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With terrifying precision a mass of land can shift and begin moving down hill creating what we call a landslide. This mass of earth and rocks have been known to wipe out everything in their paths. But why do landslides occur?

On October 3, 1963 the calm waters of the Vaiont reservoir in northern Italy were peacefully reflecting the slopes and forests of Mout Toc to the south. With sudden force the mountain moves. A rumble echoes around the surrounding peaks and 700 million cubic feet of the hillside crumble and fall into the lake. Upon impact of this hurtling mass of rock and soil a wave, 656 feet high, spreads outward from the scene of the collapse. A dam that is 870 feet of towering concrete, stands unmoving. Although the dam resists the impact of the wave, the water plunges over it and hurtles down the Longarone Valley. Village after village is swept away and the death toll reaches 2,600 people.

This disaster occurred when the water of the reservoir had seeped into the surrounding rocks, lubricating the joints and bedding planes so that the slopes became unstable. This made it possible for the great masses of rock that formed the mountain to slide over one another. It has been learned that this is the usual cause of the rapid erosional movements that we call landslides.

Most commonly landslides occur where the layers of rock dip toward the open slope. Once this has occurred the slope is very dangerous. Any piece of supporting rock that is moved from the bottom of the slope will cause the upper layers to come sliding down the bedding planes, lubricated by moisture that has seeped in.

Another familiar type of landslide is known as slumping. Internal rock structures are not involved in this landslide. An exposed cliff of soft material where the rock or soil at the foot of the cliff is squeezed outward by the weight of the rock or soil above causes the cliff to collapse. As the soil moves out it drags the overlaying masses down with it. This results in a series of curved slices and produces a steplike structure of the ground at the top. With this type of landslide, the movement may be quite slow, sometimes taking even several months. Or it can occur suddenly and disastrously.

A special kind of slumping is called a mudflow. The mudflow occurs in very loose material like mud or clay. When an unstable slope collapses in the usual curved slices like a normal slump, it then forms a jumbled tongue of disturbed material that can flow engulfing everything in its path. We often see this occurring in volcanic areas. In most cases the sides of an active volcano consist solely of soft ash that has been spewed out of the vent by the eruption. As vast quantities of steam erupt from the volcanic vent, it falls like a torrential rain. When the powdery slopes drenched and waterlogged, resulting mud rushes downhill, adding to the volcanic disaster.

A good example of this occurred about a thousand years ago in Java when a 9,800 foot volcano slumped into a nearby valley completely damming it. The jungles and temples of the valley, the home of a sophisticated civilization, were erased by the lake that formed. Even today, the folk culture of Java has tales of this civilization which perished some ten centuries ago.