Name _________________________________________________ Class ______ Date ___________________
Follow up to Convection Current Lab
Theory of Plate Tectonics and Convection Currents
The idea that Earth’s surface might be moving is not new. As far back as the 1500s, when
mapmakers started including North and South America in their world maps, they noticed something
curious. The western coast of Africa and the eastern coast of South America seemed to fit together
like pieces in a puzzle. Were these continents joined at one time?
In the late 1800s, a German scientist, Alfred Wegener, began studying this question. In 1912, he
proposed a hypothesis known as continental drift. According to Wegener’s hypothesis, Earth’s
continents were once joined in a single landmass and gradually moved, or drifted, apart. For many
years, people did not accept Wegener’s ideas. Not until the mid-1900s did scientists find new
evidence that made them consider continental drift more seriously.
Scientists now had proof that tectonic plates move. But the same question remained. How could the
plates move thousands of kilometers around the planet? The asthenosphere provided a possible
answer.
Tectonic plates rest on the asthenosphere, a layer of soft, hot rock. Molten rock in the
asthenosphere and in the mantle just below it, moves by convection. Convection is heat transfer by
the movement of a material.
You have seen convection if you have ever boiled a pot of spaghetti or if you have seen a lava lamp.
The water at the bottom of the pot heats up, becomes less dense, and rises. At the surface, it cools,
becomes denser, and sinks, only to be heated and rise again.
The rock in the asthenosphere acts in a similar way. The hot, soft rock rises, cools, and sinks, then
is heated and rises again. If this sinking and rising motion continues, it is called a convection current
a motion that transfers heat in a material.
Convection currents in the mantle are much slower than those in boiling water. The rock creeps only
a few centimeters a year. The diagram below shows convection currents circulating. The tectonic
plates in the lithosphere are carried on the asthenosphere like long, heavy boxes
moved on huge rollers. Over millions of years, convection currents carry the plates thousands of
kilometers.
Convection
In most substances, as the kinetic energy of particles increases, the particles spread out over a
larger area. An increased distance between particles causes a decrease in the density of the
substance. Convection occurs when a cooler, denser mass of the gas or liquid replaces a warmer,
less dense mass of the gas or liquid by pushing it upward.
Convection is a cycle in nature responsible for most winds and ocean currents. When the
temperature of a region of air increases, the particles in the air spread out and the air becomes less
dense.
1. Cooler, denser air flows in underneath the warmer, less dense air, and pushes the warmer
air upward.
2. When this air cools, it becomes more dense than the warmer air beneath it.
3. The cooled air sinks and moves under the warmer air.
Convection in liquids is similar. Warm water is less dense than cold water, so the warm water is
pushed upward as cooler, denser water moves underneath. When the warm water that has been
pushed up cools, its density increases. The cycle continues when this more dense water sinks,
pushing warmer water up again.
Homework
In today’s lab you saw an example of a convection current. Answer the following questions by using
your lab experiences and the above information.
1. Why did the paper hole-punches move the way they did?
2. In the beaker of water, where was the denser water? The less dense water?
3. Draw a diagram of the beaker showing how the most dense and least dense were arranged?
4. If the denser water is the layer on top, what is it going to want to do?
5. If the less dense water is on the bottom, how does it get to the top?
6. The motions you described in #4 & 5 are a continuous cycle. Use the reading at the beginning and
describe in detail how a convection current There are several examples and you have observed in
lava lamps and hot air balloons.
7. How would the motions #4 and #5 cause something on the surface of the water or on the surface of
molten rock, like tectonic plates, to move? (Use the information above)
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