Gadgetory

okay so the question was that some people would think that the mantle is liquid what we know about the structure of our planet we all know this from geophysics we know this from geophysical investigations for the most part we're talking about seismic waves so what happens when we have an earthquake it sets off an immense amount of seismic energy and that seismic energy travels to our entire planet those are very strong seismic waves and the two principal types of waves one are compressional waves they compress and expand compress and expand the material just like the sound waves do and the second type of waves are shear waves they move perpendicular to the wave movement and they have a shear movement and so these two principle type of waves are set off by any earthquake and some of them are humongous and so let's assume we have an earthquake that wave travels through the entire planet and arrives anywhere on on the planet now if we have an earth that is homogeneous that has no layered structure inside those waves would come through and would come through and would come through and you would have an arrival of waves all over the sphere that's not what's happening what's happening is those waves are refracted and reflected at different surfaces within within the compressional waves we call them also the the P waves the primary waves they travel really fast about twice the speed it will test the shear waves and they travel to any medium they travel through liquids to solids and through gases and so they arrive anywhere the shear waves you cannot cheer a liquid you cannot see a liquid what that means is the shear waves once they hit a liquid they do not propagate liquids the shear wave is a force like that and when you do this to water or to any other liquid or to air you can achieve it okay so the velocity of shear waves in any liquids is zero so they just simply get absorbed and so when waves travel through our planet if you look at cheer waves the S waves that move this way they do not arrive at the opposite end of an earthquake so there is a liquid down there we know that okay so they travel down and they get absorbed but the neighboring wave they make it so some of the shear waves they make it like this and they arrive here and they arrive at a 103 degree angle away from the origin of that wave okay from that we can calculate the distance to that to that sphere inside that's liquid we know there is a liquid sphere inside and that's our outer core okay and we can we can make easy calculations that's about two thousand nine hundred kilometers down from from where we are okay so that's where the manticore boundary is okay so that's where that is what also happens is the S waves they travel through that entire mantle okay so from that we know that the mantle is solid because else they would not travel through that okay we also know that the inner part of our core is a solid because we get S waves that travel through the inner part of our core and that's amazing because they get first absorbed and then they show back up and that's because the P waves travel through it and when a P wave P wave hits a physical boundary they is called wave conversion some of the P wave energy is transmitted into an S wave again okay so basically from from looking at hundreds and thousands of earthquakes we know exactly the structure of our planet we know the velocity with which waves travel through the different layers of our crust and of our mantle and of our core and we not only do we have seismic energy that totally proves that there is no liquid in the mantle but we also have samples of the mantle samples of the mantle and here is one this is called a mantle peridotite it consists of magnesium iron silicate and it's very heavy material okay and the seismic velocity of this material coincides with with what we know from earthquake energy and these samples they're not really all that rare because volcanoes bring them up from the mantle all the time so this is sort of undigested mantle that hasn't been melted in in volcanoes okay so we have samples of it and it's crystalline it has large crystals in it which means it cooled very very slowly and this is a result of the initial differentiation of our planet in the early days of our planet like 4.6 to 4.5 4.3 billion years ago a differentiation was the first process that it happened by gravitational forces that really heavy material the nickel and iron gravitated towards the center of the planet and then lighter layers like the mantle makes up the next big shell and then we have on the very top very thin the lightest materials of all and that is the crust okay and here's a part of the crust of the Earth's crust as typical typical composition of a granite okay so that's that's how that looks like of course we don't have a sample of the liquid of the liquid core or of the solid part of the core but we have materials that are sort of an analogue to that and those are meteorites these are nickel iron meteorites and these meteorites they have about that composition and they are and so all these lead us to to knowing that the core is made out of nickel iron and so there is no doubt the mantle is solid and there is no doubt that we know what it is we do have samples and we do know from the seismic wave propagation of s waves that they travel through it with the speed that they should travel through it and so all this there is no dispute about it the the mantle itself the earth is extremely hot okay so the earth is extremely hot and we know that the plates are moving plate tectonics plates are moving and they're moving by heat convection very slowly with about let's say you know an inch to two to three inches per year that's about the most motion it's like a boiling soup very slowly falling soup and so it's basically a big huge convection cell and so there is some plastic movement in it because of the and that heat comes from radioactive decay of uranium thorium not very long-lived radioactive elements the short-lived ones have have ceased to exist for a long time now but we have a huge amount of heat left and heat is really what drives our planet heat is what drives our drives everything without that radioactive decay we wouldn't have the magnetic fields we wouldn't have to state tectonics we probably wouldn't have an atmosphere to live in I got my PhD in Germany I graduated from one of the oldest universities not only in Germany but in the world University of Tuebingen and from there I had a postdoc appointment at the University of Michigan I stayed there for three years doing basic research fundamental research actually in the magnetic field and then I worked for nine years at the ocean drilling program at Texas A&M University before I came here and I've been here at ul for thirteen years none professor of geology