 Welcome back to OpenGeology. This is a free and Creative Commons lecture series to be used with the OpenGeology.org textbook. Fund it in part or mostly by Salt Lake Community College and made by Salt Lake Community College adjunct instructors. Today we're going to talk about igneous rocks and volcanoes and why composition varies in those. What we'll be discussing is bones reaction series, fractionation including crystal settling and partial melting and assimilation and partial melting, which is mentioned up there. So if magma is, the mafic magma is like the mantle, then how do we get felsic magma, the more silica rich magma? And why are continental plates felsic and intermediate and the ocean plates more mafic like the mantle? These are some of the questions we'll try to address today with why the composition varies in magma. And it all really boils down to, no pun intended, why or how minerals cool and crystallize at different temperatures. Ice crystallizes at a fairly low temperature of zero degrees Celsius, but there's a number of minerals that will crystallize at a much higher temperature, and there's a wide swath of temperatures that minerals can cool and crystallize at. So because of this, because of these differences in temperature of melting and temperature of crystallization, you'll get minerals separating from each other, and it'll kind of separate out the different chemicals. And some minerals will crystallize and get left behind while others stay in the melt and continue to move towards the surface. And this is what I call the distillation of magma. So three ways to make new compositions of magma is by crystal settling, partial melting, and assimilation. So let's talk about each. And these differences in crystallization, these different temperatures of crystallization, in correspondence with the different compositions of igneous rock, this is best summarized or very well summarized by Bowen's reaction series. Norman Bowen measured the melting temperature of a number of different minerals and came up with this idea. Up here you have the temperatures that melt and crystallize at the highest temperatures, and these are your ultramethic and mafic minerals like olivine. And then as you move down in temperature, you get more and more felsic until you get to quartz and muscovite and case bar, which are your much more felsic type minerals. So if a magma is cooling, which of these minerals do you think would crystallize first and which would crystallize last? We would assume that the olivine and other ultramethic minerals would crystallize before you get to the more felsic minerals. So this is what brings us to crystal settling. As your magma begins to cool, your more mafic minerals start to crystallize first and more and more felsic minerals start to come out, making your resulting melt more felsic as a result, since you're crystallizing out your more mafic minerals first. And a lot of times those crystals will sink to the bottom and just come out of the melt completely. Another similar way to change the composition of a magma is this idea of partial melt. And that is where you only melt part of the rock and some of the crystals never melt and they're left behind. And so this is the same idea except in kind of reverse of the crystal settling, and it's showing where you just start to warm up that magma and that the minerals with the lowest melting temperature, they'll start to melt first, leaving behind the minerals with the highest melting temperature, the more mafic minerals. And then finally, another way to change the composition of a magma is by introducing or melting into other rocks into that magma. And this process creates xenoliths or kind of rocks captured inside of the batholith or the magma chamber that are kind of floating around in that magma chamber. And this is a very common process. Here's a nice picture of a xenolith floating around with some dikes separating it from the original host rock that that magma body was moving through. And so this is how I think we get these different types of magmas. As your magma moves through the crust, it goes through this distillation process. So andesitic magma, your intermediate magma, is created from this differentiation of basaltic magma. And you can also create it when basaltic magmas assimilate some of your more felsic continental crust. And then granitic magmas, they can also be created this way, especially if you have basaltic magma ponding beneath the continental crust. As you do in convergent subduction zones, right, in the subduction zones. And, you know, you can also create it from differentiation of the andesitic magma, that crystal settling or partial melting of the andesitic magma. And so the next lecture will be on volcanic hazards. And that concludes the lecture on how you get different compositions in a magma. I hope this was informative.