Introduction to plate tectonics
- [Narrator] What if I told you that the earth below you is moving? You'd probably say, "Of course it's moving. We're standing on a planet that's spinning on its axis while revolving around the Sun at about 107,000 kilometers per hour. And on top of that, our whole solar system is circling the center of the Milky Way galaxy." But there's another kind of movement that happens slowly in the rock beneath your feet. Earth's lithosphere, which is made up of the crust and the upper part of the mantle, is broken up into pieces called tectonic plates. These plates move around on top of the asthenosphere, which is the section of the mantle just below the lithosphere. Don't bother racing a tectonic plate, because your victory would be guaranteed. Tectonic plates typically only move a few centimeters in a year, which is about as fast as your fingernails grow. However, after millions of years, those distances add up. This means that the Earth hasn't always looked the way it does now. Scientists believe that the continents were once all connected in one big supercontinent called Pangea. Just imagine if the continents were still connected today. You could drive from Africa to Antarctica or even take a train from South America to Europe. Over 200 million years, Pangea broke apart, and the pieces drifted into the continents we know today. So if the plates move so slowly, how do we know that they move at all? Scientists have documented evidence from various features on Earth that support the theory of plate tectonics. If you cut out the continents on a map, you could see that they almost fit together like a jigsaw puzzle. For example, the east coast of South America looks like it could fit into the west coast of Africa. Matching or complementary coastlines is one piece of evidence that continents were once in different locations. Another piece of evidence is that scientists have found fossils from the same species on different continents. There's no way those land organisms could've traveled across the ocean. This suggests that the animals lived when the continents were connected. And the plates are still moving, slowly but surely, to this day. But they aren't all moving in the same direction. A plate can collide with one plate, move away from a different plate, and slide past another. We categorize the ways plates interact at their edges as having convergent, divergent, or transform boundaries. Con is a Latin prefix meaning together, so convergent plate boundaries are places where two plates come together. Let's take a look at an example of a convergent boundary where the Indian tectonic plate collides with the Eurasian plate. The crust of the plate becomes compressed, and the Indian tectonic plate gradually moves under the Eurasian plate. However, the lower density of the crust keeps the Indian plate from sinking back into the asthenosphere all the way. This lifts up the Eurasian plate and creates the Himalayan mountains. These mountains are some of the highest in the world, and they include Mount Everest, which is the tallest mountain above sea level. And the two plates are still colliding. This causes the Himalayas to grow by more than one centimeter each year. Another kind of plate movement is called a divergent boundary. The di in divergent comes from a Latin prefix meaning apart, so divergent boundaries happen where two tectonic plates move apart. Divergent boundaries can create different kinds of land forms, like rift valleys and mid-ocean ridges. The third kind of plate boundary is called a transform boundary or transform fault, and it happens when two plates slide past each other. Okay, maybe slide isn't the best word for it, because the plates don't move in one continuous motion. You can imagine when two incredibly large, bulky, rocky things move past each other, there's a lot of friction. Pressure builds up between the plates, and once it gets to be too much, the plates slip and release the pressure. This motion causes earthquakes. And you might be wondering, what causes plates to move? Well, scientists are always learning about the Earth, since the Earth is so complex. Although Earth's internal heat may play a small role, more evidence shows that gravity is key. Tectonic plates are solid, and they're denser and cooler than the asthenosphere. Because the asthenosphere is also pretty solid, the plates rest on top of it. However, the asthenosphere is so hot that it can behave a bit like clay. This means that at convergent boundaries, the edges of the plates can sink into the asthenosphere, a process that is driven by gravity. So even though you'd win in a race against a tectonic plate, the plate will still keep on moving millions of years after you've declared yourself to be the winner.