Misconception – The Rock Below Tectonic Plates is Liquid

There are many misconceptions about geological concepts. There is a list here, developed by Kent Kirby of the University of Minnesota. This post is to debunk two of those misconceptions. There will be others. Find them here.

Misconception: Asthenosphere is liquid and/or the lower mantle is liquid

Many people know that the Earth’s surface is broken into lithospheric plates, that move around, sometimes colliding and making mountains, and sometimes associated with volcanoes.

In a previous post, I describe what these plates are and how they relate to the Earth’s crust, on top of which we reside. We know that these plates move around… somehow.

We have this vision of the plates ‘floating’ on top of molten and flowing rock, and being carried around by this flow.

In our minds, this is similar to the motion of the dried surface of a vat of nacho cheese that hasn’t been stirred for a while, but is being heated at the bottom. The cheese directly above the heat source flows up and out of cracks in the surface, pushing the dried cheese to the side. The dried up cheese crumples up into cheesy mountain ranges when it collides with other cheesy plates or the edges of the vat.

In this model, all of the cheese is liquid except for the crust on the top that’s getting pushed around. The flow of the liquid cheese is caused by convection, pulling the crustal cheese-plates along for the ride.

When we talk about the motion of lithospheric plates, we use the same terms. There’s flow in the Earth below the plates that move them along by convection. It follows then, that the Earth below the lithosphere must be liquid. Besides, it’s really hot down there. Wouldn’t it make rock melt?

Alas, the layers below the lithosphere in which flow and convection are taking place are solid rock. The uppermost of these layers is called the asthenosphere.

The lithospheric plates, composed of the crust and the very uppermost mantle, sit on top of the asthenosphere. The lithosphere is solid, rigid rock, while the asthenosphere and all parts of the mantle below it is capable of flow – but it is still rock.

Flow in the mantle is by very slow ‘plastic’ deformation. The rock is still solid, but can change its shape. Think of it like how wax might flow. It’s not a liquid. There are still individual mineral grains there, but they slowly change their shape, allowing the rock to bend.

Heat from deep in the Earth causes some of the rock to be less dense and flow toward the Earth’s surface. Cool rock that’s been close to the surface for a while gets heavy and begins to fall back toward the Earth’s center. But it still is solid rock (have I mentioned this yet?).

The only part of the Earth’s interior that really is liquid is the outer core, which is composed of molten nickel and iron. The rest of the Earth is solid, except in a few places close to the surface where there are volcanoes and a couple of places where we know a little bit of incomplete melting has occurred.

The flow that causes the lithospheric plates to move is happening in solid rock.

Pretty cool, eh?