 talking about igneous rocks and volcanoes and the first thing we're going to talk about before we get into the topic of igneous rocks is what you have before igneous rocks, which would be magma prior to crystallizing and forming rocks. We have magma. So we're going to talk about magma. Whenever a magma cools and solidifies we get igneous rocks as mentioned earlier. If that magma, liquid hot magma makes it to the surface, we call that lava and magma has essentially just melted rock, right? Here's a beautiful picture of magma or lava to be more specific, crystallizing and forming basalt. So magma contains three different components to it. Even though we just imagine like all liquefied rock there's always these other components. So of course, that first part is just liquid. That's the melt, the molten rock. The second part, those are your solids. That includes little tiny crystals floating around your magma. You may or may not be able to see those just but those are the various solids that are floating in your magma. And finally you have the gasses, the dissolved gasses, and these play a major role, your eruption style and the type of igneous rock you end up with. And they can include water, carbon dioxide, water vapor to be more specific, carbon dioxide and sulfur dioxide, right? In order for magma to go from liquid hot magma to to a igneous rock you need this process known as crystallization. And that's where the atoms in the magma start to get into an ordered arrangement and start to bond and form these chains and strings of silica tetrahedron and create these various igneous rocks, right? And the biggest thing is a play here are these silica and oxygen bonds and they form these tetrahedrons which are shown here at the bottom of the screen, right? They're kind of like a pyramid shaped arrangement of atoms and you can get strings or chains or rings or individual or pairs and this is all especially when you get the strings and long chains and stuff it's known as polymerization of silica tetrahedron. You can think of polymers. Polymers is a common term applied to plastic and it's a similar kind of idea, right? And and this process of crystallization is exactly the same as freezing water to make ice. It's just working with different temperatures, right? Igneous rocks are sensibly just frozen magma and magma just has a much higher temperature of freezing than what water does. And that gets us into this concept of how you that melting temperature, that freezing temperature, how you make magma, how the earth makes magma, right? It has a lot to do with what's known as the geothermal gradient, the increase of temperature with depth into our planet and for any of those of you who may have worked in a coal mine, especially the deeper ones, I've been in a deep iron mine in Michigan before and once you go down deep enough you start to notice a significant increase in temperature. There's some of the shallower caves and stuff might stay a consistent cool temperature a year round, but the deeper you go, the more you'll notice this increase in temperature with depth, which is known as the geothermal gradient. And on average, generally, it's about 25 degrees Celsius per kilometer, right? Once you get to a certain point about 100 kilometers down, your temperature, your geothermal gradient, you're very close to the melting point of a lot of different types of rocks, right? But right near the base of the lithosphere, into the asthenosphere, you're getting close to that a solid liquid boundary for mantle rock. And this is important because essentially all you need are little little tweaks in the system to to change things enough to start to melt rock. And this is what allows for the generation of liquid hot magma and making volcanoes and things like that. So here are the three major ways you can generate magma based on this idea of this geothermal gradient. You can reduce pressure, right? Most magma is in this situation is made at divergent plate boundaries. And divergent plate boundaries, if you think about it, are probably some of the biggest generators of of magma in general because it's just this extremely long mountain change of volcanoes streaming all across the planet, constantly making new ocean plate. The second way you can generate magma is by adding water, right? And a lot of this is caused at subduction zones where your oceanic plate that's getting going down into our planet, subducting down with it, it is bringing ocean water. And that ocean water mingles with the hot mantle as it is driven off the plate, subducting plate by heat, and that allows for melting of that hot mantle water. It's very much like applying salt to the road and during the winter, melting that ice, not allowing that ice to solidify. So it's a very similar kind of concept of introducing that water into the mantle and changing the temperature at which it melts and creating magma from that. And finally, of course, the obvious way to create magmas to add heat and a lot of times you see this at hot spots. So with decompression melting, which is reducing pressure, you basically need higher temps to melt at greater depths. And if you decrease the pressure, you're lowering the melting temperature there. And what will happen is those mantle rocks will rise to the regions of low pressure and help create that melting area. So this is specific to these divergent boundaries, right, where we're generating new magma. And then we have a scenario where we're adding water. And like I said, that subducting plate as a subducting is bringing some of that ocean water with it. And as that water is driven off, it's generating new magma, right? These ocean waters migrating up into that wedge of mantle in that subducting zone and allowing for new magma to be created. And I mentioned the analogy of brood salt for ice. And then finally, last but not least, we have a temperature increase and heat from any kind of magma source can melt a lot of the surrounding crustal rocks. And we see this, not just at hot spots, but also a lot in the crust. A lot of the melting related to crustal processes may be attributed to this temperature increase. And finally, we have our different geothermal gradient diagrams for each kind of scenario where you have your normal situation where your geothermal gradient doesn't intersect the line where rocks melt mid-ocean ridge where you decrease the pressure and it allows for a much faster increase in heat with depth and it crosses that line of rock meltage melting the rock. And then your hot spot, where you're adding heat at some depth, pushing that line over past that line of rock meltage. And then finally, adding water changes the properties of your rock and moves that line of meltage over allowing for melting to occur. Solidus is what we call it. Okay, so in the next lecture, we'll start to discuss how we classify igneous rocks. And that concludes this section of the lecture here. Thank you.