 We all know that the atmosphere connects and wind is a very obvious example of that and you probably think it connects because wind flows from high-pressure areas to low-pressure areas and if you think that you're right But why are some areas in the atmosphere high pressure and others low pressure and the reason is buoyancy? So in low-pressure areas The air is hot and moist and it's light and so it rises in In high-pressure areas the air is dry and it's dense so it sinks So it's buoyancy that causes those pressure differences And when you think about natural processes and you go back to the to to the root cause It's invariably buoyancy and buoyancy drives convection in fluids and it also drives convection in the mantle It drives magma's out of the mantle and that's what I want to talk about So let's start with a very simple case of convection between a hot and cold plate And you've probably seen examples of that your stove is a good example of convection in a fluid So let's take a Fluid between a cold plate and a hot plate The hot plate at the bottom of the fluid heats a boundary layer above the boundary layer at the bottom of the fluid and in heating that fluid It makes it light so that fluid starts to rise At the top of the tank below the cold plate The fluid loses heat To the cold plate and it becomes dense and it starts to sink and There's buoyancy differences between the bottom and the top are what drive convection There's buoyancy differences can be very small, but they still drive convection And more or less the same thing happens in the mantle with one fundamental difference that I'll come to The surface of the earth is cold. It's about 20 degrees on average The center of the earth is the core the iron nickel core And that's a thousand degrees hotter than the mantle above it. So the core acts as a hot plate It heats the boundary layer Immediately above the core and that's about a hundred kilometers thick. It makes it light And that layer starts to rise and starts to rise in the form of mantle plumes The cold boundary layer at the top Becomes denser as it cools And it starts to sink back into the mantle at subduction zones There's a fundamental difference between The fluid between a cold and a hot plate and the mantle and that fundamental differences difference Is that the upper boundary layer is cold and stiff So it can't flow back in in into the mantle in the form of negative plumes It flows back as a stiff body and those stiff bodies are called plates What drives magmas out of the mantle and the question again of the answer again is buoyancy The mantle has a density of three thousand three hundred kilograms per meter cubed Whereas basalt has a density of two thousand eight hundred and fifty kilograms per meter cubed So the density difference is four hundred and fifty kilograms per meter cubed If that magma is ten kilometers below the base of the crust Then that density difference is applied over a depth of ten kilometers And that gives you a pressure of four point five by ten to the six kilograms per meter squared So there's a huge pressure on that magma and that pressure drives the magma out of the mantle But that pressure is caused by the buoyancy difference or the density difference between the magma and the mantle