To begin talking about earthquakes and volcanoes I began developing a KWL chart with my students. Most elementary teachers are familiar with this graphic organizer that helps to organize students’ previous knowledge. This organizer is particularly good at helping students to keep information they learn organized. In filling in this organizer students formulate questions that they wish to answer as they pursue their research. The organizer consists of 3 columns titled: K -- What I know, W- What I want to know, and L- What I learned. The last column is usually completed as students find out answers to their questions or after their study is completed. This is also the place where students might correct erroneously held opinions, or add information they learned that is beyond what they had anticipated. In doing this with my class I found that there was some misinformation contributed by some; while one boy who had read about volcanoes already had a good foundation of some of the basics of volcanoes. I did not correct anyone’s mistaken ideas. That will come about as we get further into the topic.
Most earthquakes are caused when stress builds on rock and it cracks making a new fault or when the rock moves along a fault. A fault is a break in the rock that makes up the crust of the earth. Not every crack is a fault. What makes a fault unique is that one side of the rock has moved in relation to the other. You can see faults in certain geographic locations and larger faults like the San Andreas can even be seen from outer space.
Once again the block (brick) and board demonstration can be used to show the slip and slide motion that results in an earthquake. You can tie an elastic band and/or a spring to the block. If an attempt is made to drag the block by pulling the spring or elastic slowly it may resist. Frictional sliding between surfaces does not always occur in this stick-slip motion. Depending on the surface the sliding can be smooth. Along places like the San Andreas Fault in California where the Pacific and North American plates are scrapping by each other there we can be this somewhat smooth or small “creeping”which causes many small microearthquakes but no large ones.
If the block hits some resistance the spring or elastic will stretch simulating the building up of tension along the fault line as two plates try to pass one another. As the block refuses to move more and more energy is in the elastic or spring. The more the elastic or spring stretches the more energy is being stored. The force on the elastic (spring) builds. A lot of energy can build up in the elastic (spring). When the static friction holding the block in place is overwhelmed the block will slip forward and the force in the elastic (spring) drops. When enough of the force in the elastic (spring) is released so that it is equal to the force of the sliding block, the block will stop moving. What we have demonstrated with this demonstration is a very basic explanation of what is called the elastic rebound theory.
The elastic rebound theory was developed by Harry Fielding Reid who was the only non-Californian invited to study the aftermath of the 1906 Earthquake in San Francisco. Reid concluded that the cause of the earthquake was not at the source of the destruction but miles away where pressure had built up over the years. The ground underneath had become unstable and when the pressure became too much the land snapped back like
an elastic band (http://www/ucmp.berkeley.edu/geology/anim1.html). It should be stressed to students that when this movement happens it is usually a big lurch resulting from tremendous pressure and although in time most earthquakes last a few moments the result can be catastrophic.
Another possible illustration of this is to take two rectangular pieces of clay and put marbles into one side of each piece. When the two pieces of clay are put on top of one another the bumpiness of the marbles simulates the roughness of the plates as they try to slide by one another. In trying to move the two pieces children can see that the marbles impede the smooth movement of the clay and they become entangled with one another. If they try to pull one piece past another sometimes a marble will dislodge and the clay will jerk along for a split second only to be caught again by the next marble. (Van Cleave,
Earthquakes
, pg. 15)
In addition it was suggested in some readings that the stress along the fault line is like that of the rope in a tug-of --war game. As long as both sides pull with the same force the rope is taut and nothing moves. As one side gains in force some of the contestants on the other side will drop out. The remaining members try to hold on and the pressure on them increases. Finally when the stress overcomes them they will give up which in terms of a fault slippage would occur and an earthquake would ensue. This is also a good demonstration activity for students to try. It allows them to actually feel the build up of energy as they tug on the rope and experience the release of energy when they are unable to sustain the equilibrium with the other side. As one side releases the rope the others are jerked forward and there is a sudden movement of the participants. One of the ideas that is also difficult for students to understand is that the build-up of tension is usually a long process but the actual release of tension is quick. It was suggested to me that in having children replicating the building tension with the tug-of-war the children might be asked to hold the tension and build it while being timed so that they could see the relative time in tension was significantly longer than the actual release of pressure (Nankivell-Aston and Dorothy Jackson, pg. 28).
