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Author Topic: QotW - 15.12.15 - What causes the Earth's plates to move?  (Read 3417 times)

Offline thedoc

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The tectonic plates below our feet move. But where does the energy for this come from? Apparently the reason is analogous to how your porridge gets heated up in the morning. We find out why!
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« Last Edit: 30/09/2016 08:04:15 by chris »


 

Offline thedoc

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What causes the Earths plates to move?
« Reply #1 on: 14/01/2015 12:34:56 »
We answered this question on the show...



Khalil -   Fear not, John.  I'm sure this is one weíll 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 Ė 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?  Itís the same with the Earth, itís 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.  Itís these cracks that are the boundaries of the plates like giant islands floating on an ocean of hot rock.

Marian -   Itís a common misconception that the plates are actually being dragged along by the convecting mantle itself.  Thatís 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 weíre 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 weíre 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 Earthís mantle is convecting because itís so difficult to diffuse heat out of the Earth that itís easier to move the heat out by actually moving the Earthís mantle around.

Khalil -   All these heat is trying to get out because the Earthís 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 doesnít stay hot forever and will eventually cool to the same temperature as its surroundings.  The same process is happening here at Earthís core except unlike your cup of tea, the Earthís 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
« Last Edit: 14/01/2015 12:34:56 by _system »
 

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What causes the Earths plates to move?
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