Landslides occur when large masses of a hillside collapse. Landslides can occur because of rain, temperature change, volcanic activity, earthquake shocks, human modification of the land, vibrations from machinery or traffic or the weight of accumulated material on top of a pile such as snow, rock, or ore. In all of these cases the trigger mechanism for the landslide is the gravitational pull on the earth.
Usually there is an angle at which material on a slope will be stable. This is called the “angle of repose” or “angle of friction”. When this angle is overburdened the slope increases and the mass falls. Avalanches are similar occurrences only involving snow.
One of the easiest things to do with a class of children is allowing them to play with a box of sand trying to build it up in a mound and seeing how it eventually reaches a certain height at which it will collapse and flatten.
I chose to do some things with the class to show them how different materials behave when dealing with stress. The three categories that describe the behavior that most material exhibit are elastic, ductile, and brittle. If you take a thin board and try to bend it there will be some flexibility but the board will return to its normal shape through its elasticity. If you were to bend a piece of clay it would assume its new deformed shape illustrating it to be ductile or plastic. Finally, if you took the same board and bent it in a quick sharp way it would it quickly snap into two pieces. (Abbott, 31-32)
There is an interesting variation of this for students to do in the book:
How the Earth Works
by Michelle O’Brien-Palmer (pg. 170-171) in which students check for the effects of pressure on a piece of uncooked spaghetti, a graham cracker, and a craft stick or tongue depressor. Students record their findings as they bend, then try to snap the materials one at a time. How loud of a sound does it make when the spaghetti snaps? It does bend somewhat showing some elasticity before it breaks. The Graham Cracker snaps with little elasticity and finally the craft stick takes the most energy to snap. That kind of snap is what occurs when an earthquake occurs as the result of the release of stored energy.
In terms of avalanches and landslides material is usually held in place through static friction. For the slide to begin the friction must be overcome. A series of demonstrations can be done to help students see this. The block and slide experiment is suggested. To begin simply you will need to have a piece of wood about meters in height and 30 cm wide and a block of wood similar to a child’s wooden block or brick. If the block is placed on the board there is no movement and the block remains in place. If you raise the board the block will remain in place until the “angle of repose” is overcome and the block will begin to slide. Students can measure the height at which the slide occurs with a meter stick.
You can extend this demonstration by giving students some dirt. Prop the block up so it has a small incline and put a small amount of dirt on the block to show students that the dirt is holding or the “angle of repose” has been reached. If you keep adding soil little by little the pile will be overwhelmed by the addition of extra material and begin to slide. In this case the cause of the landslide would be the addition of more material.
On a third try put a small amount of dirt on the board and begin increasing the angle of the board by slowly raising it. The dirt will begin to slide when the friction has been overcome because of the increased angle of the board. You can continue this demonstration by having students introduce different material in place of the dirt or to the dirt. For example what happens if small pebbles are substituted for the dirt? What happens if pebbles or small sticks are added to the dirt? How does the addition of water change the “angle of repose”?