Plate Tectonics
Breaking, breaking! The land underneath you is moving. It is crashing into other land! Before you seek shelter, though, keep in mind that the earth’s crust has been moving your whole life and for millions of years before that.
Effects of Plate Tectonics
If you don’t believe me, let’s hop into your mind-TARDIS and go back in time 300 million years. All of our current land masses used to fit together in a supercontinent called Pangea.
The slow movement of earth’s plates caused the supercontinent to be split into smaller fragments through a process called continental drift. Because these continents sat on top of separate, moving plates, they pulled apart and slowly drifted to where they are now. As this happened, some pieces got broken up and turned into sand and dust: that’s why there is more land in the picture above than just the modern continents. They also changed shape a bit due to changing sea levels and erosion, all to make the world you know today. (The current world, of course, being the one they show from space in every movie before someone blows it up and/or saves it from being blown up.) Pretty amazing.
But, wait, there’s more! The plates are still moving. Yep, North America is moving away from Europe at about a rate of about 1 inch (2.5 cm) per year, which is about how fast your nails grow. This means I will have to travel a whole extra 2.5 cm per year if I want the good cheese. How will I manage?
The other plates are also moving at different speeds, some toward each other and some away from each other. As you read this, India is slowly crashing into Asia. That’s why the Himalayan mountains are so big. The land slowly crumbles upward as they come together.
The other plates are also moving at different speeds, some toward each other and some away from each other. As you read this, India is slowly crashing into Asia. That’s why the Himalayan mountains are so big. The land slowly crumbles upward as they come together.
India and Asia collision
Plates don’t just crash into each other and move apart, though. They also slide past each other. A famous example of this is the San Andreas fault in California. As you may know, California is pretty famous for frequent earthquakes, among other things. There is a simple explanation for this, based on the San Andreas fault: the sliding creates vibrations in the Earth, which creates earthquakes. Finally, at various hot-spots where plates come together, lava bubbles up from the Earth’s molten mantle, forming volcanoes. Some volcanoes are very gentle and pretty, like these ones:
Some volcanoes are a bit more explosion-y and destructive, like this one:
What are Tectonic Plates?
“That’s interesting,” you say, “But you still haven’t told me how any of this happens.” Fair point, enthused reader! Let me explain.
The crust is the outermost layer of the earth and is broken into 7 pieces called tectonic plates. Some of these are oceanic plates and some are continental plates. They have slightly different compositions, partly because different lifeforms live there and release different amounts of organic material back into the crust when they die. Oceanic crusts are also denser, younger, and thinner than the old, thick, less dense continental crusts. This is partly because the flow of water over the oceanic crust weathers it and keeps it fresh—kind of like how an auntie’s moisturizing face cream keeps her looking young and fresh. Oceanic crusts are also younger because the divergent faults that make new crust are in the ocean.
The crust is the outermost layer of the earth and is broken into 7 pieces called tectonic plates. Some of these are oceanic plates and some are continental plates. They have slightly different compositions, partly because different lifeforms live there and release different amounts of organic material back into the crust when they die. Oceanic crusts are also denser, younger, and thinner than the old, thick, less dense continental crusts. This is partly because the flow of water over the oceanic crust weathers it and keeps it fresh—kind of like how an auntie’s moisturizing face cream keeps her looking young and fresh. Oceanic crusts are also younger because the divergent faults that make new crust are in the ocean.
Plate boundaries-- click to enlarge
Why Do Plates Move?
Now that you know what tectonic plates are, let’s take a look at how they move. Lucky for you, this idea builds on what you already know about the Earth’s mantle.
Below the crust is a layer of Earth called the mantle, which is made up of molten rock. Mantle moves in circular patterns called convection currents, which we introduced last lesson. They look something like this:
Below the crust is a layer of Earth called the mantle, which is made up of molten rock. Mantle moves in circular patterns called convection currents, which we introduced last lesson. They look something like this:
We’ve learned that one reason for these convection currents is that mantle heats up close to the Earth’s core, because the core is so hot. This makes that mantle less dense, because things get less dense as they heat up. The less dense mantle then flows to the surface and the cycle repeats, causing constant motion of the mantle. As the mantle flows and churns, it carries the plates with it.
Another reason convection currents occur is because, as plates move, some plates are driven under others and back into the mantle. The video demonstrated this happening. Because of all the lifeforms that live on earth and whose organic material has decayed into the crust, the plates bring carbon dioxide with them into the mantle. The gas doesn’t like being stuck in hot melty rock, so it escapes. As the gas floats upward, convection currents are formed, causing the mantle to move around. This creates more recycling of crust, which results in more carbon dioxide getting stuck in the mantle, and the cycle goes on and on.
(As it happens, that hotspot of gas as the carbon dioxide is first driven into the mantle is what makes volcanoes explode, meaning that volcanoes are just Earth’s way of letting gas escape. So, basically, a volcano is the Earth farting).
How Do Plates Move?
All of these interesting things that we’ve described—mountain ranges forming, magma bubbling up as two plates move away from each other, earthquakes, volcanoes—occur at specific types of plate boundaries. You should understand the differences between them and what types of events are common at different boundaries.
Summary
This video gives a nice overview of some of the most important concepts that we’ve discussed:
You should understand:
- That the Earth’s crust is split into 7 tectonic plates, which move past each other due to convection currents in the Earth’s mantle.
- That convection currents are the combined result of mantle heating up at the core and floating to the surface and CO₂ getting trapped in the mantle as organic material from the surface is recycled.
- The types of events that occur at each of the three main types of plate boundaries:
- Convergent: Two boundaries come together, creating mountain ranges and deep trenches. Old crust is pushed into the mantle in crust recycling. Volcanoes can occur due to hotspots of CO₂ bubbling up, and earthquakes can occur due to plates sliding past each other.
- Divergent: Two boundaries come apart. Happens on the opposite side of a plate as convergent boundaries. Forms ridges, or big gaps, as the plates pull apart. Mantle bubbles up and becomes new crust in crust recycling. Volcanoes can occur due to CO₂ and mantle bubbling out of the Earth.
- Transform: Two plates slide past each other, resulting in earthquakes.
Learning Activity
Content contributors: Ismael Harouna, Suzanne Xu, Emma Moulton