Earth Movers

Podcast

Earth Movers

Transcript

How the earth moves under our feet. I'm Bob Hirshon and this is Science Update.

We've heard how earthquakes are caused by plates of the earth's crust moving around. But Duane Bryan of Anchorage, AK wanted to know what drives them.

Bryan:
The tectonic plates that move and cause the earthquakes, what causes them to move?

We spoke to Andy Freed, a geophysicist at the Carnegie Institution in Washington, DC. He says that the plates float on a layer of rock called the mantel. And that mantel acts like an incredibly slow-moving liquid.

Freed:
People tend to think of it as molten, but it's really not. It's really just more like soft rock that over millions of years—again—acts like a liquid.

And like a liquid, the mantel has currents. These are driven by a fairly simple concept—light warm things float, and heavy cold things sink. Dr. Freed says it's like a lava lamp.

Freed:
One material when it gets heated up, it rises inside the lava lamp. When it gets near the surface, eventually it gives off its heat and it cools and it sinks back down.

And Dr. Freed says the plates also have their own momentum. For instance, sometimes one plate will override another—pushing it down. And since this plate is cold and heavy, it'll keep sinking down into the hot mantel.

If you've got a earth-shaking science question, call us at 1-800-WHY-ISIT. If we use your question on the air you'll get a free Science Update Mug.

For the American Association for the Advancement of Science, I'm Bob Hirshon.

Making Sense of the Research

In many cities around the world, earthquakes are an ever-present threat. The recent earthquake in Seattle is evidence that they can strike at any time, and without warning. And while it's well known that earthquakes are caused by the earth's crust moving around, this Science Update investigates the cause of that cause—a question long debated in the scientific community.

The fact that plates have moved in the past and are moving now is beyond dispute, but details of why and how they move are more challenging to scientists. The crust of the earth is made of solid, lightweight "plates." Over millions of years, the layer of rock under these plates acts like a seething, bubbling, flowing liquid. Deep within, the earth moves with a churning motion, carrying heat from the interior of the earth to the surface where it is cooler. These motions of the material under the earth's surface in turn cause the plates to move. The plates drift so slowly—about two inches per year—that we wouldn't think of this process as shaping the earth. And yet, along with other processes, it does!

Now try and answer these questions:

1. Describe the layers of the earth.
2. What explanations do scientists offer for the movement of the earth's plates? What is occurring within the earth? What is happening at the surface of the earth?
3. How is this motion of the currents in the mantel similar to those in a lava lamp?
4. Can you think of other processes in which convection and subduction play a role?
5. Where else do you see proof of the concept that warm, light things rise and cold, heavy things sink?
6. How does the theory of plate tectonics in this story differ from previous representations of the same concept that you have seen? Is there a conflict between these representations?
7. What other processes shape the earth?

Going Further

To explore plate tectonics and to understand how mountains, earthquakes and volcanoes are related to the movements of plates, go to the Science NetLinks lesson for grades 9-12, entitled Earth Movers.

For more on plate tectonics, read Earth Forces in the Crust, part of Earthforce from the Franklin Institute. Scroll down to the bottom of the page for a wealth of additional resources related to plate tectonics and earthquakes, most notably the comprehensive site from USGS called This Dynamic Earth: The Story of Plate Tectonics.

For information, maps and diagrams related to the processes that shape the Earth, go to Plate Tectonics from Windows to the Universe.

For additional resources related to this topic, see Content Section: IV. The Physical Setting.