howdy everyone and thank you for continuing on with module three where we will explore all of the three rock types in more detail starting with ignous rocks now ignous rocks again I like to call them my OG rocks because they're formed by the cooling of magma which is the unerupted melted material beneath the Earth's surface or they're also formed by the cooling of lava which is the erupted material above the Earth's surface and we categorize ignous rocks into two different types called intrusive and extrusive which are directly related to how they cold intrusive rocks form inside the Earth's surface by the cooling of magma extrusive rocks form on the exterior of the Earth's surface formed by the cooling of lava but we'll start with intrusive ignous rocks again intrusive forms inside the Earth that's how I remember these and because they're forming inside the Earth and they're being insulated by Earth's surface they are cooling much more slowly over time and because they're cooling much more slowly over time the crystals and minerals in that melted material have a lot more time to grow and they have a coarse grain texture so these are just larger crystals coarse grain just means that the crystals are easily um viewable with the naked eye like these photos seen here you can clearly see that there's individual crystals here coarse grain texture and that's because the melt had time to cool slowly so those crystals had more time to grow to bigger and bigger sizes so the cool thing about this is that the crystal size is directly related to the cooling rate the slower cooling results in bigger crystals the faster cooling results in smaller crystals so some some of the um best examples of intrusive ignous rocks are granite which is shown here you see granite here there are this is a coarse grain texture rock there are different you can see that there are different crystals in here we have the pink felt spars we have our light colored quartz we have our dark amphubables and pyines and the reason why we can see these different crystals is because it cooled slowly enough for those crystals to grow to large sizes another example would be diorite um this is also an intrusive ignous rock made up of different compositions but you can see here that there are different um crystals here because you can see that with the naked eye again coarse grain so intrusive forms inside the earth and because it's forming inside the earth it's cooling much more slowly over time which allows those crystals to grow into a coarse grain texture now the opposite are extrusive ignous rocks form on the exterior of the earth extrusive exterior of the earth and because they're forming on the surface they're not being insulated they're going to cool a lot more quickly than if they were underground so these cool fast and because they're cooling fast it does not have enough time for those crystals to grow to large sizes like we saw before instead we're going to see different textures we're going to see fine grain textures like shown here this is an andesite where you can almost see individual crystals but it's kind of hard to see with the naked eye it's really hard to identify those crystals so this is a fine grain texture this is fast cooling but sometimes we can get very fast cooling where that melt that lava actually doesn't even have a enough time for the gas to escape completely so we preserve those gas bubbles in the texture we call this vesicular this is skoria we see those preserved gas bubbles got that spongy texture because the gas in the lava wasn't able to fully escape before it turned into stone and then we have extremely flat fast cooling where this cools so fast sometimes these things um cool before they even hit the ground they're exploded up into the atmosphere and they cool before they even hit the ground there's literally no time for any texture or minerals to form at all and this gives you a glassy texture this is obsidian so these very different textured rocks um because of how they cool and where they form on Earth so extrusive forms on exterior of the earth cools fast and we see fine grain vesicular and glassy textures now pyrolastic material or sometimes we just call them tera are the pulverized rock and lava fragments that are ejected during an eruption now these particles can range in size from fine dust to rocks the size of mini bands the smallest form of pyrolastic or tera is volcanic ash and this is the result of when a gas rich viscous magma erupts explosively like froth and as the magma moves up in the vent gases rapidly expand generating a melt that resembles froth like champagne froth that flows from a bottle of champagne and as the heated gases expand violently that froth is blown into fine glassy fragments we call ash and when the hot ash falls and fuses together it's often creates a rock that we call welded tough these aren't exactly ignous um but they do have ignous origins which is why I include them in this lecture now lei means little stones and it's a fancy word for those cinders that make up a cinder cone volcano that woo will talk a lot about in a couple modules but leill can range in size from small beads to the size of walnuts now blocks are much much larger than the pilli blocks are made up of hardened lava that are greater than 2 and 12 in in diameter sometimes these hardened lava fragments can reach insane sizes bombs are materials that are also greater than 2 and 12 in in diameter but they're ejected as hot lava because they're not completely cold and still have a semi molten state when they get ejected they take on streamlined shapes as they're hurdled through the air almost like a football now ash can travel huge distances very far away from the volcano thanks to their small size and lightweight making it easier to be carried up into the atmosphere however blocks and bombs usually fall near the vent of the volcano because of their size and weight but occasionally they have been found to be thrown great distances for example a bomb that was 20 feet long and weighed 200 tons 200 tons was blown over 2,000 ft from a vent during an eruption in Japan of Mount Osama so these are also important materials um that get ejected during volcanic eruptions and some of these things will come up again later on when we get to that module but just wanted to introduce it now since we're talking about ignous rocks all right so composition of ignous rocks this is important and some of these terms we've already talked a little bit about when we talked about plate tectonics we can categorize ignous rocks based on where they form extrusive or intrusive but we can also categorize them based on composition what they're made out of and when we we do that we can categorize them as felic mafic or ultramaic and when we use these terms we're just talking about what they're made out of their composition you can have intrusive felic rocks and extrusive felic rocks you can have intrusive mafic and extrusive mafic again this is just talking about what they're made out of it has nothing to do with where they're formed now felic rocks we talked about this remember our continental crust is felic felic rocks are rich in silica they're also lower in density and lighter in color that's what makes up our continental crust remember continental crust was lower in density it does not subduct and they're lighter in color remember granite those pink colored rocks those are lighter in color that's because they're felic they're rich in silica rich in silica minerals which are lower in density lighter in color but felic rocks are kind of important felic ignous rocks are kind of important when we're talking about hazards because felic rocks come from felic magma or felic lava and felic magma are rich in silica magmas that are rich in silica are very thick very sticky and these lead to very explosive eruptions so when we're dealing with felic magma we're dealing with explosive volcanic eruptions very hazardous now we also have mafic ignous rocks and mafic remember our oceanic crust our oceanic crust was mafic in composition that's because it's rich in magnesium and iron magnesium and iron have a higher density than silica so these rocks are denser they're also darker in color magnesium and iron tend to be darker in color remember our basalt or oceanic crust or basalt now mafic rocks come from mafic magma or lava now mafic magma is not thick and sticky like our silica felic magma instead it is thin and runny and they often are at very high temperatures to melt that magnesium and iron requires more temperature so these are thinner and runnier so these eruptions are a little bit different these are what we call ausive eruptions meaning the the lava kind of gently pours out doesn't necessarily explode every time and these things are going to come up again when we get to volcanoes so again I will beat this through your head enough this semester now ultraafic are super high in magnesium and iron super low in silica they're rare they usually originate in our upper mantle we do not see these types of rocks very often at the surface and because they are super high in magnesium and iron they are extremely dense and they're extremely dark they even appear green because the primary mineral of these rocks is olivine again these are very rare you won't really see these on a hike unless you're somewhere in Hawaii or somewhere um where we have special circumstances where we get that upper mantle at the surface um so these these are very very rare but felic and mafic important to know the difference between those two felic rocks are lighter in color and lower in density because they have more silica and they produce more explosive eruptions because of their thick magma mafic rocks are higher in density darker in color because they're rich in magnesium and iron and they produce thin and runny magmas which lead to ausive eruptions so this is our most common felic rock this is granite granite is an intrusive rock again felic and intrusive non-related you can have an extrusive felic rock but granite's the most common type and that's what makes up our continental crust so hopefully plate tectonics makes a little bit more sense now that we're understanding the composition of our earth materials now the most common mafic rock is basalt basalt's an extrusive rock it's fine grained it makes up our oceanic crust this is what our oceanic crust looks like it's dark in color it's denser rich in magnesium and iron and then our most common ultramaic rock which remember are not common at all this is peritite this is primarily made up of olivine it is super rich in magnesium and iron and appears green in color very dense this is what our upper mantle actually looks like all right so let's practice ignous rocks together we went through a lot of information but let's try to digest it a little bit and slow our thinking down starting with this rock right here everything we just learned what is your first observation about this rock let's focus on maybe color what What do you observe do you observe light colors dark colors mixture you probably see a mixture but we probably see mostly light and color we see these pinks and whites right now what does color tell us about ignous rocks light and color light and color tells us about composition right color is related to composition so light and color is light colored minerals silica rich in silica low density light color low density this must be felic right felick in composition okay so that's focusing on color let's focus on texture what's the texture of this rock we were able to very clearly pick out individual crystals in here pink white and black so this has a coarse grain texture right what does coarse grain tell us coarse grain tells us how it cold right these crystals are big what does that mean means it cold slowly right and where do things cool slowly beneath the earth's surface so that makes this intrusive this is an intrusive ignous rock it formed below the earth's surface it's a felic ignous rock it is rich in silica light in color lower in density this is a granite my friends this is what continental crust is made out of so if you're going on a walk you come across this rock what would it tell you well it tells me that this rock formed below the Earth's surface and now it's at the surface so something had to have happened to bring this rock up right that's one part of the story but we're looking at the composition we know it's an ignous rock it formed by the cooling of magma but we know this is a felic rock it's rich in silica so that magma had to be really thick and sticky and if it would have erupted which we know this one didn't because it's coarse grained and cold underneath the surface but if it erupted it would have been super explosive because it's rich in that silica it was thick sticky magma so that might tell us if we're on a hike and we see this that there might be an explosive volcano nearby or at least there was one in the past see just by looking at this rock you were able to tell an incredible story and infer a lot of hazards now let's take a look at this rock over here what's your first observation not really much right we don't have any coarse grain texture it's really hard to see individual crystals we see a fine grain texture so what does that tell you about this rock fine grain texture that means it cooled fast it didn't have enough time for crystals to grow so it cooled fast where did Where did this probably form on the Earth's surface the cooling of lava lava that means there was an eruption that means there must be a volcano so this is an extrusive ignous rock and we know that just by looking at it because it's fine grain we don't see individual crystals so we know this formed after an eruption but what about its color what do you observe about its color well it's dark in color what does dark in color tell us tells us about its composition right it's mafic what does mafic mean rich in magnesium and iron okay rich in magnesium and iron that's dark in color it also means it's denser okay we know it erupted because it's extrusive fine grain we know there's a volcano this lava must have been very thin and runny because it's mafic so this eruption probably wasn't explosive but it was it could still be hazardous right this lava is thin and runny that means this lava can travel great greater distances it can flow farther so we're still dealing with a hazard so if you run into this rock when you're on a hike then you know there's probably an ausive volcano nearby and this is basalt this is what our oceanic crust looks like so really cool you were able to tell a lot about the environment in which these formed in some of the hazards that could be associated with these rocks that are nearby just by looking at the rock see rocks are really cool they can tell you an incredible story about the Earth and its history so hopefully you have a greater appreciation for rocks now but that's ignous rocks in a nutshell i'll meet you in our next video when we talk about sedimentary rocks