 Stories of lecture videos on igneous rocks and volcanoes in the first video We talked about magma and how to generate magma in the second video We talked about igneous texture, which is one of the ways we identify igneous rocks igneous composition is the second way we use to identify igneous rocks, so that's what we're going to talk about today is igneous composition and Igneous composition, which is what you know the chemicals inside of the igneous rock those are defined by the You know generally defined by where your Magma is moving through what kind of tectonic boundary is generating your magma, right and That in turn will define the viscosity of your magma which in turn defines How dangerous of an eruption you have at your volcano, right so? Viscosity is a measure of resistance to flow, right and a magma with higher viscosity has a higher resistance to flow and Those that resistance flow can be affected by temperature think of warming up honey before you pour it makes it Less resistant to flow right it reduces its viscosity Adding more silica will make your magma more viscous. It's like adding more sugar To your molasses it makes it more viscous and finally dissolved gases Can change how your eruption occurs and it interacts with viscosity very well And I have a great video Showing that as an experiment and I'll put a link to that in this presentation And that's the that's the link to the video there So let's move on here and Then if you watch the minerals lecture you would notice that we went through the trouble of mentioning dark silicates Which are associated with mafic rocks, which is a type of igneous composition Those are rich in iron and magnesium and include olivine pyroxene and amphiboles and biotites and then light silicates, which are known as felsic type composition and those are more rich in quartz Or I'm sorry potassium sodium and calcium, right? They represent minerals quartz and muscovite feldsparas They're much more silica rich and So we can classify our igneous rocks based on the amounts of these minerals and these chemicals And you know you can have a pretty fairly significant range of silica in the crust and Based on this amount of silica. That's how we Categorize our composition into two major groups and then there's there's other groups that kind of fit in between Now on the ends of that you have a felsic type composition Also known as granitic right and these are very high in silica content. They make for a very viscous thick Magma and they erupt at lower temperatures. That's important and then you can have a mafic Magma, which has a lower silica content It is more fluid of a flow of less viscous and then it erupts at higher temperatures or fires higher temperatures for melting So here is kind of the Different major categories of igneous composition. You can have mafic. There's also ultra mafic Which we'll talk about in a little bit of detail. There's intermediate, which is the creative name for in-between the two major compositions and felsic and This mafic has the least amount of silica. Felsic has the most amount of silica and here's an ultra mafic This is a dunite or Vertitite and screen from olivine. There's the mineral olivine. I think commonly known in the gym where it was peridote and This is very close to the composition of what our mantle is like. So very very mafic ultra mafic, right? Here's some ultra mafic rocks, right? So The Extrusive version so that we talked about textures in the last lecture. That's tiny microscopic crystals Well that the salt or an intrusive version would call gabbro Fit these are rich with magnesium and ferrum also known as iron. So may thick Right dark silica. It's high density. These are essentially ocean Ocean type crust for those of you who are familiar with plate tectonics and the types of crust, right? A lot of volcanic islands are associated with these mafic rocks Here's some basalt. I mentioned that earlier Very very common extrusive igneous rock The upper layers of our oceanic crust and then gabbro the intrusive variation of that So it just has larger more physical crystals in most of the magma that erupts if you take into consideration the mid-ocean ridges is basalt, right? coming from these divergent plate boundaries where you're generating new plate and They originate from partial melting of the mantle is decompression melting and we They're commonly known as primary or primitive magnets because they're very similar to where they originate from which is the mantle, right? like low silica and Low viscosity is they flow very readily and Here's some of the the textures you can see and some of these basalt flows here And boy boy Ropey type magma and some more like a spiky pointy type magma I have a YouTube link here to some of that eruption style of mafic type eruption Right, and here's the type of volcanoes. They generate these what's known as shield volcanoes very broad gently sloping Volcanoes and that has a lot to do with the viscosity of the magma which in turn has to do with the composition, right? Has a low viscosity so flows very readily and that ready flow makes for broad dome-shaped type volcanoes and and these are Still dangerous eruptions, but they're not as explosive as Ethel sick type eruption would be right And then some examples are the Hawaiian Islands canary islands Galapagos Islands Easter Islands Which I think are all hot spots, but you can have mafic flows in other places, but these are great examples of that type of Volcano, and then you can also get these cinder cone volcanoes from mafic Compositions and some great examples are in craters of the moon a national monument Which is an Arco Idaho area and there's some in Veo, Utah me being out of Utah. It's good to mention some local areas And these are a little bit more gas rich So they have very vesicular cinders erupting out of these but still mafic in composition And I have a video linked here on pericutin which was a fairly famous Mexican Cinder cone eruption, right so in terms of composition we also have these intermediate rocks the Extrusive version being andesite which is like a grayish-looking rock It's volcanic in origin right sometimes it's porphyritic, and then it's intrusive same composition But the intrusive variation is known as diorite, and it looks like a dalmatian It's kind of a salt-and-pepper appearance, and it lacks visible quartz And it's coarse-grain for the diorite, and then finally the felsic composition, and these are light lighter in color and There's usually always visible quartz right and they're high and silica content, which means it makes them magma very Discus and here's a beautiful example of a granite with quartz and k-fels bar, and it's Extrusive equivalent known as rhyolite microcrystalline you can see some little quartz Phenocrysts floating around in that right Typically buff pink or light gray in color and a lot of times this is red weathered readily so it might be altered Sometimes altered hydrothermally so it could be hard to recognize in some cases And then you know you could talk about pyroclastic rocks like obsidian as well So this is considered a felsic type pyroclastic rock silica rich, but did not have time to grow crystals it was quenched at the Earth's surface and That leaves us to the volcano type for felsic and Intermediate rocks which is the strato volcano We'll talk about these a little bit more in the talk about volcanic hazards But these are your classic movie type volcano also known as composite volcanoes Right nice symmetrical cone And they're created from these interbed at lava flows And eruptions and Mount St. Helens is a prime example since it's erupted recently And I have some great video links in our slides on some strato volcanoes erupting and I think we'll end with this Jeff on Mount St. Helens and its ash flow and landslide Pulled together by Gary Rosenquist So thank you very much for listening on the different types of volcanic compositions and We'll talk about volcanic hazards in the next lecture