Asthenosphere: Using Silly Putty as an Analogy
Science
Lecture video about Asthenosphere
King. Another important misconception that a lot of people walk around with is to imagine this material below the plate, the mantle material below the plate, which we call the asthenosphere, imagine that is liquid. In fact, it's not liquid. There's a good reason for this misconception, actually, because you do see in some popular programs that the plates are somehow floating on a liquid part of the deeper mantle. And that's absolutely untrue. This material is not liquid. It is what we refer to as viscoelastic. It means under certain conditions, its elastic and under other conditions, it can deform as a plastic material. A good model for this is silly putty. So silly putty is a viscoelastic solid. It's not a liquid.
We will see that earthquake waves are very elastic waves. Very elastic deformations. And they're very capable of propagating through the Astana sphere of the earth. And yet under other conditions, the asthenosphere is capable of flowing. So let's imagine, for example, in this diagram, we have a volcanic island here. This volcanic island grows, it imposes a big weight on top of that lithospheric plate. And with that weight sitting there, for millions of years, this plate will eventually bow. In order to bow the underlying asthenospheric material needs to flow out of the way. And so that's a case in which we put a stress on for a long period of time on the asthenosphere and is capable of flowing.
So that's the contrast that we want people to get. A viscoelastic material when we put a short term stress on it is elastic and could a long-term stress on it, it's plastic. And that's what silly putty does. So if you take out your silly putty and you make a reasonable sphere out of it, then you can bounce it. Bounce it on the floor and what happened there was you throw the silly putty down, it hits the floor, it compresses into a pancake shaped thing to forms a little bit and then bounces right back up. So it acts as an elastic solid under those conditions of short term stress. When we take the silly putty and we just grab it by the ends and we're really gentle about it. We slowly deform it, putting a stress on it over a long interval of time, the silly putty flows.
In fact, you can make a sphere out of the silly putty, put it on the corner of your desk and just leave it there. And in about an hour, it'll turn into a pancake. So just at room temperature, it'll flow under its own weight. So now this is a permanent deformation, right? I take the stresses away, I have permanently plastically deformed the silly putty. So silly putty, fiscal last material, short term stresses. It's elastic, long-term stresses, it's capable of flowing. Same thing for the asthenosphere under the rigid lithospheric plates. This stuff, when you put earthquake waves through it, it acts like an elastic solid, when you put a weight on it, leave it there for long interval of time, this stuff is capable of flowing capable of plastic deformation