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Continental Drift

Continental drift is a scientific theory which says that Earth’s continents are slowly moving. Continents, which are part of Earth’s crust, lie on top of the mantle. The mantle is full of hot magma which the continents drift over, like leaves on water. An illustration of continental drift.
 
It’s hard to believe that the continents could be moving. For a while, most scientists couldn’t believe it either. But, the theory of continental drift has some very strong evidence behind it. As an example, we’ll look at one major discovery about the continents. 

A Worldwide Puzzle

The first clue that the continents are moving is actually right in front of us. Take a look at the world map below. You might think you know it well. But, there’s a big secret hidden in it! Here’s a hint: Compare the shapes of South America and Africa.
World Map
 
If you didn’t see it, I’ll tell you. The east side of South America matches the west side of Africa. They’re like two pieces in a puzzle! That’s no coincidence. Still, it took scientists a long time to figure out what it meant. When they did, it was almost too strange to believe.
 
They said that the two continents, Africa and South America, must have been together at some point. Then they formed one bigger continent. And if they broke apart, they must be moving! Once scientists understood this, their next question was: how? This proves a much harder puzzle than the first. Fortunately, they figured it out. Let’s take a look at the science of continental drift.

Slow Motion

From the start, it’s clear that continents are not moving too fast. If they were, it would be pretty hard to miss. Instead, continental drift relies on very slow processes in the earth.
 
You might ask: What could move a continent around? They’re huge!
 
And that’s a fair question. Continents are enormous chunks of rock. Yet, compared to the rest of Earth’s layers, they’re actually not that big. Continents are part of the Earth’s crust. The crust, which is where we live, makes up only a tiny fraction of the Earth. The layer below, called the mantle, is 84 times as large! It’s not hard for the massive mantle to move the crust around. This happens in a few ways. 

Divergent Boundaries Push Continents Apart

One way continental drift happens is by spreading at divergent boundaries. Divergent boundaries are places where the earth’s crust is splitting up. On either side of the boundary, the crust is pushed away. This is what separates continents.
 
The splitting is caused by magma from the mantle. Sometimes, it pushes up into the crust and against the surface of the earth. This thins the crust there and separates the land on either side. Magma then continues to flow towards the separated pieces of land. It acts like a conveyor belt, guiding them away from each other. 
Rift Valley Diagram.

A divergent boundary. Divergent boundaries are formed by rising magma from the mantle.

 

This doesn’t always create two new continents. That only happens if the land splits completely. Although, continents are often pushed so far that their divergent boundary turns into a valley. If this valley becomes large enough, it can fill with water. This is what creates an ocean.
 
Oceans continue to grow through what is known as seafloor spreading. They’re often big enough to completely split a continent into two. At that point, the divergent boundary which formed the ocean is considered a mid-ocean ridge. 
 
We should note, though, that separation also causes continents to grow. Some of the magma at the divergent boundary will reach the earth’s surface and become lava. This lava will cool to form igneous rocks. The divergent boundary will then push those rocks into one of the continents.

Subduction Brings Continents Together

We now know that oceans form at divergent boundaries. But, what if they close?
 
Oceans form where continents split. Yet, if a new divergent boundary forms in either continent, the ocean stops growing on that side. It tries to keep spreading, but the continent pushes back. Because the continent is less dense, it pushes the oceanic crust down into the mantle. This process is called subduction.
 
If subduction is happening on both sides of an ocean, the ocean shrinks. This pulls the continents together.
Tectonic plate diagram.

Oceanic crust being pushed down by continental crust. This process is called subduction. diagram.

Other Great Resources

 
 
 
Alfred Wegener and the World Map Puzzle:  https://earthobservatory.nasa.gov/Features/Wegener/wegener_2.php