Under the rigid layer of rock we live on, the Earth’s asthenosphere is plastic and more dense.
Because of its fluid-like properties, mantle convection can occur. Then, mantle convection is the main driver of plate tectonics.
And it’s because of convection deep down beneath our feet why we have volcanoes and earthquakes as well.
But how does mantle convection work? And how is convection related to plate tectonics and moving continents?
Convection cells start with a heat source
Imagine you are boiling water in a pot. If you boil one side of the pot, it will get hotter directly where the flame is.
At the top and right of the pot, the water isn’t as warm. This is because it doesn’t have a source of heat altering its temperature.
When hot water rises up, it will try to go into an equilibrium. So at first, that means that it mixes into the cold water above.
For Earth, the heat source is radioactive isotopes deep inside the interior of Earth. Over time, they slowly release heat accounting for approximately 50% of Earth’s internal heat budget.
Convection replaces cold water with warm water
Now that we have a source of heat, we know systems always try to go into an equilibrium. For example, hot water will mix with cold water. Actually, this happens in the atmosphere with air temperature.
So far, hot water has risen to the top of the pot. But it’s even colder directly to the right. Next, the hot water will start moving to the right and mix with colder water.
Eventually, this pattern creates a circular motion. This circular motion is the convection cycle which happens inside Earth within the asthenosphere.
Mantle convection cells in the asthenosphere
For liquids and gases, the convection cycle can happen because particles can freely flow. But in our rigid lithosphere which is a solid, particles cannot freely move.
Beneath the crust, the convection cycle is what’s happening in the mantle. Because the plastic-like “asthenosphere” acts as a liquid, it makes the convection cycle possible
Based on mantle convection, each plate tectonic moves in a specific way. For example, there are divergent, convergent and transform plates.
The rigid lithosphere sits on the plasticky asthenosphere
As mantle convection rises, it breaks apart the Earth to form mid-oceanic ridges (tensional force). When it sinks down, it breaks it apart (compressional force).
These tensional and compressional forces are what drives plate tectonics. They break apart the whole lithosphere into 7 major plate tectonics and 12 or so minor ones.
Now, that we know mantle convection tears the brittle lithosphere apart which is also called “slab pull”… But what happens if there’s a continent within one of these plates?
Continents ride on plate tectonics like conveyor belts
We know that plate tectonics are moving because of mantle convection. You also know that plates can diverge, converge or slide across one another.
But how does this relate to continents? Are continents actually moving?
You should think of it as continents riding on the plate. Because the whole plate is actually moving, it’s like standing on a conveyor belt.
Is it moving? No. It’s the conveyor belt that transports the continent in the process of continental drift. So continents passively move on plate tectonics.
Plate tectonics form mountain chains, volcanoes and earthquakes
Plate tectonics help answer what we didn’t know about volcanoes, mountains and earthquakes.
Because plate tectonics crash into each other, this collision creates chains of mountains. All the greatest mountains in the world are the result of continents crashing into each other.
It’s from the tensional and compressional forces from mantle convection that drives plate tectonics.
Each continent rides on plate tectonics like a colossal conveyor belt. And they move about 5 to 10 centimeters per year.
Radioactive isotopes put Earth on a light simmer
Four radioactive isotopes inside Earth account for about 50% of Earth’s internal heat.
Like a slow cooker, they constantly release heat within the planet keeping it on a light simmer.
These four isotopes (uranium-238 (238U), uranium-235 (235U), thorium-232 (232Th), and potassium-40 (40K) generate 50% of Earth’s radiogenic heat.
The majority of internal heat transfer occur at mid-oceanic ridges. Whereas, the least amount of heat transfer is from the continental interiors.