It is believed that all of the dry land on Earth was once part of a huge landmass which we call “Pangaea” which means “all earth”. Alfred Wegener (1880-1930) was a German scientist who was the first to propose this theory which he called “Continental Drift”. Not only do the current shapes of the continents appear to fit together like puzzle pieces, but there is also fossil, rock and glacial evidence to support this theory.
But WHY have the continents changed position over millions of years? You can view the continents as huge rafts of land that are floating upon a molten sea of magma. As the magma moves, due to convection currents, the continents move also. The can move apart from each other, or collide. The map at the top of this article shows where the different plate boundaries are. The San Andreas Fault that runs through California is one of these boundaries. When stress builds up and is suddenly released, an earthquake occurs. The more western side of the fault is moving northward and the eastern side is moving to the south. The two plates are griding against one another and some day Los Angles will be in Northern California. Millions of years ago, what is now India crashed into Asia, creating the Himalayas.
So, over millions and millions of years, the continents are drifted to their current positions and they continue to move today. The Atlantic Ocean gets about an inch wider every year, while the Pacific ocean becomes smaller. This picture is one rendering of how the continents may look millions of years from now. Notice that all of Europe is now inland and Africa is now across the Atlantic from North America. Notice how Baja California is much farther north in this picture than it is in reality today.
There are 3 basic ways that two plates can interact with one another. The first on is by CONVERGING. A convergent plate boundary is a place where two plates come together. How the plates interact depends on if the plates consist of oceanic crust or continental crust. Oceanic crust is much thinner and less dense, so it tends to slide under the continental crust. The plate that slides under the other is said to be “subducting”. It will re-enter the mantle of the Earth and melt. If two continental plates collide, then the smashing of the continents can cause the plates to crumple and thrust upward creating mountains (like the before mentioned Himalayas).
Other type of plate boundary is DIVERGENT. This is a place where two plates are pulling away from each other. There is a boundary such as this in the middle of the Atlantic Ocean, which is what is causing the Atlantic to widen each year. At the plate boundary, you will find the “Mid Ocean Ridge”. As the plates pull apart, lava comes up from in between causing new crust to form.
The last type of plate interaction is called a Transform Fault. In this type, two plates slide and grind past each other as in the San Andreas Fault, described earlier.
Teachers: It is important for students to MODEL the different plate interactions. Long pieces of foam can be used to show two plates moving together and crumpling upward to demonstrate a continental convergent plate boundary. The two pieces of foam can be placed between two school desks and pushed upward to demonstrate divergent plate boundaries. Place in a block on the foam as it emerges from the crack between the desks shows how a landmass might spread apart from another as a result. Sliding the two pieces of foam next to each other demonstrates a transform fault. Placing a block on each piece shows how the land masses change position. These demonstrations are effectively done as centers during two 50 minute science periods. Students should be asked to show their understanding by answering questions and drawing the models, using arrows to show which way the plates are moving. Alternatively, students can model the plate interactions with graham crackers by sliding them around on a paper plate coated with frosting. This idea came from an old issue of Mailbox Magazine. Younger and older children alike enjoy this, but be sure to include a written assessment of some kind so that the focus is the science and not the food.
https://www.learner.org/interactives/dynamicearth/ This site, linked below, would be a great activity for middle school students to explore this topic at their own pace as a computer/lab activity. It could also be done in pairs. It has a quiz at the end to use as a lab grade or assessment.
Bill Nye the Science Guy videos are available on this topic, as are short video clips from www.brainpop.com. BrainPop requires a subscription but will be worth the expense for most teachers (upper elementary and up). Each video clip comes with a 10 question quiz to be given after viewing. Students can watch them individually and take the quiz on line, or a whole class can view the video on the TV, projector or Smartboard. Both Bill Nye and BrainPop are funny and high energy presentations that children enjoy.
- Wegener Continental Drift Theory
- Future continent positions
- U.S Geological Survey
- Dynamic Earth Interactive
- Bill Nye Earthquakes Teacher Guide