hi guys this lecture is going to be on volcanoes tectonic settings for volcanic activity so we know that at convergent plate boundaries you have subduction zones where you have ocean content --all convergence and at ocean ocean convergence those two scenarios you will have subduction and you guys know that when a plate descends under another one it will then melt in this atmosphere and then the melt will rise up to create volcanic eruptions notice that content or content will convergence is not on the list because there is no subduction zone for content or continental convergence so when you have ocean floor colliding with the continental crust you will have volcanic activity and remember those were continental volcanic arcs and when you have ocean floor collide with ocean floor the resulting volcano is a volcanic island arc and of course you know these are examples of two tectonic settings for volcanic activity it's at the subduction zones the convergent plate boundaries that have subduction zones also a divergent boundaries you do have volcanic activity so the divergent boundaries on the ocean floor the mid-ocean ridges remember when we said that mid-ocean ridges are actually volcanoes on the ocean floor where the ocean floor splits apart and you have lava coming up above the ocean floor and so when molten rock is introduced above the crust we call that a volcanic eruption or volcanic activity and so you got two types of crust contento crust is when the eruption happens on land and you can see the lava and an eruption when you have lava going through the oceanic crust those are underwater eruptions and so if you have lava and if you have lava on the ocean floor or lava on and either you know popping up through the oceanic crust or the content to across those are both considered to be volcanic activities and so again mid-ocean ridges at you know divergent boundaries you have volcanic activity happening there and the other one that we haven't talked about is hot spots also called intraplate but for the exam I'm not going to use the word intraplate I'm just gonna use the word hot spots and hot excuse me hot spots have nothing to do with subduction zones it's just basically you have volcanic activity located somewhere inside of a tectonic plate you're not dealing with the area where two plates move away or where two plates collide not at all you just have volcanic activity somewhere inside of the tectonic plate for example Hawaii I'll go ahead and circle it on so we're gonna be looking at this map right here and notice where Hawaii is Hawaii is a volcano that's located inside somewhere in the middle of the Pacific tectonic plate and when you look at the North American tectonic plate you have this Yellowstone volcano right here and so these are nowhere near the plate boundaries you don't have any divergence or convergence zones here so Hawaii and Yellowstone are both hot spots basically hot spot is when molten rock wants to vent out somewhere in the middle of a tectonic plate has nothing to do at subduction zone so you can have hotspots on the water an example of that is Hawaii and you can have hotspots on land and an example of that is Yellowstone so where do volcanoes occur in California so notice that you have you know Mount Shasta Lassen Peak and you also have Clear Lake and so from here to here you know this is Northern California so you guys know that the San Andreas Fault runs through here and then so along the San Andreas Fault you have this plate moving up this plate moving down so you don't have any subduction going on here so there's no volcanoes due to so duction or anything like that and so if you do you know this volcano this coastal volcano here this long Valley caldera over here these are volcanoes that have nothing to do with subduction zones right there's no ocean floor subducting under the North American plate here and so long Valley caldera and COSO volcano these are hot spots in California and you know San Francisco field Bandera filled craters of the moon and Yellowstone these are volcanoes that are hotspots ok where the San Andreas Fault ends like from here going up this is all convergent plate boundary where you know you guys remember that this area is the one-day Fuuka plate right this is where the Juan de Fuca plate dives under Washington dives under Oregon and Northern California and also Canada and so the Juan de Fuca plate is subducting underneath Northern California Oregon Washington and Canada and it's creating chain so it's creating a chain of continental volcanic arcs so these are all volcanoes created by convergence by subduction but you do have volcanoes in the United States that has to do with hotspots they're not they're not you know content of volcanic arcs or anything like that so let me go ahead and erase all this like it's not so messy ok cool now so Mount Saint Helens is a continental volcanic arc right over here right so we got Mount st. Helens in this area in Washington and it erupted in 1980 ok so that's the most recent eruption in general and of course it's because of the subduction zone and everything the Juan de Fuca plate is melting to create the Mount st. Helens volcano contento volcanic arc the last volcano to erupt in California is Lassen Peak and last he corrupted in 1915 so it's not too far you know if you look at geologic time it's didn't happen you know that long ago over here Yellowstone Yellowstone the last time interrupted was 640 thousand years ago in Wyoming and it was very devastating Yellowstone is a supervolcano where the magma chamber underground is so big that it covers parts of Montana Idaho and Wyoming and when that thing blows up very very devastating we'll get into that later so here is Washington and somewhere in Washington you have this mount st. Helens volcano that has that favors violent volcanic eruptions and of course this is a content of volcanic arc the Juan de Fuca plate is subducting under to create this volcano so this is what Mount st. Helens looked like before the eruption in 1980 I believe it happened sometime in May of 1980 but right after a directed look at what happened look at this crazy eruption it said that this ash cloud that went to the air crashed on to the people at 300 miles per hour very very devastating this happened in 1980 a lot of people died the eruption was so loud that people that were nearby their eardrums blood and they went deaf so look at how much of the volcano is gone imagine like how loud it is when you have your vacuum you know on and it's pretty loud and annoying but imagine if a huge chunk of this mountain blows up like that's really really loud and so remember when we talked about in igneous rocks we talked about the pyroclastic texture will you'll have rocks forming from fragments of Ash Rock debris blown up by violent eruptions into there so you do find some pyroclastic texture rocks in this area so this is a digital view of Mount st. Helens before the eruption but this is after the eruption look at how much material was displaced and this is in 2003 notice if you go back in 1980 after after the eruption in 2003 it's filling back up you guys see that it's filling back up and then in 2007 it's filling back up again I don't have a recent photo of it now but I can only imagine that it's almost filled up and you know Mount st. Helens is alive it goes through many many it has many many eruptions in its lifetime and so it's due for another eruption because member the Juan de Fuca plate is descending underneath Washington and this is constantly creating melt and the melt will rise to fill up the volcano again it's kind of gross but it's kind of like you have a pimple and you pop your pimple and your pimple now is half the size but then you wake up the next day and your pimple is big again so it keeps filling back up the magma keeps filling up the volcano and so this is the Juan de Fuca plate that I covered in this is the San Andreas Fault right here and you know the Juan de Fuca plate is going under this area and so the melt is going to create a chain of continental volcanic arcs in this area let me go ahead and draw them as little triangles and so it makes sense if you have volcanoes in this area you know it's because of subduction but if you have a volcano over here you know has nothing to do with subduction zone so they have to be I'll go ahead and write it in hot spots okay in years 1700 there was an earthquake that caused a tsunami in northern California and this occurred along the Cascadia subduction zone so remember let me go ahead and erase all this so the volcanoes that you have in this area they are part of the Cascade Mountain Range or the Continental volcanic arts and Mount st. Helens is one of them and this area remember were year but you have subduction you have tsunamis right and so in year 1700 there was a tsunami in Northern California and because there was um you know as a megathrust earthquake the magnitude was a 9.0 is strong enough to cause the waters to D to be displaced and crash onto the land the waves of the tsunami were 50 feet high and it's due for another megathrust earthquake so you know the shores of Canada and Northern California these are all tsunami zones so I personally wouldn't buy a house on the coastline in those areas so what is a volcano oh okay so you know in order to be a volcano you have to have molten rock molten rock is you know without the molten rock you just have a simple volcano I'm sorry without the molten rock you have visible mountain so there are two types of mountain your mountain that has no magma in it at all no molten no association with volcanic activity no association with magma or lava and then you have a mountain that has volcanic eruptions has magma has lava has all that an example of a regular Mountain would be the Himalayan mountain range today where the mountain range is just created by content or content or convergence with the land raising up to create mountains squeezing the land together to create mountain ranges but notice that the Himalayan mountain ranges were created with no association with volcanic activity is just compressional forces squeezing rock together uplift to create the mountain range there are no volcanic eruptions in the Himalayan mountains but example the Mount st. Helens mountain that's about mountain Ana volcano so in order to be a volcano you have to have a magma chamber and the molten rock in the magma chamber will rise up through the conduit and then it will vent out so you can have a main vent or you can also have side vents as well so remember that subduction zones so what some volcanoes are created because of subduction zones where you have a plate descending under another plate and the descending plate melts and the melt will rise to create a volcano and at divergent boundaries you have volcanoes that were created by divergence where you have the ocean floor ripping apart and lava rising up to create volcanoes the mid-ocean ridges and also hotspots this is uh remember hot spots have nothing to do with subduction zones these are volcanoes that just are located within the interior of a tectonic plate is just molten rock wants to vent out inside within a tectonic plate an example of that would be Yellowstone and another example of that would be Hawaii so I want you guys to remember Hawaii is not a volcanic island arc you know Japan is a volcanic island arc because it was created by ocean floor subducting under another ocean floor to create the the japan island in japan is a volcano hawaii is a volcano and hawaii is an island but it was not created by ocean ocean convergence hawaii was created by simply molten rock wanted to rise up in the middle the pacific plate and then with continuous molten rock rising above the ocean floor a grue grue above sea level and now you have an island that's visible and so Hawaii is an example of a hotspot volcano so this portion right here that I'm coloring in did not exist ok so pretend this is all flattened out pretend that there's no island visible here and so molten rock wanted to rise up somewhere within the interior but Pacific plate no notice that you have no subduction zone here and so lava came up to this area hardened and cooled and then more lava came up hardened and cooled and they turned into a seamount ASIMO is like an under water island that's not above sea level yeah but then with continuous eruptions the lava came up Harding cold and now it's above sea level and so now it's an island and people live on the island let's go ahead and watch this video showing how the Hawaiian Islands formed and I just want to mention that when you have molten rock rising at a fixed point deep beneath asthenosphere and that molten rock is rising up through the oceanic crust keep in mind that the hot spot source of the hot spot the source of the molten rock is fixed and doesn't move but what moves is the lithosphere above so keep in mind when you're watching this video it shows you why we have a chain of Hawaiian Islands or a chain of hot spot volcanic islands not all volcanoes form at plate boundaries some form instead in the interior of a plate where geologists call them hot spot volcanoes or simply hot spots not all collectively instead in the interior of a plate where geologists call them okay let's start that over again that was not cool right so let's go ahead and make this bigger and start from the beginning not all volcanoes form at plate boundaries some form instead in the interior of a plate where geologists call them hotspot volcanoes or simply hot spots one explanation of such activity is the presence of a mantle plume a regionally restricted column of hot rock rising inside the asthenosphere when it reaches the base of the lithosphere the rock partially melts sending intrusions of magma upwards into the plate some of this magma erupts to form active underwater volcanoes or if activity is sustained an active volcanic island an entire chain of such islands forms because the plate carrying the volcanoes moves a the stationary hot spot inside the asthenosphere this movement slowly carries the active volcano off the mantle plume causing it to go extinct a new island grows in its place over the plume when the plate motion changes so does the direction of the volcanic island chain the inactive older islands cool and subside with distance to the heat source and experienced significant erosion with time former hotspot Islands turn into smaller shallow islands and ultimately become underwater sea mounts you through geological time the direction of the plate motion is recorded by this hotspot tract of islands and underwater sea mounts all right that's awesome and so this video is really nice because it shows what we're going to talk about here and so this is your well first of all let's put where everything is so this is your Astana sphere so everything beneath this line red line is this Hemisphere and you know this is your crust and this is the uppermost part of mantle so we know that this is the oceanic lithosphere right here and so the oceanic lithosphere travels above the asthenosphere so keep in mind that the asthenosphere is fixed and it is the lithosphere it's everything above this red line let me make it a little bit more solid everything above this solid red line is moving to the left and everything beneath the solid red line is fixed and it's not moving it's stationary and so here down here you have a stationary source of hot liquid molten rock always always always rising above this area right here it's always coming up here and so any little sphere above it will have a volcano but we know that the plate is moving and so this big island of hawaii currently here today it's not always gonna stay there 2 inches per year it's gonna be where Maui is and then another you know later Maui is gonna be where Molokai is Molokai will be aware or Oahu is and then Ajo will be work away is and then Kauai will end up being you know a seamount underwater right and and it'll be under sea level and so the new Big Island of Hawaii went the moon the moment when the Big Island of Hawaii goes where Maui is the new Big Island of Hawaii is going to be low he currently law he's below sea level but by the time when law he reaches where the new Big Island of Hawaii is it will be the size of Hawaii so notice that the volcanoes that are older they you know they're smaller and there's reasons for that one of the reasons is Maui Molokai Oahu and Kauai are no longer being fed by you know there's no longer molten rock coming they're making it bigger they're no longer being fed and so they're not getting any bigger and then because they're older volcanoes far away from the source of molten rock they you know through time erosion will occur weathering will occur the rocks will break down by physical and Mackenna and chemical means and so notice that the older volcanoes they are smaller and more rounded because of the erosion so kawaiii is older than oahu Oahu's older than Molokai Molokai is older than Maui and Maui is older than Hawaii but the new Big Island of Hawaii is going to be llahi this is far far into the future so Hawaii is an example of hotspot volcanic activity on the ocean floor Yellowstone is an example of hotspot volcanic activity on land we're going to end the volcano's lecture here this will be volcanoes part one and volcano volcanoes lecture part two we'll start at pyroclastic flow thank you guys for listening and I'll see you guys soon