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Plate Tectonics

Plate tectonics is a theory which says that the Earth’s surface is made up of moving parts. We call these parts ‘tectonic plates,’ or plates for short. Plates move for many reasons. But, convergent boundaries are central to their movement. As plates drift around, they run into each other a lot. This creates plate boundaries which have a huge effect on the land around them.
Map of the Earth's plates.

The Earth’s plates.

Theory of Plate Tectonics

Sometimes, the world can seem very mysterious. We see huge landforms like mountains and wonder: How did those get there? And, that’s a good question to be asking. It helps us think about how scientific processes shape our world.
Often, they are more important than we think. We can see that the earth is constantly changing if we study it closely. In fact, the theory of plate tectonics tells us that our entire world is in motion. It says that the earth is made up of drifting plates. 
These plates include both continental crust and oceanic crust. For example, the South American plate includes both the continent of South America and some of the Atlantic Ocean. These different kinds of crust are stuck together as part of a plate. If the ocean moves, so does the continent. 
Snow capped mountain illustration

When plates collide, they can create mountains.

And tectonic plates are always moving around and colliding. We call this movement continental drift. Continental drift is actually what creates mountains! If two plates collide, they might push up into a mountain.
We know this because of plate tectonics. Yet to use this theory, we have to know what exactly plates are!

Tectonic Plates

We say that the world is broken up into plates, and that’s kind of a weird saying. Plates are small and fragile. We use them to eat! So, why would we use the word ‘plate’ to describe parts of the earth?
Porcelain plates.

A stack of dining plates. They’re nothing like tectonic plates!

Well, what scientists actually mean by ‘plate’ is that these parts are quite thin. At least, they’re thin in comparison to the rest of the Earth. Tectonic plates account for only a tiny portion of the Earth’s size.
We call this part the lithosphere. The lithosphere is a layer of the earth which includes the crust and some of the mantle. It rests on top of a layer of magma, called the asthenosphere. Because the lithosphere is solid and this liquid magma is dense, plates float over the magma. This is what lets them move in the first place.
But what’s pushing them around, over this magma? As we’ll see, there are a few ways to answer that question.

What Drives Plate Tectonics?

Three major forces that cause plates to move. Let’s start with the most important.

Slab Pull

Slab pull is what happens when oceanic crust is pushed under continental crust. This process is called subduction, and it takes place at convergent boundaries. We call the sinking oceanic crust a ‘slab.’ Remember that this slab is attached to a tectonic plate. So, as the slab sinks, it will pull the whole plate with it.
A diagram of plate subduction.

Plate subduction diagram.

Ridge Push

Another, less powerful force moving the plates is ridge push. In the middle of most oceans, there is a mid-ocean ridge. This spits out lava constantly which is burning hot when it comes out. But, it cools quickly in water. When the lava gets cold, it turns into a solid and merges with oceanic crust. We call the solid igneous rock.’ This dense, heavy rock will slide down the ridge. In the process, it pushes oceanic crust, and the plate it’s a part of, along with it. The resulting movement is referred to as seafloor spreading.

Magma Movement

The last force moving the plates has to do with how hot magma moves. As we’ve discussed, tectonic plates make-up the lithosphere. Below the lithosphere is a layer of magma, called the asthenosphere. Like most liquids, this magma isn’t motionless. Instead, it’s constantly swirling around as it gets warmer or colder. 
When magma gets hot, it rises closer to the lithosphere. We call this rising magma a mantle plume. The hot magma in this plume will actually cool down as it gets closer to the earth’s surface. Then, it sinks back downwards in an arch. This arch-shaped motion of hot magma pulls the tectonic plates floating on top of it. 

Other Great Resources:

Video on Plate Tectonics: 

Written by: Noah Louis-Ferdinand.