The Evolving Theory of Plate Tectonics

SciWorks Radio is a production of 88.5 WFDD and SciWorks, the Science Center and Environmental Park of Forsyth County, located in Winston-Salem.

Why does the Earth's surface look the way it does? How can we explain mountains like the Appalachians or valleys like the Dan river valley? If you're in the NC piedmont, you are standing on what was once one of several Island Arcs, like Japan which broke away from their parent continent and collided with what is now North America. That was followed by a collision with what is now Africa. This continued division and collision can be explained by the theory of Plate Tectonics. But that's only a theory, right?

It's actually been elevated to observable fact. Since the development of GPS we can actually measure the plate velocities and it turns out they're moving in the directions and at the speeds that were predicted by the plate tectonic theory.

That's Kevin Stewart, Associate Professor in the Department of Geological Sciences at University of North Carolina in Chapel Hill. He wrote the book on the geology of the Carolinas.Its called Exploring the Geology of the Carolinas. its co-authored by Mary-Russell Roberson.

The outermost part of the Earth consist of rigid rock…

We call this the Lithosphere

…that is actually broken up into pieces that constitute the plates of plate tectonics; this way of explaining many of the features on the surface of the earth by the interaction of all these rigid plates. A lot of the really interesting features that we see in North Carolina are a result of when the plates are coming together.

For example, the appalachians are a result of continued collisions.

New lithosphere is created at a divergent plate boundary where the plates are spreading apart. And there's a wonderful divergent plate boundary out in the middle of the Atlantic Ocean.

You can see this on your favorite on-line map. It's an underwater mountain range that snakes its way along the ocean floor.

We have a hot upwelling of mantle rock.

The Mantle is the layer of earth below the lithosphere.

So when that hot mantle is rising at the divergent plate boundary we are making new oceanic lithosphere.

That rock is rich in Iron, and we call it basalt.

In other places the plates are sinking back down into the mantle and these are the convergent plate boundaries.

Convergent boundaries are where the plates are coming together. So, why is this all happening?

It's tempting to think about plate tectonics being driven by large circulating cells of mantle.

Which is what you probably learned in Science class. We now think the driving mechanism of Plate Tectonics is a different process.

There is this sinking slab going down to the convergent plate boundary but at a certain depth the slab then becomes a lot denser than the surrounding mantle.

Thats because the high pressure and temperature at this depth compress it and turn it to a very dense rock called eclogite.

And so it acts like a sinker and starts to pull the slab down.

But if eclogite is formed 50 to 60 miles down into the Earth, how do we know it exists, and why would we assume it is causing what is known as “slab pull”? It turns out that North Carolina has provided some of these answers.

In the past we were the location of the convergent plate boundary and we've had a subduction zone…

Where one plate dives under the other.

And we also do have some of this strange rock this eclogite exposed in western North Carolina. And the fact that they are far from the coast reflects that they were created during a relatively old continental collision and we've had a lot of rock added to North Carolina since then.

Rocks are a record of Earth's Geologic history. Go outside and pick one up. What could it tell you about your place in North Carolina, or North Carolina's place in earths history?