Rock Identification with Willsey: Volcanic Rocks (Tuff, Obsidian, Pumice)

okay hello there and welcome to the College of Southern Idaho the Evergreen building uh the geology classroom where I spend a lot of my time and we are going to continue our next video series in the rocks with we'll see series look focusing on rock types and how to identify rocks and hopefully good helpful tips that will help you as you're looking at rocks out there on your own remember you can if you like these videos you can always help out by either clicking on the Donate button at the top of the YouTube page or underneath every video description there's a PayPal link and some other information there so let's get right to it this is going to be our last video on igneous rocks and then from this point forward the plan is to focus our attention on the sedimentary rocks so probably do a a video focusing on the sedimentary rocks kind of an introduction to those rock types and then we'll start looking at specific sedimentary rocks one at a time but today we're going to focus on three rocks that I think a lot of people know and to some degree they're pretty easy to identify although we'll see there can be some subtle differences or another rock type you might confuse with each of these so we're going to look today at pumice obsidian and tough these are three extrusive igneous rocks so these are rocks that erupt from volcanoes they come to some degree from different types of volcanoes although there's some commonalities so we'll we'll look at that and we'll see how these kind of compare to each other because I think these three rocks have a lot of um a lot to tell us remember that our main goal here is to not just be able to identify rocks but to determine the story so every Rock has a story that's something that we need to remember and that our job hopefully is not just identifying The Rock which is which is fun and nice and gives us something to kind of hang our hat on but but more importantly looking at the story behind the rocks and what the rocks can tell us about Earth's history at that period of time so let's start with our first rock which is tough t-u-f-f this is a rock that is in terms of its texture we went over igneous rock textures so this has a pyroclastic texture mean meaning it's totally made out of Ash so volcanic Tufts are composed dominantly of Ash and Ash is just a particle size it's less than two millimeters in size so if the particles are are less than two millimeters by and large and they're Ash then you're probably looking at a tough now where a tough can get confusing is it can look a lot like some fine-grained sedimentary rocks like a fine-grained Sandstone maybe even a mudstone potentially and we'll get to that we get to the sedimentary rocks but what you would want to do is look closely at those particles which would probably necessitate um you know some sort of uh a hand lens or uh some sort of something that gives you some magnification and what you'd want to do is look at those particles and see if they look like ash particles or if they look like uh little grains of of rock fragments so maybe rounded or maybe not rounded but just you know broken rock fragments so remember to make Ash there's a fundamental process involved here we have to basically have an explosive eruption that actually is fragmenting and shredding the lava into little tiny pieces so just having Ash in a rock lets you know that the processes involved part of the story was one of explosive conditions where we were actually had so much gas present in the magma or locally that we're able to break apart the lava so it's explosive like a bomb going out off if you will you might ask well what happens if the rock is you know pyroclastic it forms by particles breaking during the eruption but what if it's larger than this this criteria we have here well then we sometimes call that a volcanic breacho we'll come back to this word when we use it in a sedimentary context but if you have larger particles in your volcanic rock and it's dominated by those then you might use this term here but we're going to stick with tough because that's a little bit more common one here so Tufts can be either hard or soft depending on how much welding occurs if you can imagine Ash accumulating in an environment as it sort of stacks up on top of itself and if it still retains a lot of heat that can actually allow it to weld into a more cohesive Compact and more dense type of rock so some Tufts are soft you can just wipe the ash particles with your with your thumb and your fingers other Ash or Tufts will tend to be much more hardened resistant this isn't the same as another word you hear out there tufa and these are actually confused a little bit on the internet and other places so this is a type of limestone this is a sediment truck made out of calcite we'll we'll probably tackle this when we get to sedimentary rocks but even though the spelling is very the same bear in mind that a tufa and a tough are totally different processes and totally different rocks so what kind of environments might we be able to accumulate Ash in well either Ash can fall out of the sky during explosive eruptive events as the eruption drives the ash up into the atmosphere eventually that driving pressure wanes and dissipates and so that Ash can and then fall back to Earth and that's what we would call an ash fall tough If It's just ash that's accumulated falling out of the atmosphere or we might get a tough that forms when Ash is actually flowing along the the Earth's surface kind of like an avalanche of hot ash and rock and debris and we sometimes call those surges or pyroclastic clothes I've done some videos on those Yellowstone and a couple around Southern Idaho so we've talked about that process a little bit before in terms of composition most of our Tufts are going to be felsic to intermediate so if you remember what composition means that's going to indicate that they're going to tend to be a lot lighter in color and so they would be rhyolitic and acidic and composition so they're going to tend to be light Grays pinkish that sort of thing but realize that you can have basaltic Tufts mafic Tufts it's a lot less common because we know that mafic magmas tend to erupt non-explosively but you can get a basaltic tough if that magma or lava is interacting with water or in some cases ice so if you get that kind of interaction between the lava and the water you can create explosive conditions where a tough might form so let's look at some some rocks here uh but let's start with maybe looking at a little bit of Ash I'm sure you've all seen Ash before but just to give you a little bit of context it's essentially like flower-like consistency I can feel a little bit of grit between my my thumb and forefinger here but this is incredibly fine material um it looks pretty innocuous it doesn't look you know it looks like possibly dough or something like that or flour I suppose but um this is actually nasty stuff Ash when it's inhaled in your lungs can mix with the moisture in your lungs it can set up like concrete you can become asphyxiated it's mainly made out of since it's mainly felsic it's mostly silica and composition so it can have long-term health effects on your lungs so the point is that nasty stuff to breathe in you wouldn't want to expose yourself to breathing in a lot of Ash over a long period of time but let's look at some different types of tough I think I want to start with I have got three right here and this one here this one and another one over here that all came from the same area but I think they can teach us some cool lessons and I hope I have them in the right order I didn't mark them but just looking at them I think I've got them in the right order where these rocks came from was in eastern California east of the Sierra is a place near Bishop called Owens River Gorge and Owens River Gorge is a spectacular setting to see a thick exposure of volcanic tough from The Long Valley Long Valley Caldera that erupted um it's about 760 000 years ago and as you start down the grade there you can walk the old road down into um into the canyon you'll see that the tough near the top um has you know Crystal particles in here let's see if we can find there's a nice little Crystal like like there we go there's a nice Crystal of quartz um in it but you can also see there's fragments in here right we've got just chunks so not only do we have crystals but we have fragments and chunks of other things the other thing we have are these kind of creamy colored materials here with with kind of this fibrous nature in them it's a little hard to tell just by looking at this piece but these are actually fragments of pumice there's another nice quartz crystal over here um and so the point here is just to look at the general texture of this rock even though it has chunks of other things in it it's dominated by the ash which is kind of the the background kind of grayish pink color here and so that's what qualifies this as as a tough but as you'd walk down Owens River Gorge you're walking into sections of the ash that um are deeper in the pile and so they were compacted more by the Heat and the weight of the ash above and so then you see um ones that maybe look like this where there's actually again not just crystals a lot of these irregular pieces here these are pieces of pumice again notice I can kind of scrape it away so it's a little bit soft um we still see some crystals in here in places uh let's see where's a nice one looks like there's one right there nice little quartz crystal coming through so it looks highly fragmented but in places you can also see around some of these fragments kind of a little Halo and these are thermal alteration effects so basically this ash was so hot that these particles that maybe weren't hot to begin with maybe these were just chunks of rock carried up to the surface by the eruption actually developed some of these little Halos around them as well but the point here is maybe seeing that some of the the particles are a little more squished we're seeing a lot more elongated and flattened particles in here even though we do have some that are kind of randomly oriented and then by the time we get to this one here which would represent the Rocks you see at the bottom of uh of the Owens River Gorge zoom out a little bit um what you can hopefully see here is that the particles are even much more compacted and in fact so much so that the a lot of the pumice is even flattened as well and just these little glassy lenses are kind of all you get in here we still get crystals so we can still see some of the original crystals in here but the idea here was to see with these three rocks just from welded this rock as I feel it my hands is incredibly hard and resistant if I go back to the first one here it's a little bit softer if I can kind of scrape off yeah a little bit of a little bit of the material here with my finger and so from you know you can see the degree of welding here with these three was kind of the idea in showing you this from Owensboro Gorge let's look at a couple others I just grabbed randomly here's uh one from a core so this is a drilled core but this also nicely shows some of the crystals in here here looks like we've got one there try to catch the Light just right yeah a couple little crystals in here again you can kind of see the fine grain nature this is like a typical tough here kind of light gray again we might get some other particles of rock entrained in the ash especially if it's an ash flow tough even if it's an asphalta if you might just have rocks sitting on the ground they get Incorporated and maybe melt or not melted but welded in with the tough as well again this one's kind of they're not always soft so don't take that as a cardinal rule but you see how powdery the residue is on my hand there so this one also feels a little gritty and powdery because the ash particles are coming off same thing with this one here this one actually feels a little bit light to the touch this is a much more fine-grained Ash and again the the trick you're going to have in determining let me put that down so I can zoom in without holding it and give you a little better view determining if this your sandstones or Ash Tufts is going to be to get a good look more more magnification than I can provide with my camera right now but get a really close look at this stuff and see if the particles look like um little shards of glass and what we call Ash particles which you'd expect them to be various shapes if you see a lot of rounding in the Rock that's probably indicating it's some sort of sandstone or mudstone or some sort of sedimentary rock realize too that you can have it can get messy because you can actually have Ash sized particles Incorporated in a Sandy environment so there's some Ash and some sand so we sometimes call those rocks tafatious sandstone so it's it's has the tough particles in it but it's but it also is made out of sand or sand sized particles in the sedimentary environment so we'll get to that more maybe when we talk about sedimentary rocks um what else another one here uh this one's showing again some of the crystallization this is a more welded tough as well looks like that might be a nice Crystal of my might be a tight I can't quite tell uh let's see what else we've got in there yeah there's definitely a little bit of biotide in there I can see some of those black crystals flashing looks like there's another Crystal of quartz right there catching the light remember those cleavage planes so that's our way to maybe identify a few minerals so all of these Tufts I've showed you are felsic maybe a little bit intermediate but mainly felsic and composition um I had to go outside and look for the only uh piece of basaltic tough that I I had and I apologize it's not the best one but this is a basaltic tough black kind of grainy it's probably got some of this yellowish orange material in here this is something called polygonite This is common when we get basaltic magma interacting with with water it forms this byproduct called polygonite in it so I don't know if I have a whole lot of cool things to show in this rock but just letting you know what a a basaltic tough Mite looks like might look like and that they're they're possible as another type of tough out there a couple particles of maybe oops sorry um larger chunks embedded in it maybe as well but there you go just another example of a different type of tough so so there's our Tufts let's move on to um our next two rock types and so we're going to focus on obsidian and pumice so these actually even though they look totally different and I'll go ahead and give you a quick visual here in case you aren't familiar with these too much everyone knows obsidian black shiny right and then this one here is um is uh pumice and so this one here and this one actually have quite a bit in common um and so we're gonna kind of look at both of these real quick this is these are actually technically both made out of the same stuff so if you take obsidian and pumice even though they look really different um if you grind these up into a powder they're actually the same composition this is the same material the exact same composition one to another but yet they look so different so they're both felsic they're both glass in terms of what they're made out of so let's start with obsidian uh obsidian's kind of an odd duck because we stick it with the igneous rocks but really it's not a rock because it's not made out of minerals and it's not really a mineral because it doesn't have ordered structure with uh with the crystals in it doesn't even have crystals or any crystallization whatsoever so I guess you could call it a mineral Lloyd although that's kind of a stretch too but for whatever reason we've decided to stick it with the igneous rocks probably primarily because it forms with volcanic environments and because it has that association with volcanoes and magma it makes the most sense to stick it here so realize that classification systems aren't perfect they always break down at some point so obsidian has no crystals it has no so you know what minerals are in obsidian none it has no minerals it doesn't have quartz it doesn't have Mica it doesn't have Olivine because all the elements in it are unordered and kind of random and again it has this glassy texture for which it's known for of citians are always felsic in composition even though they're dark in color and we learned that color was a good guide to determining composition this is one notable exception where the color does not help you the color does not lend anything to understanding the composition of the parent magma material so obsidian actually is only from felsic magmas basalts and mafic magmas do not form obsidian When lava runs down the hill in Hawaii and goes into the ocean and cools very quickly it is not creating obsidian so where does the dark color come from it's just small impurities of iron give it its sort of darker color which is kind of interesting so again kind of an odd duck this obsidian usually as an eruptive product we see obsidian forming it's always a lava flow so it's not Material thrown up into the sky as tephra or what we sometimes call pyroclastic material obsidian is always associated with some type of flow of love offer usually it takes place near the end of an eruption when the residual magma is cooler in temperature going to the next bullet point and lower in water content so if it's got more water content in it it's going to tend to make rhyolite and rhyolite lava flows than versus obsidian so you've got to basically get the water drained out of the magma it's what we call like dewatering or kind of what we call a dry magma to form obsidian and that temperature's got to be low not too low that it crystallizes but right near the the low part of its um or the upper part of the the crystallization spectrum and then one interesting about obsidian is over time it doesn't do well it doesn't last it actually what we call devitrifies or if you want to call it deglassifies basically it's characteristic glassy composition or Texture changes over time it absorbs moisture in the atmosphere it takes that water into its structure and it starts to actually create crystals so even though it doesn't have crystals initially it can actually develop crystals over time and because true obsidian then doesn't have these crystals and over time it de-vitrifies or basically the glass breaks down because it's not stable there essentially is no such thing as old obsidian all the obsidian you've ever seen is oh shoot probably younger than like let's say 20 million years or so there's so there's no way you'll find a beautiful chunk of uh you know you know 400 million year old obsidian because it will have already de-vitrified into rhyolite or something similar as it becomes hydrated over time it starts to develop little crystals and we sometimes call that snowflake obsidian that type of obsidian but those little those little snowflakes in there actually uh called spherilites so let's go ahead and jump to uh obsidian let me move some of my rocks around here and then we'll come back and look at pumice so here's a nice classic obsidian here you can see the black glassy um luster you can also see one of the characteristics of the obsidian that makes it such an important material for making Tools in primitive cultures or these curved fractures that run through it this is called concoidal fracture and because it breaks along these curves it allows someone with the skill to actually pressure break this with pressure called pressure flaking and start to shape it and form these really sharp edges here you can actually see it's becoming a little bit more let me zoom in a little bit more translucent over here you can actually start to see through it when it's in a little bit thinner pieces and again you can see how it kind of breaks here and forms these uh these sharp edges here so this is a pretty nice piece of obsidian here of course we talked a little bit about some of the color you can get in obsidian so this is a piece of obsidian with a lot of the kind of Reds in it this is what's called pumpkin obsidian so the iron here is partially already oxidized so it gives it some of that nice color kind of reddish brown orange color a similar piece here with just sort of a little cool concentric band of the more oxidized iron in the center of the obsidian piece there so another another chunk there it's got a little bit more of it on the back side but I'd say these first three here are just classic obsidians here is what obsidian looks like when it's just barely starting to de-vitrify a little bit so if we zoom in here um hopefully you can see that it's starting to form the glares a little rough here but it's starting to form little specks there's little white specks I don't know if I can try to get that closer yeah you might see it's got a few of these little specks running through it as well and these are these kind of developing sphere lights you can still see up here uh the conquital fracture pattern that's still forming but it's hitting as it's hitting some of these little tiny uh well these are actually crystals in here these little specks these little crystals um it's not as glassy in fact when I rub the the the this metal tip on the obsidian it moves and slides very easily because it's so glassy but when it hits one of these obstructions it's actually kind of sticky there and there's another one right there so these are actually crystallization points within the The Rock so this is an obsidian that's starting to de-vitrify it's starting to crystallize a little bit I'm not a flint knapper but I would guess this stuff would not be as good a material as the stuff I showed you right off the bat here this really nice material here so um and then over time it can actually become uh full-on snowflake obsidian where you can see these fairy lights forming that kind of uh have these crystallization points to kind of radiate out from around them so this is a good example of snowflake obsidian or a de-vitrifying um obsidian you can also see how weathered the surface is here that it's kind of looking a little bit this is what it would look like on the surface so as you're if you're looking for this stuff you know that's what it's going to look like it's not going to be typically super shiny and awesome unless you break it open with a rock camera and then you'll see some of the more kind of characteristic look there and then I have one more obsidian that's kind of odd and I can't remember a student brought this into me and now I can't remember where it came from but I think it's somewhere in Southern Idaho this is what it looks like on the outside um kind of an unremarkable dark rock with these um it looks like the little concordial fracture lines are partially filled in with some sort of material I haven't given this a ton of scrutiny but what's fun about this rock is when you break it open so here it is again on the outside when you break it open you can see it it is Obsidian but it also has a lot of again those specs in there so it's a partially or it's starting to a little bit so pretty cool um let's go to our last rock type and that is pumice so pumice is just the same as obsidian but now imagine obsidian that's just incredibly infused with gases so it's kind of frothy and foamy so this is essentially a gas Rich obsidian that's what pumice is um it's going to tend to have oh it's typo here this should be vesicular just switch to I and the S sorry about that vesicular remember vesicles are the gas bubble so it's going to have a lot of these gas bubbles it's going to be typically really light in color because it has very thin walls between adjoining vesicles pumice sometimes might have crystals sometimes not just depends on if there was some crystallization of that magma that took place before eruption pumice unlike obsidian gets thrown out of the volcano into the air so this is usually the beginning of the eruption the more gas Rich explosive phase of the eruption will tend to form pumice and then this is going to come from our our laundry list of naughty volcanoes right so stradal volcanoes may be called air forming eruptions lava domes these are the the suspects we'd expect to form pumice and so let me come back to our final group of rocks here but I want to actually make the connection a little stronger with um obsidian that site's good there so there's a nice piece of obsidian and now I'm going to put a chunk of hummus on top and I want you to think of this as the same material in fact think about when you pour a mug of let's say root beer or it could be an adult beverage but you've got the liquid a root beer down below which does have some gases in it but it's dark syrupy has a sugar and yummy tasting stuff in it but sitting above that root beer just as you pour it is the foam right the froth the head of the root beer or the beer and that's kind of like what happens in these magma Chambers the gas Rich portion of the magma Chambers is sitting above the more dense gas pore portion of the magma chamber and so as the gas Rich stuff is driven out of the volcano so this pumice is going to go flying out of the volcano collecting along the sides and then later we might get this uh gas pore felsic magma oozing out of the volcano to form the obsidian flow and so these can form from the same eruption and so I don't know I think that's a nice little analogy there so think about when you're next time you're pouring a big mug of root beer or an adult beverage Root Beer Works better because it's kind of similar color um think of that with the foam and the froth up above that's the pumice and then the more dense gas pore liquid down below that would be like the obsidian so so a couple pieces of pumice here to kind of conclude so here's one this one I remember I collected from the Salton Sea area in Southern California so you won't be able to tell us in the video but it's incredibly light as you well know pumice will float on water it has all sorts of uses because it's made out of glass it's very abrasive so as I rub this with my finger it feels rough it's used to clean things in your house you can scrape off the rough spots on your feet it's used industrially as a as an abrasive as well but the point is it just has so many holes so many vesicles in it and that's what makes it makes it so lightweight here's a little bit different color but also let me zoom out a little bit so you can see that this is from Southern Idaho about 40 minutes north of Twin Falls on your magic Reservoir so another piece of pumice they actually Quarry this pumice in smaller pieces uh probably crush it a little bit too and it's also used on the infield of some of the baseball and softball fields in our community so it's also used outside sort of as a landscaping rock in different applications so you can see some of the holes there um here's one this one I think this one's from just outside Yellowstone but this one you can actually see down so you can see it a little better this one actually should have some got my little pointer lots of crystals in it so we can see some of these crystals with Quartz in it actually this one has quite a few crystals in it if you kind of just kind of keep looking as I rotate it in the light you can see a lot of these crystals in here there's one down here at the bottom there's another big one right there I think that was a crystal anyway that might just be a shiny section but nonetheless you can see some of the crystals in here as well this one's from the oh which eruption from Yellowstone was this this might be quote Beyond this this might be the Mesa Falls tough which is the 1.3 million year old eruption so another another lovely piece of pumice again kind of abrasive um and then these last ones here are kind of fun this one actually shows the bubble walls so you can see uh these vesicles these gas bubbles in here and the walls between them and just sort of the glassy but also the the sticky uh viscous nature of this material and then there's areas in here where it's a little bit more rhyolytic so it just forms these kind of Gray gray bands in here in between these more pumice-like you can see these bubbles kind of getting stretched right in here this is a pretty cool piece here I believe I got this from oh eastern Sierras Long Valley Caldera area Once Upon a Time there's some of the bubble walls there and this one was in The Collection I'm assuming this one is real but it could be man-made glass but it nonetheless it just shows sort of the texture with the the bubble walls and in this case the bubble walls are really thin which makes them translucent to transparent to some degree um but again just kind of a big piece of foam just a big piece of foamy glass if you will that's pretty much all this this pumice stuff is so um hopefully that was helpful hopefully you learned a few things we covered tough we covered obsidian and we covered a couple samples here of hummus learning their stories their history how they're related to each other and to igneous rocks so for now thanks for joining me here at the College of Southern Idaho we're gonna close the chapter on our igneous rock series and we will do our next one next week we will start an introduction to sedimentary rocks and we'll explore the wonderful world of sedimentary rock so thanks so much for your time enjoy your day and we'll see you next time

Channel: Shawn Willsey

Views: 26,688

Keywords: shawn willsey, rock identification, stories of rocks, rock id, pumice, obsidian, snowflake obsidian, tuff, volcanic ash, volcanic rocks, igneous rocks, extrusive rocks

Id: xX6YGpk3DWk

Length: 31min 35sec (1895 seconds)

Published: Tue Dec 06 2022