 Hi, I'm Maureen Feynman and I'm Heather McKeown-Droon and this week we're going to be talking about igneous rocks of a few different types, specifically basalt, diorite, and porphyry. So up until this point in class we've been talking about minerals most of the time. Mineral such as this crystal of quartz here has a five-part definition. It is a homogeneous inorganic solid with a definite chemical composition and a defined crystalline structure. And those are the things that make a mineral a mineral. A rock on the other hand is an assemblage of minerals. So it has all of the definition of a mineral, but it is made up of more than one grain. And although it is possible to have a mono-mineralic rock that is made up of all one type of grain, it is more common to have rocks that are made up of multiple different types of minerals altogether. So for example, the basalt that you see here, this is an example of a basalt that was erupted under water or ice in Iceland. It has a little bit of a glassy top to it. This basalt is made up of the minerals olivine, pyroxene, and plagioclase. And in some cases might even have a tiny bit of quartz in it as well. So we move now into looking at these rocks, these assemblages of minerals. And in particular the rocks that we're going to be looking at in this module are igneous rocks. Igneous rocks are rocks that form by crystallizing from a liquid magma. So we take a rock from the interior of the earth, melt it to form a molten liquid magma. That magma then either erupts at the surface to become a lava or it cools slowly beneath the surface of the earth. And in either case, over time it crystallizes to form different minerals and becomes a solid igneous rock. You can see quite clearly when you look at the texture of this Pohoihoi sample here, this is a type of basalt, similar in composition to this one. But you can really see the textures of the liquid flowing lava that formed this particular rock here. We call this sometimes a ropey basalt for the texture that it takes on as it flows and wrinkles up. These are both volcanic rocks that cooled very quickly at the surface. In contrast, diorites are rocks that cool very slowly over time. And that slower cooling underneath the surface of the earth allows larger mineral grains to grow. So you get some larger crystals forming that you can see with the naked eye, as opposed to the microscopic minerals in the basalt. And in some cases, you get a combination of the two. So this is a porphyritic rock. This rock has some large crystals of feldspar in it. And then those are held together in a fine grained matrix of smaller crystals. These rocks are very hard, which leads to some of the ways that they are used in an architecture through history. So as we'll find in art history, what makes these particular igneous rocks important is their hardness and their permanence. For something like basalt, one of the first types of uses that we have for it is exactly what's expressed in this little souvenir model I have from Mayen, Germany. And that is that these were used as grinding wheels for grain mills. And such grinding wheels were used way back into the Neolithic period by many groups of peoples. And even today, if you buy yourself certain types of food grinders or mortars and pestles like the traditional Mexican mocajete, the mocajetes tend to be made out of basalt. So you might even have had some guacamole made with such mocajete. So this is a very utilitarian type of rock. It was also because it has a tendency sometimes to form into these great pillars of rock, particularly favored for large-scale monuments that were intended to be permanent in the ancient world. And you might be familiar with the Law Code of Hammurabi, which is something that we'll discuss in our case studies. That's made from a basalt pillar. Similarly, the hardness and permanence of diorite were incredibly important in the ancient world and even later. And we also have both hardness in porphyry. And in this particular variety of porphyry, which is purple, we have a color that has a very symbolic meaning to it. And this was also quite rare. This was only found in one quarry in Egypt in the ancient world. So we'll be talking about that.