 Hi, and welcome back to another video on the rock cycle. This is our second video on igneous rocks where we'll focus on the composition of igneous rocks and the processes that control the different compositions. You know all about how we control texture in igneous rocks, so now it's time to take a deep dive into the composition. But before we get started, what do I even mean by the composition of igneous rocks? This is talking about the chemical composition of the minerals that interlock to form these igneous rocks. Let's get started. Magma, where does it even come from? Well, it comes from melting the lower crust or the upper mantle. And intuitively, you just think, well, to get magma, molten rock, you just heat up rock. But interestingly, because our geothermal gradient, and what I mean by that is the temperature at which it increases at depth, is about 25 degrees per kilometer, this isn't actually enough to melt rock without something else happening. So let's have a look at what else is happening to generate our magma. Temperature is not the only variable that we need to consider when we're talking about magma generation. We also need to consider pressure and volatiles. But what are volatiles? These are simply other things within the melt, such as gas or water. Let's talk about the two types of magma generation. The first one is called decompression melting. And this is how we generate our mafic melts, which create basalt and gabbro and our mafic igneous rocks. The second one that we'll discuss involves volatiles. And this is how we create our felsic igneous melts, which produce our rocks such as rhyolite and granite. So let's dive in to our decompression melting, which creates our mafic igneous rocks. This magma is generated directly from the mantle. How cool is that? And as the name might suggest, it's created by decompression. So therefore I mean lowering the pressure of the mantle above it. How can this happen? For this, we need to think about plate tectonics again. So if you consider two ocean plates, they're locked together. And once they start pulling apart, you're reducing the pressure in the center bit. And this allows magma to up well, because suddenly the pressure is much lower on the mantle underneath it. And this makes the mantle, which is a semi-solid rock, unstable. And it starts to melt because it's not actually stable at the lower pressure. Which I think is pretty cool that simply by moving something apart, it can create something beneath it to melt. Now let's consider the effect of volatiles on magma generation. So why do we even care about volatiles on magma generation? Why can't we just create all our magma from decompression melting? Well, by introducing volatiles into the system, it decreases the melting temperature of the rock. Therefore the amount of temperature required to melt to generate your magma is much less. The common environment where we'll get volatile generated magma is in a subduction zone. Because as the plate subducts under the other one, it releases its water and gas content into what would call the mantle wedge. Which would be this little triangle here between our subducting plate and our overriding crust. So what this does, it releases its volatiles into that section. Meaning that that little bit in the mantle wedge can begin to melt. And because it's hot, it rises buoyantly into the overlying crust and begins to melt that as well. And these magma chambers become more and more buoyant as they melt more and more crust. And the reason for this is it's actually incorporating lighter material. Because the continental crust is created from lighter material than the subducting oceanic crust. So the more that it incorporates the continental crust, the more it can rise into the crust. So in summary, the composition of igneous rocks is controlled by the composition of the magma from which it cooled from. And there are three main ways that we can get different compositions of magma. The first one is by increasing the temperature of the rock beyond which it can be a solid at. Which creates the rock to melt and form a magma. Another way we can create magma is through upwelling deep rocks from the mantle, decreasing the pressure above it, forming decompression melting. And the third way which we can form magma is the introduction of volatiles to a melt. And this decreases the melting temperature of the rock so that melting can occur commonly within subduction zones. In our next videos we're going to move on past igneous rocks into the exciting world of sedimentary rocks.