PLAES notes
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TECTONIC PLATES
Earth/Space Science
The motion of tectonic plates is very complicated and not completely understood by any
geologist. The best your students can get is an idea of what’s going on. Since plates are
big classical objects, they aren’t too hard to model with ordinary objects.
1. Scale and sizes of things. It is hard for students to imagine the size of the Earth
and its structures. The problem is made worse by lots of bad diagrams they have
seen. You can start out by asking them to draw a cross-section of the Earth one
foot in diameter. Ask them to try to draw to scale and include:
 The tallest mountain
 The deepest point in the oceans
 The deepest hole or mine dug by humans
After they do this, you can give them this data:
 The Earth is a little over 12,000 km in diameter (so 1 inch = 1000 km)
 The peak of the tallest mountain (Everest) is 9 km above sea level.
 The deepest point in the oceans is 11 km down.
 The deepest hole dug by humans is 12 km deep.
 Note: a crater in South Africa has exposed rocks from 25 km down.
How did they do so far? Have the students figure out for themselves how big Mt.
Everest and the oceans should be on their diagrams. Most are astounded to know
that if the Earth were 12 inches across, the mountains, oceans, and diamond mines
would be unnoticeable. [Ask kids to think of a bowling ball without finger holes.]
2. What’s inside? Point out that we haven’t been there. What we know comes from
the “shadows” of earthquakes on the other side of the world. When an earthquake
happens geologists look at seismographs from all over the world to map the stuff
that the waves passed through on their way through the Earth. [Note: volcanoes
do NOT spit up stuff from the Earth’s core. They spit up melted bits of the Earth’s
crust.]
Got Hostess Cupcakes? Got Peanut M&Ms? Non-diabetic kids will love this, so
make sure you have treats for the others. Slice a Hostess cupcake in half. Tell the
kids to imagine that the cupcake is spherical and that the icing goes all the way
around. Place a peanut M&M in the middle of the gooey white filling. If you have
a chart of the Earth’s contents, the students now have a model in their cupcake.
 The icing is about the thickness of the Earth’s crust (all that humanity has
ever seen!).
 The soft cake is about the size and consistency of the Earth’s mantle (the
mantle is not liquid!).
 The gooey filling is about the right size to be the molten core.
 The peanut M&M is about the size of the solid core.
Tell the kids they have to transfer their new knowledge to their 12-inch diagram
of the Earth BEFORE they get to eat their model. Kids will never forget this!
PLAES notes
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3. Why do plates move? A hotplate and any soft food that forms a crust will
demonstrate this. Melted cheese, melted chocolate, or thick gravy heated in a
saucepan will form a crust on top that cracks and moves as the goop is heated.
Note that the goop does not have to boil in order for “convection currents” to
move the surface! Very slow convection currents in the mantle (remember, the
mantle isn’t even liquid) are much of what fuels tectonic motion. [We also know
that the Earth’s mantle is not uniformly heated and this adds to the motion.]
4. Why are there oceans? Ask your students why there are oceans. Why are there
such huge areas covered by water instead of lots of land with lots of smaller
lakes? Why is the Earth’s crust lower under the oceans than under the continents?
Collect answers. [Many students will say because the water is holding it down and
that is an okay answer at the start, but it doesn’t answer the question. When North
America gets wet the water runs off into the oceans. The water doesn’t push down
North America. There must have been a reason why the oceanic crust was lower
in the first place. Revisit these answers after this exercise.]
 Get some same-size samples of oak and pine boards (or oak and poplar).
Ask the students to make some observations. Even with their bare hands
they should be able to tell that the oak boards are heavier for the same
size. Ask the students to use the boards to model the Earth’s crust and use
some water to model the mantle (which isn’t really liquid, but oh well).
Place the boards in the water. What do they see? [Teacher’s notes: All
boards float but the denser oak floats lower in the water. This is the
difference between oceanic and continental crust. Continental plates have
lower density so they rest higher on the mantle.]
 Puzzle: When Mount Saint Helens erupted, the stuff that came out was
composed of minerals that had (on average) a higher density than the
continental crust around us. Where does that mean it came from? How did
it get there? [Oceanic crust is pushed under the North American plate by
plate tectonics This is what causes the volcanic activity of St. Helens.]
5. When plates collide. Find a flexible material to use as a model for tectonic plates.
Mouse pads work great. Place magazines of different thickness under two mouse
pads. (Which one is the oceanic plate? The lower one.) Let the mouse pads
overhang the magazines a bit so that the edges are free to deform up or down.
Push the pads together. Make observations. Note that the higher one usually
bulges upward away from the edge and both (usually) bulge down at the edge.
Repeated trials will show all kinds of behavior including mountain ranges,
trenches, and a combined form of bulging and sagging followed by a sudden shift.
The sudden shifts are earthquakes. The bulging and sagging phenomenon is what
happened to cause the Indian Ocean tsunami of 2004 and it is precisely what is
happening off the coast of Washington.