What is a tectonic plate?

Chris asked Geologist, Owen Weller, to shake things up with this question from Clare on Twitter. 

Owen - Well, that’s three questions in one so I’ll start with the first one. A tectonic plate forms part of the outer, relatively cold and rigid part of the Earth that we call the Lithosphere. Essentially, as you go into the Earth the Earth warms up quite quickly and the effect of increased temperature and rock strength is to decrease rock strength. The lithosphere is the relatively cool part where rocks are relatively strong and relatively rigid but as you enter the Earth they become much weaker and they can, in fact, flow on geological timescales, this is flowing in a solid state. So tectonic plates and the lithosphere is just the strong outer layer and below this sits what call the Asthenosphere. So the analogy is the tectonic plates are like icebergs which are flowing under the asthenospheric mantle below.

Chris - Vagner had the idea that this might be happening more than a hundred years ago, but it took quite a lot of data to emerge subsequently to prove that this was actually happening. What about the other part of that question which was “other planets.” Is the Earth unique in having this geological activity or do other planets appear to do the same thing?

Owen - That’s a great question because, in fact, it’s an ongoing area of research. Certainly other planets show evidence for geological activity. For instance, we can see on satellite images of Venus that we have enormous volcanoes and that we can date the surfaces of planets by how pockmarked they are. So we can see that some planets are being essentially resurfaced by volcanism. However, what we don’t see is the sort of geometric arrangement or jigsaw of tectonic plates that we observe on the Earth today.

However, one really exciting new area of research is on one of the moons of Jupiter called Europa. Researchers there are suggesting that we in fact have an earth-style plate tectonics where you have the creation and destruction of plates but these plates are, in fact, made of ice, so we call this icy tectonics. It’s a sort of ongoing area of research.

Chris - So the bottom line is you need what we have on the Earth - a squidgy interior with something fairly rigid on the outside and not too heavy to bob around on that squidgy interior and that moves things and jockeys things around and remodels the surface? So as long as you’ve got that sort of configuration it doesn’t really matter where it is or what it’s made of, the same processes will play out?

Owen - Absolutely. It is a little bit of a Goldilocks situation where you need the right thermal profile and you need the right chemical properties. So, for example, Venus has a very similar size and thermal structure to the Earth, but it’s thought that there’s not quite enough water in the deeper parts of Venus such that it cannot flow. It essentially too strong on geological timescales. Whereas on Earth, we have just enough water buried in the mantle which allows these rocks to flow in the solid state.