good morning or today I'm going to introduce you to igneous rocks and volcanism which is lab number four now some of this material I've covered at least in lecture but I want to make sure I give you a brief overview of the specifics that you need to know to help facilitate the understanding the lab ensure that you complete the lab in the hour and 40 minutes that you're in the laboratory now the first thing we want to get across here is igneous rocks you might remember that igneous rocks are those rocks that are formed directly from crystallization from a magmatic melt now igneous rocks can crystallize both above the ground such as shown in this PowerPoint slide you can see a lava that is cooling at the surface this lava happens to be basaltic in composition but it's crystallizing at the surface Magma's can also crystallize below the surface and so the important thing we're looking at today in igneous rock lab is we're going to look at two aspects of igneous rocks that will facilitate the classification one is going to be the composition and that's the mean largely controlled by the source of the magma did it start as a partial melt of the asthenosphere and up well to form ocean lithosphere which is comprised of basalt was it the result of subduction of ocean basalt and partial melting of the ocean lithosphere to create intermediate composition or was the result of continental collisions where it may be partially melted continental crust and so that would facilitate the three different compositional character you should be looking at today the second component will be textures did it crystallize above ground or below ground remember very simply the if it crystallized above ground it's cooled very rapidly you're going to have finer textures smaller minerals if it crystallized below ground it's will cliff cooled slower you'll have coarser textures now if we're looking at the compositional characteristics of igneous rocks we can make classify them into three basic compositional forms basaltic and acidic and granitic which you can see here shown by these three pie diagrams now if you were taking an advanced igneous petrology class there would be a lot more subdivisions you would have subdivisions that might fall in between okay basalts and Anna sites and subdivisions that fall in between and the sites and Granite's but for the purpose of our introductory class we're just going to learn the three basic subdivisions and I will introduce one additional subdivision right here in-between granite and Anna site note the compositions of the three magma types and the three major compositions of units rocks are to find large you buy silica content basalt is about 50 percent silica content andesite is about sixty five percent and we get over to granite and it can be as much as 70 percent silica now one thing I want to get across here when we're mentioning silica content I mentioned this in lecture silica content does not equate to quartz content quartz is a mineral that is comprised of a hundred percent silica but you know that in basalt there's no quartz or typical there's no course the silica content you see basalt is the silica tetrahedra that are found in the other minerals like pyridine and calcium-rich plagioclase very important remember that the other important thing to note compositionally is that silica content goes up the iron magnesium content goes down and iron magnesium there's the brown and the light orange is shown right here that's iron magnesium and note that in a granite that our magnesium content is inversely related to the silica content now once you know the silica content and you know the composition you'll note that there are some other properties related to properties of lava or magma that are important and relevant to the slope angles of the volcanic landforms that are relevant to the explosivity of the volcanic eruptions and one of these major components is viscosity viscosity is defined as the resistance to flow and so high viscosity lavas tend to be very thick and sluggish they have a high resistance to flow viscosity as you remember we discussed in lecture is depend upon silica content and temperature the viscosity is directly related to silica content meaning that the higher the silica content if you look here the rhyolitic magma has a very high silica content it has a higher viscosity the basalts have a very low viscosity because they have a very low silica content note that viscosity units are shown logarithmically that they are not a linear scale because there's such a diverse variation in the height that excuse me the magnitude of viscosity the other important note here is temperature the temperature of the lava is inversely related to viscosity the higher the temperature for instance the salts have higher melting temperatures than rhyolites you're going to have lower viscosity and so we'll come back to viscosity at the very end of this this preview lecture we talked about explosivity of volcanic eruptions now where dove assaulted composition Magma's form and remember we talked about four different tectonic settings you can get basaltic composition Magma's anywhere you have partial melt of this dino sphere that can be in a mid-ocean ridge spreading zone which is shown here that could be a mantle hotspot shown here like the hawaiian islands you can all fit partial melt of this dino sphere behind the subduction zone here in a backlog spreading basin like the Columbia Plateau or the Sea of Japan or the Korean sea both those geographic terms been used to describe the same body of water you can also believe it or not get some basalt sometimes leaking up to subduction zones itself you get extension and sort of as the magma rises and causes the overriding crust to rise and extend you can get some basalt that might leak up along some of these extensional cracks remember we're looking at the salt that it tends to have a low viscosity but as the salt begins to cool it can begin to attain a higher viscosity in this case here we can see two different properties of basalt flows the pahoehoe flow the ropey texture one is the lower viscosity basalt flow that forms initially as the basalt erupted from the vent as the basalt begins to cool and D gasps its viscosity Rises and it gets a more blocky textured remember our flow and I mentioned in lecture think of walking on it and going ah ah ah to remind you of which flow type represents the higher versus low viscosity characteristics now if we're looking at the intermediate composition igneous rocks we talked about those forming at subduction zones this is your and acidic composition as far as lava types are concerned here we're looking at a partial melt of the ocean lithosphere which is being subducted along with that subducting ocean lithosphere you have marine sediments being taken down ocean water is also being taken down and as this gets abducted you start to squeeze the water out of the subducting plate that's gonna lower the melting temperature enhanced melting and that partial melt is going to rise the surface it gets erupted along volcanic vents along these subduction zones both in ocean ocean convergent margins such as Japan or ocean continental convergent margins such as the Cascade or the Andes Mountains now one thing that we note here is as I mentioned the intermediate composition Rock is described as an aside I want to introduce a second intermediate composition magma or lava type and the reason I bring this up is if you look at Mount st. Helens you're gonna find when you go to Mount st. Helens as you have over the past few weekends that there the rock type for Mount st. Helens is de site well it turns out that andesite and de site are both intermediate composition rocks the only difference we're looking at here is andesite is slightly more mafic than de site which is slightly more felsic and so de site would have higher silica content in it lower iron magnesium content in it okay they're both intermediate composition Magma's and so here we see Mount Saint Helens it's also intermediate composition subduction zone volcano but it has slightly higher fat content likely because there's been some crustal contamination as as the magma rose up through the North American plate you had some contamination from the higher silica content continental rocks here we can see in the case of subduction zone volcanism where we have the Magma's crystallizing underground you end up with a coarser grained Vulkan excuse me igneous rock and this igneous rock we see are comprising elcapitan in the Sierra Nevada is Grande or diorite and granite I write it turns out is the coarse-grained equivalent of de site and we'll get to that a little bit more detail ok so those are your intermediate composition Magma's forming from subduction zones and partial melting of the ocean lithosphere and including marine sediments and squeezing out of water and water lowering the melting temperature now when you have continents collide we get partial melt of continental crust and remember continental crust has an average composition like andesite so now we're partly melting on the site when you partly Montana site as we've been discussing all quarter you separate the melt phase from the solid phase the Mount phase is going to have a different composition than the solid phase that remains behind so when two segments of continental crust collide you get partial melting partial melting of the andesitic composition continental crust forms granite so granitic composition magma is generated from partial melt of continental lithosphere now you can see this partially melted continental lithosphere if you visit the Appalachian Mountains in Vermont or New Hampshire or any of the other states that have the Appalachian Mountains present along the eastern margin of United States and here's a quarry that is located in Vermont and you can see this granitic quarry here you have these extensive granitic rocks being comprised in this area of the Appalachian Mountains this is the old plutonic core that underlay the Appalachian Mountains just like the Cascades just like the Himalayas many these mountain ranges of course have plutonic bodies that are incorporated in their cores and the Appalachians as you know formed over 300 million years ago you've had count you've had continued uplift to the Appalachian and weathering erosion now exposing these old granitic plutons to the surface now you can go to Yellowstone Park and you can actually see another tectonic setting where continental lithosphere can begin to melt here it's slightly different in Yellowstone Park we have a hotspot it underlies this area of Wyoming the basaltic magma that's created at the hotspot rises up through the continental crust and begins to partly melt it as it partly melts this mag will start to rise well what's different here then say the Appalachian Mountains is that this magma this partial melt of the of the continental crust as it begins to rise it reaches the surface so it crystallizes at the surface and so it forms a composition of lava that is identical to the granite or magma that crystallized beneath the Appalachian Mountains and it forms the rock type rhyolite in this case rhyolite tuff I'll talk about tuff a little bit later okay but clearly it is the equivalent composition of granite it's just that it crystallizes more rapid at the surface here you can see as you remember we talked about viscosity properties of rhyolite viscosity is the lava type that is the highest viscosity you'll note here it has a very steep front to the flow it's almost vertical that flow is very thick and sluggish you can see as it was flowing out from the Yellowstone Caldera okay so those are the three different compositional forms of the Magma's or lava types that you're going to be looking at today that define the igneous rocks now I want to look a little bit closer at actual composition the minerals that comprise igneous rocks this is Bowens reaction series chart and I discussed this in lecture but just to reiterate what the most important aspects to get out of this bones action series chart is that certain minerals will crystallize okay at similar temperatures let me put this circle this in dark so you can see it better and so if we look at this up here in the red zone the minerals up here olivine calcium-rich plagioclase empiric seen all will tend to crystallize at higher temperatures in the intermediate these would be our subduction zones you're going to find these minerals present in andesite and diorite remember basalt gabbro is the higher temperature andesite diorite is the intermediate temperatures and this is comprised of ansible certain emerge plagioclase your some of your Mica's and maybe a little bit of potassium which feldspar when you get down to your lower melting temperature your partial melt of continental crust you're going to have largely magma is a drive from quartz potassium rich feldspar and your mic isn't maybe some sodium-rich plagioclase and so here we see again your three compositional forms of magma your three compositional characteristics of igneous rocks basalt gabbro and a side i right rhyolite granite and remembered a site and granodiorite would fall somewhere in between and a site and Riley now you might ask well what does this term mere continuous series and discontinuous series now remember I address this in lecture but just to reiterate because this is a little bit more confusing or complex let's just make sure we understand this the content continuous reaction series part of the bones chart is between calcium-rich plagioclase insert image pledge okay's remember what I mentioned in the lecture that with this continuous reaction series calcium and sodium can ionically exchange for one another in the minerals as it is crystallizing so as long as the melt phase is in contact with the solid phase the crystals you can have ionic substitution of sodium atoms replacing calcium atoms now if it's a high low temperature magma you're going to largely get the calcium rich phase crystallizing first and if you sit excuse me if you separate the calcium-rich phase early on okay from that male face then your rock is going to have more calcium-rich plagioclase in it but if it stays in contact they can continue to ionic exchanges one another now as you early on as you Pring out the calcium-rich phase the residual magma that gets separated will be more enriched in sodium so in later phases of crystallization you're going to tend to have a higher proportion of sodium-rich plagioclase crystals now keep in mind very important this is continuous you can have any combination or proportionality of calcium to sodium in your magma okay higher temperatures will tend to be higher in calcium rich lower temperatures higher in sodium rich but they will ionically substitute for one another in this crystal you can see here which we you can look at this closer on your own time what you'll note here is that this plagioclase crystal is zoned and you'll note that the if you if you look at this here you'll see that on the interior here you will see that the calcium-rich plagioclase the more calcium rich phase did it crystallized first and the more sodium rich in the outer zone of this plagioclase crystal showing that it was constantly ionically substituting you had a whole range of compositional forms of his plagioclase now the discontinuous reaction series as i mentioned in lecture is an either-or situation that we start out here with our partial melt of this deena sphere and as the magma starts to crystallize the first minerals that start to crystallize out are the olivine with iron magnesium and the silicate structure and as that madman continues to cool you'll start to then get Pyrrhic seen crystallizing out and remember I said it's an either-or situation you can have all of these crystallizing out and then as the temperature reduces then Pyrrhic seen olivine and pyroxene for a period of time can crystallize together but there's no hybrid mineral or any ionic substitution between the olivine structure and the pyridine structure it's an either/or situation though if minerals can form together in the raw but their individual minerals and as you see here when we look under polarized light you can see here distinct crystal boundaries the early formed olivine and then surrounding this here in the gray the later formed peer at scene crystal so it's discontinuous and then you can go on down through Pyrrhic seen an alpha ball the PRC in football and the Micah's and you can go on down to that discontinuous reaction series and get progressively the lower melting temperature minerals starting to crystallize out as the magma begins to cool and you separate the melt face from the solid phase now you might ask yourself how do we fraction it how do we separate memories how do we get the different compositions of igneous rocks if they if the melt face rays and contact it's also engineering to get this sort of ionic substitution and it's often in this exchange but what happens of course is we can separate the solid phase from the melt phase you can actually have minerals form they become dead so they can settle out onto the bottom of the magma chamber as shown here you can also have convection cells in the magma chamber and the minerals can sort of stick to the upper chamber walls and there you can separate the solid phase from the melt phase you can actually have you know sort of constrictions where the melt can move out and you can have it these concentration of crystals move with it and so there you can separate the melt phrase and the solid phase the most important thing to remember here when you separate the melt phase from the solid phase the solid phase will always be comprised of higher melting temperature minerals than the melt phase remember we've always we kept addressing this partial melting of this deena sphere the MAL face that melt is going to have slightly more silica it's going to be a lower melting temperature than the solid face that stays behind we're always going melt phase more silica rich solid face more iron magnesium rich relatively speaking okay very important you can also separate Magma's by starting out with that solid rock heat it up and then start to have it undergo partial melting that was going to happen the lower melting temperatures would melt first you'd separate the MAL phases solemn face and have two different compositions of rock as the melt phase crystallizes it's going to be more silica-rich than the solid phase that never melted that remained of the higher melting temperature minerals okay fractionation or separation by fractional crystallization or starting out with a rock and under basic fractionation fractionating by partial melting now this diagram here really shows clearly what we've been addressing with Bowens reaction series and this fractionation process we start out with a magmatic melt okay here in the partial melt of the steno sphere note that if you look here we have iron magnesium we have calcium of aluminum we have sodium potassium and silica all different color coordinated when we start to crystallize out early on what's going to happen here is you're going to tend to have higher concentrations of iron magnesium and your silicate structures and calcium and so your mafic and your altar mafic are going to be comprised largely of iron magnesium and then calcium starts to add into this forming your calcium-rich plagioclase and in your Pyrrhic senior basalt and but what's going to happen here the melt face stays behind is going to get progressively more and more enriched in silica aluminum and your lighter element sodium and potassium so by the time we go through the fractionation process of separating your melts excuse me your solids from your melts by the time we get to that residual melt and that partial melt of continental crust it's going to be largely comprised of silicon and oxygen some aluminum and iron excuse me not iron sodium and potassium your lighter elements and so it makes sense the minerals that make up your granite or your rhyolite are only going to be comprised of these elements that remain behind these your lighter elements are going to form your lower density minerals and your lighter colored minerals like sodium-rich plagioclase potassium feldspar muscovite mica and quartz now we now understand how we fraction eight we know how we separate melt phases from solid phases and how the mineral certain minerals are Hannibal that will form together at certain freezing or melting temperatures now you're ready to classify igneous rocks and so I mentioned the cygnus rock chart in lecture and I said it will be on the exam and so I'm not giving it away it's not like oh geez what a cakewalk all I'm telling you if you understand this igneous rock chart then you'll understand how to classify igneous rocks note the same thing that showed with Bowens reaction series that if we look at this igneous rock chart you'll note that in this area right here let me put it in black so we can see it okay that is your granite rhyolite zone we come over here this is your andesite diorite zone your intermediate composition and over here is your mafic basalt gabbro and over here is your ultramafic your asthenosphere rock which is largely olivine now the way that we classify igneous rocks is we simply look at the mineral compositions to read this chart it is basically based on the percent volume of these minerals then you can read over here each of these units is 20% so the way to read this if we look at this rock X right here and go straight down and look at its composition you're gonna see that there's quartz in it potassium feldspar sodium-rich plagioclase and some biotite mica and made a little bit of ansible and this is an ideal composition Rock okay will note that it falls in the granite rhyolite composition and the way to determine the percentage is here is remember each of these units represents 20% so granite excuse me this granite right you note there there's 20% there's 20% quartz and another 10% so there's 30% quartz in this part of this rock diagram down here we might have 40 percent or 50 percent potassium feldspar and so we've gone through this you can read this diagram the bottom line is when you're looking what's in a granite there is quartz potassium feldspar certain rich Biagio clays and maybe a little bit of muscovite or biotite mica and perhaps ansible and you can do the same thing with the salt too coming over here it's largely Pyrrhic seen calcium-rich plagioclase and maybe some in it and if you think about it what minerals if you have a a basalt what minerals will form the phenol Chris what if you have a phenocryst a portrait of basalt what minerals going to form the FINA crest it's going to be the highest melting temperature mineral what minerals the highest melting temperature mineral and basalt well the highest melting down to mineral goes as we move towards the right it's going to be your olivine so olivine will form the FINA Kristin your basalt if you're looking at granite or rhyolite what's going to be your your FINA Kristin your rhyolite well maybe it's Anibal or maybe it's biotite that has your higher melting temperature okay now the last thing we want to look at here is the rocks themselves so when you're going to look today you're going to classify your igneous rocks we've got the three compositional characteristics remember basalt gabbro diorite andesite okay and let's just put this here okay the coarse-grained equivalent is excuse me fine-grained equivalents basalt the fine-grained equivalent of this is andesite and the fine-grained equivalent of this is rhyolite remember for you Mount st. Helens buffs you have grano diorite is the coarse-grained equivalent of the intermediate rock that's right in between here and de site is the fine-grain equivalent or porphyritic because remember volcanic rock can be porphyritic and so you will go through today and you will look at the different igneous rocks so here we can see that's very easy if you can see the minerals of your naked eye it's a coarse-grained igneous rocks now what is it well you can go to your rock ID chart in the appendix of your lab and you're and see okay I can see this really clear let me put that in dark that is potassium feldspar the pink feldspar I can also see the corpse the questions I'm not quite sure what that dark mineral is I'm going to give you a little clue of how to be able to identify these dark minerals and maybe middles you don't know so I've got a piece of granite here now it's not me perfect okay but you can get an idea I'd like to show you something a flat surface but for the purpose of this lecture what I'm going to do is I'm going to take this granite and I'm going to go into the dark mineral you can already know the felt the potassium feldspar it's pink it's really clear you can see the course but what's the mineral well the dark mineral could be one of three minerals it can be amiable okay antha bowl it could be Furyk scene or it could be biotite the flaky biotite the sheet silicate the mica well what you can do is come to your rock take a harder a probe say the number eight or number six go to your hard mineral on a sheet of paper put little flat but for now to show you on the film I'm just gonna I'm gonna flake it I'm going to start to break that bark mineral off now you can't see it I apologize because of the details here but what flakes off here were little tiny golden black sheets right away that tells me the dark mineral in that granite was biotite because it's the only sheet mineral that breaks off in sheets now what happens at the mineral if it didn't break off in sheets what ends of it it didn't it stayed in the prismatic crystal form that leaves us with amp a bowl and pyridine now this is my little cheat sheet to remind you we do this all the time okay I just want to back up here again to take you to the chart okay and if we look at the the ID chart here the mineral ID chart if you go to your granite and so you have a dark mineral which is not flaky that comes off in sheets that leaves you with a choice of a mobile or Pyrrhic scene well based on the presence of the pink potassium feldspar which of the dark minerals is more likely going to be compatible with it well it's going to be the alpha bowl so even if you can't see the cleavage plane you'd be probably correct to surmise that that dark mineral that doesn't break off in the little sheets is amiable because you have potassium feldspar so use the compatibility of minerals to help you maybe identify sometimes minerals that you don't know like phenocrysts and so forth okay what minerals are compatible with one another and if it's a phenocryst what minerals are compatible with one another which one of those has the higher Mel temperature that would most likely be the phenocryst okay alright so just going back today you're gonna look at the salt the fine-grained igneous rocks we all say don't use color to identify rocks then we say use color but salt is a dark black massive rock sometimes you might see some phenocrysts in it you might see some applied yoga clays crystals or the little whitish gray ones or maybe you might see small little green olivine crystals in it you get to end a site you're going to see the dark little crystal there well you go to andesite what minerals it most likely gonna be it doesn't flake off like a sheet silicon is mica well then it's probably going to be affable and then the big white crystals can be applied to a clays forming the phenocrysts rhyolite you can see is kind of a gray buff colored obsidian forms it's different we have some special textures with the igneous rocks the glassy textured one that breaks off in Concord all fractures that looks black in a in a fixed section is actually a very as a volcanic glass and it's pure silica and that's why it breaks off concordantly what makes up city and dark trace amounts of magnetite it just looks dark in a thick section other specialized igneous textures subbing this rocks form with high gas content those gas bubbles are preserved if it's a phallus it content 1:1 that is equivalent to rhyolite or granite we call it pumice if it's a mafic or intermediate we call it scoria and now she's kind of cool you guys can see here look at this from olivine phenocrysts you can see in that scoria it's got that green color diagnostic of olivine sometimes you have very explosive volcanic eruptions and that Ash and that coarse volcanic plastic material it can wall together and that will form a volcanic tuff if it's fine grained ash that's welded together this is the bishop tuff down in Eastern California and then if it's really coarse-grained volcanic classic are welded together such as this tuff that you see in lipari island in Italy you're going to have much coarser class and some of these are going to be cobbles to Boulder size this is a volcanic breccia the last thing I want to mention to you guys that you know that will not take much more of your time is the fact that the composition of the magma is going to have a very strong control on its viscosity which is in a very strong control on its morphology at the volcano and so you're going to look at Hawaiian volcanoes you're going to look at cascade volcanoes you're going to notice the Hawaiian volcanoes have very low slope angles and why do they have low slope angles because of the low viscosity basalt flows here you'll see that this might have only about a oops a seven degree slope angle that you can see here okay about seven degrees very gently sloping okay and then just let me move along here and apologize here there we go and then finally we can look at the Cascades or the Aleutian Islands subduction zone volcanoes note here if you look at their slope angle you'll see that they make slope angles a 25 to 35 degrees large because of the pyroclastic and lava flows of the anda sites have a much higher viscosity so they can retain much steeper slope angles and the volcanoes reflect that along that same line of thinking if you're looking at the explosivity of the volcanic eruptions why do the Hawaiian island eruptions why they so quiescent or quiet and gentle you can stand right beside the lava fountains because the salt has a low viscosity has relatively low gas content so the lava fountains get erupted and the gas is released easily gently and then when you come down to look at Mount st. Helens or a subduction zone volcano there you have relatively high viscosity intermediate lavas with high gas content that's not a good combination thick viscous lavas high gas content gas is expanding as the lavas upwelling or rising very explosive and so that sort of gives you an overview what you'll be doing this week in laboratory anyway I'll see you next week when we talk about sedimentary rocks and processes and you all have a good day