw cycles (physical weathering,) can actually dissolve some of the minerals that rocks are made of. This is called chemical weathering and it also affects the stability of rocks. Sometimes large mass wasting events (or large numbers of smaller events) are associated with heavy rains. Heavy precipitation can initiate certain types of mass wasting by creating hydrostatic pressure and serve to lubricate slides once they are in motion.
Certain human activities are known to affect mass wasting, many of the potential problems can be avoided by using good land use planning techniques and engineering solutions where appropriate. Drainage diversion and irrigation projects may destabilize slopes and increase the probability of mass wasting by increasing the moisture in vulnerable areas. Likewise hillside excavation which changes slope gradient could destabilize slopes making them prone to mass wasting. Blasting, often associated with mining or construction activities, can result in vibrations and air blasts which may trigger mass wasting events. Other human activities which could potentially impact mass wasting include transportation vibration, increasing the load that the ground must bear (especially on slopes,) changing vegetative cover, and pumping.
Mass wasting events are caused by gravity in combination with a variety of geologic, climatic, and in some cases human factors. One other factor which is often associated with dramatic mass wasting events is vegetative cover and particularly sudden changes to vegetative cover by fire or human activities. Particularly large scale mass wasting events frequently follow forest fires and heavy rains on steep slopes. Although it is impossible to anticipate all mass wasting events being aware of the factors which contribute to such events is essential to avoiding them.
Frequently Asked Questions
How often does mass wasting occur?
While people think of mass wasting as isolated, sporadic events, a major rainstorm or earthquake event can trigger tens of thousands of landslides and rockfalls over a broad area in a brief period of time. Less dramatic events like creep occur almost constantly in certain areas of the country.
What is the typical setting or circumstance in which mass wasting occurs?
Mass wasting occurs in a wide variety of settings including annual events in each of the 50 states. Most mass wasting however occurs on steep slopes composed of weak or fractured rock following high moisture events.
What are the dangers associated with mass wasting?
Mass wasting can cause; loss of life and injury, property damage, blocked highways, roads, and trails, broken gas lines, oil lines, water mains, and or power lines.
What damages are associated with mass wasting?
Loss of capital assets (homes, buildings, structures, transportation networks, utility interruptions to water, gas, and electric supplies), injuries, deaths, and interruption to economic activities.
What monetary costs are associated with mass wasting?
In the United States landslide related losses are estimated to cost $1-2 billion per year with 25 to 50 associated deaths. The 1983 landslide at Thistle, Utah blocked the Spanish Fork River, formed a lake, inundated the town and cost an estimated $400 million.
What can be done in terms of hazard prevention?
- Avoidance is the best method for limiting mass wasting hazards. It is recommended that people stay away from and limit development in areas prone to rockfalls and landslides if possible.
- Good land use and geohazard planning employing appropriate geologic maps is recommended prior to development activity in areas prone to mass wasting.
- Engineering solutions may be appropriate for minimizing mass wasting hazards in certain situations, examples of such solutions include; removing the hazard through blasting or stabilizing the hazard using, rock bolts, mesh, steel, retaining walls, gabions, tension cables, grout, drains, etc. Other engineering solutions may not eliminate or stabilize the hazard but rather work to minimize or control rockfall and landslide run-out areas with ditches, specially designed tension cable nets, and talus walls.
- Ensuring that projects in mass wasting prone areas have proper drainage is also crucial to minimizing potential hazards.
- Proper maintenance and inspection of mass wasting prone areas is essential to spotting potential hazards.
- Finally, education is important for understanding mass wasting hazards. Interpreters work in partnership with the scientific community to ensure that complex information can be conveyed accurately, and in a form that is comprehensive and relevant to non-specialists.
Quick Facts and Trivia About Mass Wasting
- Landslides, flows, collapses, and other forms of ground failure are a hazard that virtually all states have in common.
- In 1998 the El Nino-related landslide damage in the 10 San Francisco area counties was assessed at more than $140 million.
- Worldwide, landslides are responsible for at least 600 deaths annually.
- In the 20th century, catastrophic landslides have destroyed entire villages, killing upwards of 20,000 people at a time.
- During historical time more than 400 rock falls have been documented in Yosemite National Park.
- The eruption of Mount St. Helens in 1980 triggered a 2.8-km rockslide/debris avalanche that rushed 24 kilometers down the valley of the North Fork of the Toutle River, destroying nine highway bridges, miles of roads and highways, and many public and private buildings.
- The airblast (atmospheric pressure wave) generated by the Happy Isles rockfall in 1996 in Yosemite NP created a wind comparable in velocity to that of a tornado or hurricane (110 meters/second = about 250mph) which snapped off about a thousand trees with the area.