In beginning the unit my first concern is to present to my students information about the structure of the earth and how that leads to the natural occurrences we will be discussing. In order to lay a foundation for students the unit begins with a simple explanation of plate tectonics as a basic cause of volcanoes and earthquakes. I can see that it is a surprise to most of my students that the Earth is so dynamic - meaning it is constantly moving. For them the notion that we sit on moving plates floating on a thick asthenosphere is incomprehensible new information. I realized that the best place to begin a discussion of plate tectonics is with a quick overview of the interior structure of the Earth.
Basically, I wanted to use as simple an explanation of the interior of the earth as possible. I used an illustration from a web site called the Nevada Seismological Laboratory. It shows the Earths layered into four sections: the inner core (solid), outer core (liquid), mantle (plastic), and crust (rocky). I then further explained that the top of the mantle was composed of the crust and a stiffer material called the lithosphere. The lithosphere includes a series of rigid plates that float on the asthenosphere which is extremely hot and has a plastic like consistency.
At this point, I also told my students about Alfred Wegener, the German scientist who in 1915 thought that all the continents were once one super continent called Pangaea (pan-gee-ah). The reason Wegener and others developed this theory was in answer to a question they were trying to answer. How could fossils of the same plants, and animals be found on different continents? There was no way to explain how they could be on such widely separated land masses unless all the land was once joined.
I think Wegener’s story is important because most of my students do not understand how scientists develop hypothesis and theories which may or may not be true. My students are use to finding or learning the right answer. The possibility of there not being an answer or of it being wrong is not acceptable. At the third grade level and even younger students have already let their own thinking be subverted by the need to have the correct answer. Taking a stand based on what they believe is correct is a foreign idea. They would rather follow the crowd and be wrong than risk being laughed at and possibly correct. It gives them some more insight to hear that Wegener was maligned by others who thought his ideas were crazy. Wegener’s theory became more plausible when it was revealed that the seafloor was spreading apart making a new seafloor while the older part of the seafloor was dipping back down and being absorbed back into the mantle as part of the movement of the earth’s plates. (Abbott, pg.53)
As part of this explanation, I decided to do a hands on activity concerning Pangea. There is a classroom activity sheet on prehistoric land masses from DiscoverySchool.com that is perfect for students’ use. The page includes the outline of the continents. I gave one to each child and had them cut them out and try to see if they could fit them together as Pangaea was once thought to be. After some hesitation students began to manipulate the pieces and they each glued their pieces to construction paper. The Enchanted Learning web site has a section called “All About Plate Tectonics” (http://www.enchantedlearning.com/subject/astronomy/planets/earth/Continents.shtml) which gives some basic information and illustrations. Following this I showed them an animation of Pangaea and how it split up and moved to its present configuration. There are quite a few available on the web. One such site (http://www:ucmp.berkeley.edu/geology/anim1.html) is basically an animation of the progression of Continental Drift through different eras. It also offers a quick summary that would be helpful to teachers of what happened in each period.
I printed out a world map from a Free Maps website (http://www.yourchildlearns.com/geography.htm) that when connected was 3ft by 3ft. On that I outlined the basic 13 plates and named them. I also indicated in which direction each plate was moving. Students could then see that the plates do not travel in a synchronous fashion. They have over the years bumped -- sometimes crashed into each other which accounts for the variety of topological features on the Earth. In order to illustrate the basic movement of the plates I chose to use a demonstration from
by Blobaum which requires Graham Crackers and waxed paper spread with a thick layer of frosting or peanut butter. I tried the peanut butter.
The demonstration went as follows:
1. Two crackers (simulating plates) are put side by side on the peanut butter and are slowly moved apart. There is a rift or crack where the plates separate and magma (peanut butter) oozes up from below and a new ocean floor or mountain.
2. Two crackers are put side by side and moved toward each other. One cracker should slide underneath the earth. The bottom plate will be dragged down into the earth’s mantle and is melted down by the heat below. New magma comes up from between the plates and becomes a volcano. Mt. Saint Helen’s is an example of this type of plate movement.
3. Wet the edge of one cracker and place it next to another cracker. Move them toward each other. A ridge of cracker will push up much like what happened when mountain ranges such as the Himalayas (including Mt. Everest) were formed as a result of the crashing of India into Asia.
4. Put 2 crackers side by side and slide one up away from you and the other down toward you. When plates move against each other like this they usually get stuck on one another. There may be a sudden movement when the plate jumps. The powerful vibrations sent out by the plates result in earthquakes.
All of these occurrences are caused by forces which are nothing more than pushing and pulling that makes things move. We would begin with a simpler occurrence which most students have experienced namely, landslides.