Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
by Patrick A. Estes
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.