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Roger Weller, geology instructor

by Patrick A. Estes
Physical Geology
Fall 2009

Mass-Wasting & Landslides


     Nature expresses its emotion in many ways; nurture through light rains, anger through storms, appreciation through sunny days, and terrifying ability through occurrences like earthquakes, tsunamis and landslides.  Although simple enough to red-flag, mass-wasting can happen without expectation and with deadly severity.  This will be our focus; on landslides and their cataclysmic capabilities to alter the landscape and lives of Earthís peoples forever.


   Landslides, in any form, are almost always caused by four basic processes.  Weíll touch on these four processes briefly, get an understanding for how they manifest themselves, and possible solution for the prevention of loss of property and/or life in the event thereof.  When one thinks of a hillside or other elevated landmass as a slope, and considers the composition of the land as Horizons A (zone of leaching), B (the zone of accumulation) and C (decomposed rock), one gets a better understanding of how mass-wasting and landslides can occur.


   First, a slope must be steepened.  Steepening can be either by industrial excavation for the intent of building roads, the mining of ore and minerals, or for new housing developments whose worth is increased by their scenic, vista views.  On occasion, one will be driving alongside a hill and see it covered by concrete, cyclone fencing, or some other means to prevent or soften the effect of a landslide.  These measures may seem drastic, but if one has ever witnessed a building being torn into shreds or falling from a cliff, a certain respect must be given to these and preventative efforts.


   Second, the slope must be overloaded.  Overloading can happen many ways.  Most often, new housing developments are built along ridges to give them vista views, and housing developers higher asking prices.  Entire neighborhoods are constructed; hundreds of two and three-story houses systematically placed side-by-side in rows at the edge of an elevation.  Factor in automobiles, RVs, sheds, and other heavy housing accessories and it becomes easy to see how weight limits can be reached and exceeded in these areas.  Residents may want to have swimming pools installed, which require the excavation of relatively huge amounts of land.  They may wish to extend their houses outward with decks, observation towers, or additions being built-on.  In these cases, the thin line between structural integrity and disaster is thinned-out and the stage is set for a landslide!


   Third, the slope must be lubricated.  This may seem hard to do on such a grandiose scale, but it isnít.  Those very same swimming pools may crack and leak into the earth below.  A gardenís over-watering may create excessive amounts of water in the zone of accumulation.  And letís not forget about Mother Nature, who has a number of tricks up her sleeves to dampen the landscape.  Storms, flash-flooding, and erosion can also contribute to the lubrication of a slope.  We now have all the players for a landslide to happen; steepening, overloading, and lubrication.


   Last, but certainly not least, we need a seismic shock to take place.   It may seem as though youíd need a great force to cause an entire hillside to slump down into a talus slope, but as steepening, overloading, and lubrication happen, the magnitude of seismic shock necessary to cause a landslide is reduced respectively.  Earthquakes, the most common source of seismic shock, are to blame for a good percentage of landslides, but the roots of flora, frost/salt-wedging, and even meteorite impacts can also give the slope the push it needs to give-way and collapse.


   When one understands the four processes necessary to produce a slopeís collapse, nearly every landslide occurrence in history can be examined scientifically and have the processes attributed to it.  Moreover, understanding of the processes and furthering efforts in prevention will help to educate of as a people and will continue to save lives and soften blows to homes and property.