What causes tectonic plates to move?

16 December 2014


Where does the energy in the movement of tectonic plates come from? What causes them to move?


Khalil - Fear not, John. I'm sure this is one we'll be able to crack. To dig up the answer, I spoke to Marian Holness from the Geology Department at the University of Cambridge. Marian - The Earth is divided up into an inner core which is mostly made of iron and then there's an outer silicate part, most of which is called the mantle, and then the crust is the skin that sits on top. Now, the mantle is completely solid, but because the Earth is so hot inside, what that solid mantle does is convect - a bit like porridge in a pan on the stove.

Khalil - Imagine you're making your morning porridge. It gets hottest at the bottom because this is where the flame from the cooker is, right? It's the same with the Earth, it's hottest down at the core. As the bit of porridge at bottom of the pan heats up, it rises to the top. As it gets further from the heat source, it cools and sinks back to the bottom. This circular motion is called convection, and there's a similar thing happening below our feet right now. Just like you get a crust on your porridge if you leave it, the Earth also gets a crust except convection currents cause cracks. It's these cracks that are the boundaries of the plates like giant islands floating on an ocean of hot rock.

Marian - It's a common misconception that the plates are actually being dragged along by the convecting mantle itself. That's not actually true. What happens during this convection is that you get mantle being dragged up mid-ocean ridges, and that melts and makes new oceanic crust, and then at the other end where you're destroying your oceanic crust, you're subducting it back into the Earth under its own weight. What we're actually seeing is that the plates are being dragged along by gravity, pulling them back into the mantle of subduction zones. So, the kind of speeds that we're talking about for plate tectonics are essentially the same as the rate at which your fingernails grow.

Khalil - Areas where plates collide are often areas with lots of earthquakes and volcanic activity. But where does this heat energy come from to move these giant bits of rocky crust?

Marian - So, what's driving this? Why are the plates moving around? The Earth's mantle is convecting because it's so difficult to diffuse heat out of the Earth that it's easier to move the heat out by actually moving the Earth's mantle around.

Khalil - All these heat is trying to get out because the Earth's core is so much hotter than the rest that the heat needs to even itself out. Heat always dissipates. This is why a cup of tea doesn't stay hot forever and will eventually cool to the same temperature as its surroundings. The same process is happening here at Earth's core except unlike your cup of tea, the Earth's core is around 6,000 degrees. But why is the centre of our planet so hot in the first place?

Marian - What's the source of this heat? Well, there are two sources. The first is the heat that's generated by radioactive decay within the Earth. The second is primordial heat which was present at the very formation of the Earth.

Khalil - Earth: 4.5 billion years old and still hot. I hope our journey to the centre of the Earth has helped answer your question, John, and tune in next time when we take a healthy bite out of this question from Sarah Ward.

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