Grade/Subject Area:

Not too long ago people thought the surface of the earth couldn't change, but some scientists were puzzled by places separated by thousands of kilometers that seemed as if they had been together at one time. Fossils and mountain ranges ending at the east coast of South America matched those beginning on the west coast of Africa, but with a wide Atlantic Ocean between them.
   We now know that the earth is covered by huge systems of rock called "plates" that slowly move and interact with each other. "Oceanic plates" are mainly found beneath oceans and most of us live on the dry land of "continental plates." The way in which these plates interact depend upon the difference in their density and how they meet.
  The cause of their movement was first found beneath the Atlantic Ocean where molten rock from deep within the earth comes up through the ocean floor and pushes the older sea floor aside. This is called a "divergent boundary" and creates new rock that is commonly found on the bottom of the oceans but can also be found on land. The Great Rift Valley in East Africa and the Gulf of California are examples that can easily be seen on a map.
  Since the divergent boundary's new rock needs room and the Earth can't get any bigger, something’s got to happen to the old rock. A "convergent boundary" is where the old rock is being changed and here’s three ways it happens:
  When oceanic and continental plates meet the oceanic plate will slip under the less dense continental plate and melt deep in the earth's crust; a great source of material for volcanoes. Two oceanic plates will bend downward when they meet and form deep underwater trenches, many of which are found in the western Pacific Ocean. And where two continental plates meet they'll bend upward to make huge mountains like the Himalayas between India and China.
  In some places the plates slip by each other at a "transform boundary." The North American (continental plate) and Pacific (oceanic plate) plates grind by each other along the San Andreas fault that runs from Mexico into the ocean near San Francisco.
  Many earthquakes are the result of the interactions of these plates. Energy from their movement against each other is released all of the time. Sometimes this energy is released in amounts large enough for us to feel and even cause damage.
  

Picnic Table Top Tectonics

You'll need:

• 3 paper plates (the easily bent kind work best)
• a brown, green and blue crayon
• a world map, or globe.

Lesson:

1. First, draw lots of green lines across one of the plates then tear the plate in half and put the 2 pieces back together. See how the lines match? Slowly move the pieces apart but no matter how far you move them they'll still match when brought together again. You can easily move these pieces apart with your fingers, but molten rock is needed to move the earth. Find Africa and South America on your map, or globe, and you'll see that even though they've been moving apart for millions of years they still would fit back together pretty well.

2. Next, color the inside of the two other plates with a brown crayon to make "continental plates." Slide these plates together on the table until their rims meet and are pushed up into the air. You've just made mountains. Find China and India on your map, or globe, and the tallest mountains in the world (the Himalayas) between them. They were formed when two continental plates pushed, and continue to push, into each other.

3. Now, turn your paper plates upside-down and color the bottoms with the blue crayon to make "oceanic plates." When you slide them together their rims push each other until they dip down. Under the ocean this would be called a "deep-ocean trench." Look on your map or globe for the Mariana Trench, between Japan and Australia, and its Challenger Deep, at 11,000 meters, is the deepest place in the world.

4. This time, slide a continental plate (brown side up) into an oceanic plate (blue side up) and see what happens. The oceanic plate slips under the continental plate and dives deep into the earth where it melts. Some of it can come back up in a very dangerous type of volcano which can explode when the water trapped in its rocks turns into steam too fast. Coastal Oregon, Japan and the west coast of South America are a few of many places where an oceanic plate is slipping under continental plates and creates lots of volcanoes. (Just think: As the Atlantic Ocean gets larger the Pacific Ocean becomes smaller until millions of years from now Asia and North America will become one big continent again!)

5. Place the palms of your hands together and gently rub them back and forth, like you're making a clay snake then press your hands together tightly and try the same thing. Your hands will stick together then suddenly slip, just like a "transform boundary" between plates. Our San Andreas fault is famous for storing energy when it "sticks" then releasing it when it "slips" causing large earthquakes.

References:

Lesson © Bob Culbertson

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