McGillX | MCGATOCXT314-G000700_TCPT
PROF. JOHN STIX: Today, I'd like to give you an idea of what volcanoes are all about, an introduction
to how volcanoes work essentially. So here's a nice photograph of an erupting volcano taken in Hawaii.
And this volcano shows a lot of nice things. First of all, you can see the magma coming to the surface,
erupting as a small fire fountain, little droplets, and intense incandescence. So the magma is really hot,
and it's about 1,200 degrees Celsius, or around 2000 degrees Fahrenheit, super hot.
And then you can see there's some gas, and shown in red over here, and there's some gas flowing
from the eruption over there, next to the magma that's coming out. And then you can see the magma
flowing down as lava down the slopes of the volcano located right here. So there's a lot of interesting
things you can see from this single picture right here.
And this is another example of an eruption actually, an artist's depiction of the eruption of Krakatoa in
Indonesia in 1883. And it's an explosive eruption. And instead of a lava gently flowing out on the
surface, the magma is coming out from underneath, and being fragmented into particles, small
particles, big particles, and so forth, and then getting lofted into an eruption column. So you can see this
big eruption column developing here, as a result of what we call explosive eruption.
So it's important to make the difference then between quiet eruptions, which put out lava, and violent
eruptions, which put out a fragmental material, or what we call pyroclastic material. And sometimes,
very interestingly, some volcanoes actually put out both types of material at the same time. So there's a
lot of complexity and interesting features related to volcanism and volcanoes.
Some volcanoes are truly enormous and you might know that Yellowstone in the Northwest of the
United States is actually a very large volcano, or what we call supervolcano. And it's super, because
when it erupts in a very large fashion it puts out a huge volume of magma.
So the last eruption at Yellowstone was about 600,000 years ago, and put out maybe a thousand cubic
kilometers of magma. And to put that in perspective then, the eruption of Mount St. Helens in 1980,
May 1980, put out about one cubic kilometer of magma. Some of you may remember the eruption at
Mount St. Helens, and that was a pretty spectacular eruption, but it was puny compared to these super
volcanoes, which do erupt very infrequently.
And these super volcanoes form a large, large crater, which we actually don't call a crater. We actually
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call it a caldera. And this is the caldera right here, a depression formed when the magma chamber is
emptied. And this crater or caldera at Yellowstone is on the order of 40 or 50 kilometers in diameter,
absolutely enormous.
So these are two wonderful photographs then of explosive activity producing pyroclastic material on the
left hand side of this eruption of Mount St. Helens in the United States, a photograph probably taken in
the early 1980s, and an eruption of Kilauea volcano in Hawaii.
And then you can see, the small fire fountain that's forming at the summit is coalescing into a series of
lava streams or lava flows, which flow away from the summit of the volcano. So a beautiful contrast
here between violent activity, producing pyroclastics, fragmental material, and lava flow activity, quiet
activity.
So as volcanologists, what we want to do is understand why some volcanoes are quiet, and other
volcanoes are violent, or why a single volcano may be erupting in a rather quiet manner, and then
transform itself into our violent eruption or the reverse.
So these are key questions that are still posing problems and questions for volcanologists. And it's a
very, very interesting question. And it's very important practically also, because in terms of volcanic
hazards. If a volcano produces lava, that's a certain type of hazard. But if a volcano produces ash and
fragmental material, the hazards can be quite different.
So let's look at the main controls then on explosivity, what we call explosivity, or violence of eruptions.
First of all, it's the composition of the magma, which is very important. So there's basically three types
of volcanic rocks, basalt, andesite, rhyolite. And they have different amounts of silica. So basalt has
about maybe 50% silica in its chemical makeup, andesite about 60% silica, and rhyolite about 70%
silica.
So the very interesting thing about silica is that as you increase the silica content of a magma, the
magma gets more viscous. It becomes more sticky. And as a result, stickier magmas, more viscous
magmas generally produce more violent eruptions. And the gas is more difficult. It's more difficult for
the gas to escape from this highly viscous magma.
Also basaltic volcanoes tend to be relatively gas poor compared to andesitic volcanoes, and rhyolitic
volcanoes, which tend to be relatively gas rich. So the less gas, the less violence to an eruption. The
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more gas, the more violence to an eruption. Andesitic magmas and rhyolitic magmas tend to produce
explosive volcanism whereas basaltic magmatism produces generally quiet volcanoes. There's of
course exceptions to these rules. But that's the general pattern.
So just to look at viscosity in a scale here. There's some common examples of viscosity here. So here's
water, which is fairly low viscosity. Corn syrup over here, a little higher viscosity. Balsaltic lava is even
more viscous than corn syrup, even though it looks quite runny. And then you get up to composition
such as rhyolite, which are highly viscous compositions. So there's a very large change in viscosity from
basaltic magma to a rhyolitic magma.
And I've actually brought a demonstration here. And I want to show you two things. I've got a can of
Coke, and a mound of silly putty here. And essentially, everything you need to understand about
magmas is contained in these two objects right here.
So let's look first at maybe the Coke can. So the Coke can, you understand that there's a lot of
dissolved carbon dioxide in the Coke can. And when you open the Coke can-- and I'll do it right now. So
that noise you heard was the can opening of course, and then the release of the CO2. And the CO2
was released, because there was a pressure change, a pressure decrease with the opening of the can.
And that's exactly what magmas do as well. So when magma is stored at deeper levels in the crust,
there's more gas dissolved in the magma. But when the magma moves up, either fast or slow, to
shallower depths, there is less gas dissolved in the magma. So the solubility of gas in a magma is a
strong function of the pressure of the magma.
So as a magma moves up, it loses gas. It loses dissolved gas, and bubbles form in the magma. And
that's really a driving force behind eruptions. Another thing which I could demonstrate with the silly putty
is that when magma is sheared or moves relatively slowly, it flows just like I'm showing you in the silly
putty right here. But say, a magma, instead of moving up slowly, is moving up fast. OK. Let's do that
right now.
So here is silly putty that's going to move very fast. It didn't work. Let's do it again. One, two, three, it
broke. OK. So the point here, is that if material is moving slowly it may flow. But if it's moving very, very
fast, instead of flowing at some point, it might break. And that's what occurs with real magma as well.
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McGillX | MCGATOCXT314-G000800_TCPT
PROF. JOHN
Now we're going to look at where volcanoes form on Earth-- and where they don't
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form. So, here's a plate tectonic map of Earth. And think of the earth as an egg. And
we're looking at the eggshell here, and the eggshell is broken. And all the places
marked with jagged black lines-- for example, right here-- are the margins-- the
boundaries-- between one plate and another plate. And so, all the red dots that you
can see on this map are active volcanoes. And you will see that the great majority of
these dots are located along these plate boundaries. There is a few exceptions, but
the great majority of volcanoes are located along these plate margins.
And so, where I live, in Montreal, right here, I don't worry about a volcano erupting
today in my backyard. If I were living in Vancouver, well, there would be some
volcanoes not so far away that could affect me and the city were they to become
active tomorrow.
So, in the next few slides, what I want to do is show you three different types of
plate boundaries that produce three different types of volcanoes. So, first we'll look
at a divergent boundary, a boundary where two plates are moving apart. And just to
show you on the map right here, the mid-Atlantic ridge is a good example of a
divergent boundary, or what we call a spreading center. The second type is a
subduction zone, and that's where, instead of two plates being created and moving
apart, one plate is actually going underneath another plate. And that's called
subduction. And the best example of that is the Pacific Ocean. In it, for example, the
Western Pacific, these are the whole range of subduction zones, right here. And
finally, we will look at a hot spot volcano. And the best example of that is Hawaii, in
the middle of the Pacific Ocean.
And here, on this map, before we get into the details of the plate tectonics and the
volcanoes, I want to show you that the Pacific Ocean in particular is of great interest
to volcanologists. So, all the area shown in red is what we call the Pacific Ring of
Fire. And this Pacific Ring of Fire is where a lot of volcanoes occur, and these are all
subduction zones. And this is where a lot of big, explosive, dangerous volcanoes
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occur. So, in terms of hazards, in a sense, this is where all the action is-- or where
much of the action is-- in terms of explosive, violent volcanism.
So, here's an example of two plates moving apart. Magma comes up from the
mantle, so magma is coming up, in this sense here, from the mantle below, driven
by convective forces. So, this convective motion moving magma upward. It gets
injected into the shallow mantle and crust, and then the two plates move apart.
So, what's produced here as a function of the magma moving upward from the
mantle is basaltic magma. And, as I said in the previous lecture, basaltic magma is
generally silica-poor very runny, low viscosity, and not very much gas. So, in
general, basaltic magma erupts as lava, rather than violently, as pyroclastics.
Although there are exceptions.
And the next-- the example that I want to show you here-- the next slide shows a
beautiful example of basaltic lava at a volcano called Erta Ale, in Ethiopia. This is
actually a spreading center on the African continent where the two plates are
moving apart-- or trying to move apart-- and actually create a new ocean basin. It
hasn't happened yet, but they're in the process of trying to make it happen.
And so, here we see a crater, the pit crater here of Erta Ale. And in the pit crater is
lava-- basaltic lava-- and it's in a lava lake. You can see the incandescence of the
lava. What's the temperature of the lava? The lava is at 1,200 degrees Celsius. And
you can see the black skin of the lava that has crusted over and cooled. But
underneath that skin, which may be-- how thick? Maybe a half a meter, something
like that. And beneath that skin is the incandescent liquid lava moving around, also
convecting in its own little lake of lava. Really amazing. Really spectacular. You
probably could walk on the black part of the magma. It would probably be OK. But I
don't suggest it. It's very hard to get down, so don't do it.
OK. Now, let's go to subduction zones. So, here we have one plate going
underneath the other plate, because the plate that's going underneath-- which is an
oceanic plate, it's composed of oceanic crust-- this is going underneath a
continental plate in the diagram-- the North American plate-- so this is the North
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American plate right here. The plate that's going down, the oceanic plate, is denser
than the continental plate. So, it goes down relative to the North American plate,
which is the overriding plate, or the upper plate.
So, the process of subduction means that the plate that's going down is getting
hotter and hotter and hotter, because it's being driven back into the mantle. And at
a certain depth-- around 100 kilometers, give or take-- there starts to be a process
of partial melting. The plate that's going down starts to melt a little bit, producing
magma. And so these magmas move upward, and some of them make it to the
surface and produce volcanoes.
So here's the subduction zone right here. Here's the subduction zone right here.
And the volcanoes that are produced form an arc or a line of volcanoes which
parallel the subduction zone. So, the line of volcanoes here, shown here, would be
roughly parallel to the subduction zone, but landward, of course, of the subduction
zone, because the subducting plate needs to get down deep enough.
The subducting plate is actually full of water. It's been hydrated from being in the
ocean. And so, there's a lot of water and other gases and volatiles which are being
driven down by subduction. And that's probably the primary reason volcanoes
associated with subduction zones are so explosive. Mt. St. Helens, Mt. Pinatubo,
Krakatoa-- Katmai, the largest eruption of the 20th century in Alaska-- all these are
driven by subduction processes.
And here's a wonderful example of a subduction-related volcano-- Mount Fuji, in
Japan. And it has this perfect shape-- beautiful, classical, conical shape. And it's
very steep. And the reason it's very steep is that the volcano is made up of a
combination of pyroclastic material-- fragmental material-- which tends to form
angle of repose deposits, for example, like in a cinder cone. And then lavas. There's
also some viscous lavas, andesitic lavas. Composition of this volcano is andesitic-it's associated with these subduction zones-- and it's an alternation of lavas and
pyroclastics. And that gives it its relatively steep character.
The last type of plate tectonic boundary which is important for volcanoes are what
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we call hot spots. It's actually not a plate tectonic boundary. It's actually in the
middle of a plate. So, there's a hot spot, what we call a mantle plume, which comes
up-- the magma comes up from deep in the mantle and moves up vertically. And
the overriding plate, the plate above, is moving at a certain rate relative to what the
hot spot is. The hot spot is stationary. The plate is moving. How fast is the plate
moving? Plate's moving a couple centimeters-- a few centimeters-- per year. Pretty
fast, geologically speaking, although not very fast on human terms.
And so, this process generates a series of volcanoes that are aligned as a series of
islands. The best example of this are the Hawaiian Islands, that show this behavior.
So, the current hot spot is producing volcanoes on the island of Hawaii. In the
future, the island of Hawaii will move away and a new island will form, and so forth
and so on. So, if you look at the Hawaiian chain, the island of Hawaii is the youngest
island, with the youngest and most active volcanoes, and all the other islands are
progressively older to the northwest, and the volcanoes are progressively less
active and eventually become dormant, and then extinct. Really, a very interesting
process.
And this leads to volcanoes, such as Mauna Loa in Hawaii, which is actually the
tallest volcano on Earth. It's about 14,000 feet tall from sea level to its summit. But
there's another roughly 14,000 feet underneath the ocean. So, it's actually a
volcano which is approaching 30,000 feet in height.
And so, compare this picture then to what we just saw of Mount Fuji, and this very
broad summit here. Where's the summit? The summit is right here, but it's so, so
broad. If you were standing on the summit, you would have difficulty knowing that
you're actually on the summit.
The reason the volcano is so broadly sloping is because it's mainly made up of lava,
with few or no pyroclastics. Why? Because this is a basaltic volcano. Hot spot
volcanism typically produces basaltic volcanism, and the basaltic volcanism is
expressed-- manifested-- by basaltic lava flows.
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McGillX | MCGATOCXT314-G000900_TCPT
PROF. JOHN STIX: Now, I'd like to show you five different types of volcanoes, which are very
interesting structures. So we'll look at calderas, cinder cones, shield volcanoes, stratovolcanoes and
lava domes.
This is a nice photograph of a cinder cone in the USA, in New Mexico. And it has this very classic
shape, conical shape, caused by the eruption. So these cinder cones actually erupt only once in their
lifetime. And then future activity will form another cinder cone maybe five kilometers away or two
kilometers away. So they are one off type volcanoes. And they may be active for maybe a few weeks or
a few months or a few years.
Although, we haven't really observe very many of these types of eruptions in person. There's lots of
these cinder cones around the world, but we've actually only observed a very few number of eruptions.
So these cinder cones are balsaltic in composition, but they're also explosive.
So this is an example of a basaltic volcano, which is explosive , instead of being quiet and producing
lava flows. It's a very small scale of activity. This is a very small structure here. You can climb to the top
of it in probably 20 minutes, and walk around the crater rim in another 15 minutes. So it's a very small
volcano, and its level of activity is quite low. But nevertheless, it is explosive.
So it shows you that sometimes balsatic magmas can contain a fair amount of gas, producing explosive
activity. So the balsatic magma gets ripped apart, in this case, fragmented, producing this cinder cone.
And the cinders sit at an angle of repose, forming the very steep slopes of the cinder cone. It may be a
little difficult to see in this photograph, but there are also lava flows over here and over here associated
with a cinder cone.
So the cinder cones are also very interesting, because they can show both explosive activity and lava
flow activity sometimes at the same time. So when a cinder cone is in activity, you might have the
central eruption, explosive eruption coming from the cone, but at the base lava is coming out. And that's
a very interesting phenomenon, and is telling us something about the structure, and the plumbing
system of the volcano underneath the surface.
A shield volcano tends to be a very, very large structure. This is Mauna Loa in Hawaii. The summit is
located right about here, 14,000 feet. In winter, sometimes you can go up there and ski on the summit.
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It's really quite amazing. It's something that I haven't done, but I would really like to do that sometime.
And this very broadly sloping volcano is the result of many, many lava flows being erupted from the
summit and near the summit region. And these lava flows are basaltic in composition, the gas content is
relatively low, the silica content is relatively low. It's about 50% silica. So the lava that comes out is hot,
low silica, and it's very runny, fairly low viscosity. So it's able to run fairly easily and forming these very
gentle slopes.
And sometimes, the eruption rates of this volcano are very high as well. And because the eruption rates
are very high, that also allows for the lava to flow out very quickly, and form these very gradual slopes.
So these shield volcanoes form this very characteristic gently sloping surface in their morphology.
Stratovolcanoes are typically found in subduction zones, and are comprised of both lava, andesitic lava
in general, fairly viscous andesitic lava, 60% silica, and also pyroclastic rocks, fragmental pyroclastic
rocks. And here we're looking at a photograph of Mount St. Helens volcano in the state of Washington
in the United States. And Mount St. Helens had a very cataclysmic eruption on the 18th of May, 1980.
And we're looking at the volcano soon after the eruption. And you can see the crater area right here, a
bit of gas coming out, still an active volcano. And also all that black material flowing down the volcano in
this direction here, and flowing down the volcano in this direction here, are actually mud flows that are
being produced by a lot of melting of the snow, and mixing with a lot of loose solid debris on the slopes
of the volcano.
So these stratovolcanoes tend to be andesitic in composition. So the connection between subduction
and andesitic volcanism is very, very strong-- very, very typical to find these volcanoes. And these
volcanoes can be extremely explosive and violent volcanoes.
Calderas are another manifestation and example of extremely violent volcanism. So this is a caldera,
rather than a crater. And we call this a caldera, because it's a surface depression that, in this case, has
been filled by a lake, Crater Lake. The surface depression has been formed, because the magma
chamber beneath has been partially emptied during the eruption.
So a large eruption begins, explosive eruption. The eruption column goes way, way up. It's a violent
eruption. The magma chamber gets drained or partially drained, partially emptied. And at some point,
the roof above the magma chamber can't be supported anymore, because the magma chamber is
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partially empty. And so the roof falls in, and you form a surface depression, which is called a caldera.
So this is this wonderful example, Crater Lake in the state of Oregon in the US. And this volcano, this
caldera formed about 7,000 years ago. And before the caldera formed, there was actually a big
stratovolcano called Mount Mazama that rose to very impressive heights. And when the caldera
formed, essentially the top of Mount Mazama collapsed into the caldera. So where is the top of Mount
Mazama, or where is most of Mount Mazama today? It's buried at the bottom of Crater Lake. You
would have to drill down, but eventually you would intersect parts of Mount Mazama.
Crater Lake happens to be a relatively small caldera. The diameter of the caldera is on the order of 10
kilometers across, but there are other systems which have even larger calderas. Yellowstone, for
example, a caldera system with a depression, a surface depression, extending 50 to 80 kilometers
across. Lake Toba in Indonesia, another gigantic caldera system. So these calderas show us that very,
very large eruptions, super eruptions can occur from time to time. And these are very violent eruptions,
which can have regional or even global impact on climate and other phenomena as well.
And finally, here's an example of a lava dome. Lava domes are like lava flows, except instead of flowing
out horizontally, they grow vertically. So why do they grow vertically? They grow vertically, because of
what they're made of. They're typically made of andesite, or rhyolite, very high contents of silica, dacite,
which is somewhere halfway between andesite and rhyolite. So because of the very high silica contents
of the magma, the magma is highly viscous. And when the material comes out as lava, it can't easily
flow laterally, so it builds up vertically. So you really do form a dome of lava.
So this is a dome of Chaiten volcano in Chile. A dome that was formed after a big, relatively large
eruption in 2008. So this dome is still partially de-gassing right now, and still active, very, very
impressive structure.
Sometimes when the dome grows fast enough, pieces of the dome can break off and fragment. Then
you can have some violent activity as well. So domes are very interesting and quite dangerous,
because there can be transitions between this growth of lava, this essentially vertical growth lava, and
also transitions into explosive activity. So people are quite interested in looking at domes from this
perspective.
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McGillX | MCGATOCXT314-G001000_TCPT
PROF. JOHN
After looking at the different types of volcanoes, let's look at the different types of
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volcanic activity which can occur. So there's actually quite a diverse range of activity
and materials which come out of volcanoes that l will illustrate to you in the next few
slides. It's really quite amazing and quite spectacular at times.
So you may have seen photos such as these. Lava flows flowing down a volcano.
So this is an excellent example of a basaltic lava forming on a Hawaiian volcano. It's
very runny. It's basaltic lava. It's red hot. It's about 1,200 degrees C. The lava
moves down the countryside. It doesn't move very fast. You could probably walk
faster than lava in most cases. But if it does hit buildings, of course, it will burn the
buildings and the buildings will be completely destroyed. So there are hazards
associated with lava flows in terms of where to site a building, for example.
In terms of basaltic lava. There's two types of lava. One called pahoehoe and
another called aa. So this is a wonderful photograph of pahoehoe lava. It has these
very beautiful ropy textures, smooth surfaces, very fluid material. So what is
happening here is that the surface of the lava is crusting over and then the material
underneath is still hot and plastic and moving and causing deformation of the
surface of the lava.
Sometimes we see transitions from pahoehoe lava into aa lava and aa lava is
composed of a blocky material as you can see here. There's a lot of blocky material
on the surface of the lava. So we see transitions from pahoehoe to aa, probably
caused mainly by cooling of the lava, crystallization of small minerals in the lava as it
moves down slope. So the surface morphology of aa lava can be a little misleading.
The blocky surface is not characteristic of what's inside the interior of the lava.
So if you made a cross section, if you were able to cut through a section of the lava- which I strongly suggest you don't do-- but if you were able to do that, you would
actually see that in the interior of the lava flow, it would be quite massive and not
blocky at all. So the blocky morphology of these lavas is a surface phenomenon that
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is not occurring inside the lava flow itself. Again, this is the basaltic magma, basaltic
lava, and if you were able to stick a thermometer inside the lava-- which you can do
sometimes if you have the proper equipment-- you would see that the temperature
would probably between 1,100 degrees Celsius and 1,200 degrees Celsius.
Sometimes basaltic magma, as we've seen already, can contain some gas. And if
there's enough gas, you can get these beautiful fire fountains, what we call fire
fountains. Where you have explosive eruptions of very fluid magma with lots of
droplets of the basaltic magma lofted into the atmosphere, although not at very
great heights. So here's a beautiful photograph of a fire fountain occurring on
Kilauea in Hawaii in the 1980s.
And then what is happening is that the fire fountain erupts and then the material
actually falls back and it still-- it hasn't solidified completely. It's still partially liquid.
And if it's liquid enough, the material can coalesce and actually form a lava flow that
moves away from the volcano. So this is actually a wonderful paradox. Because the
fire fountain activity is small scale violent explosive activity, but the explosive activity
as the fire fountain collapses, leads to a lava flow which is quiet, normally quiet
activity. So if you were able to only see the lava flow maybe two or three kilometers
from the source where the fire fountaining is happening, you would generally not
know that this lava was derived from a small explosive eruption. So this is a very
interesting phenomenon that occurs in fire fountaining activity.
We've been looking at examples of small scale basaltic lava flows. But there are
examples of much larger lava flow activity in Earth's history. And the part shown in
green right here is forming a basaltic lava plateau. And this type of material we call
flood basalts. These very, very large outpourings of lava which occur over a few
million years. So we're looking at the Colombia River basalt right here shown in
green. And you can see that the lava is covering much of the state of Washington. It
covers close to half or maybe half the state of Washington and significant portions
of the state of Oregon as well.
And so these volumes of magma are absolutely enormous. It doesn't take very long
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geologically to erupt this material. A few million years. And in some cases, these
events, which have occurred all throughout Earth history, can be associated with
mass extinctions. Large die offs of life forms. So that is a very interesting area of
research in volcanology going on right now.
Lava domes. Lava domes, as I've mentioned before, form when the magma from
below has more silica than basaltic magma. So this is a lava dome in the crater of
Mount St. Helens in the center. So the lava dome is located right here, OK? In
actually a horseshoe shaped crater of the volcano. And this lava dome is andesitic
to dacitic in composition, 60% to 65% silica. And it's growing dominantly vertically.
Not in this case, but other lava domes, Unzen in Japan, Soufriere Hills volcano in
Montserrat. Sometimes the lava dome is growing so fast pieces break off, and you
generate explosive activity. What we call pyroclastic flows. Flows of fragmental
material. And when that happens, these pyroclastic flows-- they move very fast, you
cannot outrun a pyroclastic flow-- and they can be very, very hot also. So this is a
hazard sometimes with lava domes that are growing very, very actively.
And sometimes, in extreme cases, you might have a transition from effusive lava,
lava that's coming out as a lava dome. At some point it reaches a threshold or a
transition where the lava activity changes to explosive activity. So if there's a lot of
material coming up from below, you might have a transition from lava dome activity
to explosive activity. And that has been seen to occur at certain volcanoes. And, of
course, this is very, very important in terms of hazards. A lava dome is a quite
restricted feature, as you can see in this photograph, but if you're producing a lot of
ash from an explosive eruption, the hazards are quite different.
So these volcanoes in general tend to be hazardous volcanoes. Out at Mount St.
Helens, a classic case of a subduction zone volcano. So to underline this point
between making the link between dangerous volcanoes, violent volcanoes, and
subduction zones, it's a very, very important point to remember.
So here's Mount St. Helens erupting during its climactic eruption in May 1980. 18th
of May, 1980. And you can see the eruption column building up and a lot of ash
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being lofted into the atmosphere. So this ash goes up very high. The eruption
column at Mount St. Helens went up to about 20 kilometers in height into the
atmosphere. And just to put that in perspective, where do commercial aircraft fly?
Commercial aircraft fly at around 10 kilometers. So this was much higher than
where aircraft fly in the atmosphere. So this eruption column put up ash to around
20 kilometers height. And that ash was transported to the east. Many, many
thousands of kilometers. Even the city of Montreal, where I live, had a very slight
dusting of ash. And downwind of the volcano tens to hundreds of kilometers
downwind from the volcano, there were significant accumulations of ash from this
eruption.
If the eruption column actually loses a bit of energy, it can collapse. And when an
eruption column collapses, it can produce another type of pyroclastic flow. So this
pyroclastic flow, which is a flow of hot fragmental material, can rush down the
mountain slopes, the slopes of the volcano, and anything in its path will be
destroyed. And these pyroclastic flows can move very fast. Tens of kilometers per
hour, up to hundreds. Even in extreme cases, hundreds of kilometers per hour.
Very, very destructive. Much faster than lava flows. You can generally out walk a
lava flow, you cannot out walk a pyroclastic flow.
Lahars. Lahars are very important features at many volcanoes. A lahar is essentially
a mixture of 50% water, 50% solid material. And they are very mobile, meaning that
they can flow very far away from the volcano. And because they have a lot of solids
material being transported in the flow, they can be very, very destructive in many
different ways. So lahars are a real problem at certain volcanoes that have water
associated with them.
So what type of water? A water in a crater lake. Water in the form of snow and ice
on glacier clad volcanoes. And sometimes you might have neither, but you might
have a volcano that is subject to monsoon seasons. So you may have intense rains
during several months of the year. And so if there's a lot of loose material on a
volcano, say from an eruption, pyroclastic material from an eruption, if there is water
that gets mixed into the loose solid material, they can form these features called
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lahars. Flows that move through river valleys and can be very destructive.
So this photograph is a very nice photograph of the deposit of a lahar that came
rushing through the Guali River Valley, I think, if I remember correctly, in Colombia
from the eruption of Nevado del Ruiz in 1985. And so this is a spectacular picture of
the bottom of the valley being eroded. So there was a lot of tree destruction, for
example, right there. This is not normal stream flow. There was a lot of solids debris
transported through this valley. And we'll look a little later at Nevado del Ruiz in a
little more detail.
So lahars can be highly hazardous features.
Another very typical phenomenon that occurs at volcanoes-- although not that
frequently, generally speaking-- is that the volcano sometimes simply fails. Parts of
the volcano, chunks of the volcano fail and slide off the volcano as landslides. They
start as landslides, but then they break up into flows. And there's a lot of solid
material that's transported in what we call debris avalanches away from the volcano.
So volcanoes in general are unstable features. They're steep. They're made up of
fragmental rocks. The rocks can be highly altered due to high temperature gases,
for example. There may be faults that cut through the volcano, further weakening,
fracturing, breaking the rock. So volcanoes are extremely and inherently unstable
features.
And so during the lifetime of a volcano, a volcano may last a million years, for
example. And during the lifetime of that volcano, it may experience failure several
times, creating these debris avalanches. And these debris avalanches can travel
large distances away from the volcano. And this photograph here on the top of a
debris avalanche deposit from Mount Shasta in northern California shows this
hummocky very characteristic of the debris avalanche deposit. And each hummock
represents chunks of the volcano's interior that have been transported many tens of
kilometers from the volcano.
And once a volcano experiences a collapse and creates a debris avalanche, there's
a scar in the crater which is essentially a horseshoe shaped scar that is open on
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one side. So it has a very characteristic morphology. And here, the lower
photograph here of Mount St. Helens, here's the little lava dome inside the crater.
But this is a horseshoe shaped crater and I'll just show you the horseshoe there.
And it's open in the direction of the viewer. And this was produced by a debris
avalanche that occurred during the 1980 eruption of Mount St. Helens volcano.
And to finish this portion of the lecture, I want to show you something called the
volcano explosivity index, or VEI for short, which is similar to the earthquake scale,
the Richter magnitude earthquake scale for earthquakes. So it's essentially a scale
showing eruptions that are small relative to eruptions that are large. So it's rated
from VEI zero all the way up to VEI eight. From small to very large volcanoes. And
it's also shown in terms of the volume of material that's erupted.
So, for example, the Mount St. Helens eruption in 1980 located right there, OK?
Erupted about one cubic kilometer of magma. And it had a VEI of somewhere
between four and five. And then the Pinatubo eruption, the Mount Pinatubo eruption
in the Philippines, which erupted five to 10 cubic kilometers of magma in 1991,
located right here. It had a VEI of around five to six. And the very, very largest
eruptions, the super eruptions, which occur very infrequently, from Yellowstone, for
example. From Toba in Indonesia. These are VEI seven to eight eruptions. And you
can see the volume of these eruptions here. Here's Yellowstone with a volume of
about 1,000 cubic kilometers. 1,000 times the size in terms of eruptive products
compared to Mount St. Helens.
So it's very useful to grade these eruptions in terms of VEI and in terms of the
amount of material that the volcano erupts from its magma chamber underneath.
6
McGillX | MCGATOCRT314-G000200_TCPT
PROF JOHN STIX: Hi. Today we're going to do an experiment. We're going to erupt a volcano. And
what we're going to do is we're going to put the quintessential Mentos in Diet Coke,
create a lot of bubbles, and hopefully create a huge spout, a huge eruption spout
which will go very high. Sometimes this works well, and other times it doesn't work
so well.
So my trusty assistants, Gregor here and Jason, who are my graduate students in
volcanology at McGill University, are going to help with this experiment, and I'm very
grateful for their help.
I'll just put it like this here and just start putting these things in. We can do it
together, actually.
GREGOR:
So all of them go in.
PROF JOHN STIX: Yeah, every single one. We're gonna make-- we're gonna try to mega fly this thing.
OK. So now, what we have done is we've loaded this little chute, or piece of paper,
rolled up piece of paper, with a bunch of Mentos, OK? And we're gonna put the
Mentos, place them on here, with a little sliding card, and then Gregor is going to
slide the card away. And the Mentos, hopefully, will slide right into the Coke. And
then things should happen, we hope.
So we're going to proceed here. Gregor and I will talk a little bit, and so forth. So
you can open the thing now.
GREGOR:
All right.
PROF JOHN STIX: And Jason's also filming this. We have two cameras rolling, and we're gonna film it
with a normal camera and also with a high-speed camera, so we can look at the
details. So hopefully we can show you the details after the experiment goes, if the
experiment works.
And I have a backup. I have a backup can of Coke and a bunch of extra Mentos
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because in my experience, experiments fail half the time, so I'm prepared. So I'm
gonna move this up here. So now you can take control of the card. Just move the
card out a little bit. OK. So what we're going to do-- so I'm going to count to three,
and on three, Gregor's going to pull away the card, the Mentos are going to slide in,
and the volcano is going to erupt. One, two, three.
That was pretty good.
GREGOR:
Yeah. Great success.
PROF JOHN STIX: OK. So I would judge that experiment an 80% success. First of all, I'm a little wet.
I'm a little covered in Diet Coke, but that's OK. But what happened? What
happened? So this was a simulation of a volcanic eruption. And what the Mentos did
was to nucleate all the carbon dioxide bubbles in the Coke very, very rapidly. And
this rapid growth of bubbles in the Coke caused the Coke to be torn apart, just like
real magma. The magma, the Coke magma, got torn apart into little particles. And
we had a pyroclastic eruption, a violent eruption, of Coke fragments, OK?
So now this experiment is not exactly what happens in nature. So in nature, what
happens is that magma comes up from depth. And the gas dissolved in magma,
mostly carbon dioxide and water, is a strong function of pressure. So when the
pressure is very high down here, there's a lot of gas dissolved in the magma. And
when the magma moves up, the gas comes out of solution. It bubbles, OK? So
that's what drives things in nature-- rapid movement of magma from deep levels to
shallow levels. The magma foams up.
Here, what happened, the magma foamed up. But it wasn't due to a pressure
decrease. We simply opened the bottle of fluid, and then we introduced the Mentos.
And the introduction of the Mentos caused this rapid bubbling, this rapid foaming of
magma. But essentially the eruption process is identical. And so this is why this
experiment is so good to demonstrate the rapid vesiculation, the rapid bubbling of
magma, causing an explosive fragmental eruption.
2
McGillX | MCGATOCXT314-G001100_TCPT
PROF. JOHN
What are the hazards from volcanoes? We'll look at this in a number of ways by
STIX:
looking at maps at different scales. So what we'll do is first look at a very large scale
map of the world and then focus downwards to countries and then individual
volcanoes.
So this is the plate tectonic map of the world showing boundaries between the
different tectonic plates and the locations of active volcanoes shown by red dots.
And in essence, this map is also a hazards map which is telling us a lot of
information about the proximity of certain areas to volcanoes in relationship to
global plate tectonics.
So, for example, if we look at the mid-ocean ridge here. This is the mid-ocean ridge
system and all along the mid-ocean ridge system, there are basaltic volcanoes,
which are active. But in a sense it doesn't really matter to us because there's
nobody living there and the volcanoes are underneath water. So these are, of
course, interesting scientifically but they're not really an issue in terms of hazards
because there are very few people in this area.
For example, if we go to the Pacific Ring of Fire now, we can look at areas of very
high hazard along subduction zones, for example. So let's look at the Aleutians up
here, the Aleutian Islands, Amchitka up in the Northern Pacific here. And then let's
look at the subduction zone down in this area right here. So both of these areas are
on plate boundaries. And both plate boundaries are subduction zones. So we know
that the volcanoes that are generated by the subduction process are going to be
large, violent, andesitic in composition, highly explosive, commonly, and so forth. So
they present a fairly high hazard.
We've talked about hazard and risk at the beginning of the class. Now let's look at
an example of contrasting hazard and risk. So both these areas circled have
relatively similar hazard. The Aleutians up here and this specific Western Pacific
subduction zone down here. Their hazard is about the same on a global scale, but
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the risk is very different. Because the number of people living in the Aleutians
compared to the number of people living in this area down here is vastly different.
Nobody-- essentially very, very few people are living in the Aleutians, so the risk is
low. But there's lots of people living to the south and so the risk is much higher.
So I think that's a good example of the difference between hazard and risk in a
volcanic context.
Now let's look at North America as a whole. So you can see that much of North
America, all of Eastern North America, and all of the central portions of Canada and
the US are not directly affected by volcanoes. Where's the action? All the action is
occurring along the west coast. The Aleutians right here, for example. The cascade
volcanic arc as a function of subduction here. So this is a subduction system here
and this is a subduction system located right down here. And if you go down to
Mexico here, this is the Mexican volcanic belt, which is a slightly more complicated
tectonic situation, but also the role of subduction is important right here. So all these
areas have large subduction related volcanoes. And all these areas have a certain
amount of hazard associated with them.
A country like Japan has many, many volcanoes, active volcanoes, because it sits
astride a series of subduction zones. So all the red triangles shown for Japan are
active volcanoes. And, of course, the area of greatest concern is probably this area
right here where we have the confluence of many active volcanoes and a large
number of people living in the region. When we go up to island of Hokkaido and
when we go down to the island of Kyushu, well there's lots of active volcanoes right
there. But there's not as many people living in those regions so the risk is somewhat
less compared to central Honshu in terms of volcanic hazard.
Indonesia is a very interesting country. It has many, many active volcanoes. And the
subduction zone extends all along the margin of this part of the Indonesian
archipelago. This is the island of Sumatra located right here, for example. And what
I want you to focus in on then, is the island of Java. And I'll circle the island of Java
right here. And this is essentially a risk map. A combination of a hazard map of
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volcanoes and population. So what we're looking at is this very, very high population
density on the island of Java with volcanoes all along the spine of the island created
by the subduction zone. So the island of Java has many, many issues related to
volcanic hazards and volcanic risks. And the Indonesians are particularly concerned
about this island in particular.
So now let's go to the scale of an individual volcano. So this is a hazards map of a
volcano in Colombia called Nevado del Ruiz. And we can see the summit of the
volcano located right here. And this area near the summit region has high hazard
from phenomena such as ash fall, from lahars, mudflows, volcanic mudflows, from
pyroclastic flows, and so forth. So there's an area near and around the volcano that
is subject to these various phenomena.
But as you get farther away from the volcano, the hazard of pyroclastic flows
decreases significantly because pyroclastic flows don't flow that far from a volcano.
The amount of ash falling from the volcano goes way down and what we are left
with are essentially the hazard from volcanic mudflows, from lahars. So these areas
over here, for example, shown by the different river drainages. So all the areas in
gray show the different valleys and river drainages and these are where lahars will
be channeled, channelized. And these are areas of very high hazard.
Lahars at Ruiz are an issue because the summit is at a very high altitude. Greater
than 5,000 meters. And there's snow and ice on top. So an eruption can melt the
snow and ice, transforming it to water. The water mixes with loose debris and
creates lahars. So it's a classic example of a volcano. A snow clad, an ice clad
volcano, which has a very high lahar hazard at very large distances from the
volcano.
So look now at the scale. Here's the scale bar right here. 15 kilometers. So you can
see that the lahar hazard extends many tens of kilometers away from the volcano.
In places where people don't even realize that they might be affected by an active
volcano. So this is very, very important information. And that's why maps such as
these can be very useful for city officials, for emergency planners, emergency
3
management people, and the general public as well. These types of maps can be
posted in schools, can be placed on websites, can be transmitted by social media.
So they're crucial pieces of information informing the public about areas that are
dangerous in terms of volcanic activity and areas that are not dangerous in terms of
volcanic activity.
And here's another example. It's a very similar map of Mount Hood in the state of
Oregon. Part of the cascade magmatic arc. Part of the cascade volcanoes created
by subduction in the Pacific Northwest. And here we see the hazards located here
close to the volcano. And what's the scale here? Here's a scale in miles. Five miles.
So roughly 10, 10 kilometer diameter in this area. If you were in this area and the
volcano was active, you might be affected by lava flows, ash fall, pyroclastic flows,
lahars and so forth. But then move away from the volcano and we see the same
thing that we saw for Nevado del Ruiz. Lahar hazard up here. Lahar hazard over
here. And lahar hazard over here. Why is the lahar hazard high at Mount Hood?
Because Mount Hood, like Ruiz, is covered by snow and ice.
4
McGillX | MCGATOCXT314-G001200_TCPT
PROF. JOHN
Now I'd like to show you a case study-- a very instructive case study --showing the
STIX:
effects of a relatively small eruption. And the point here, I think, is that small
eruptions can have, sometimes, very large impacts. So we'll look at the case of
Nevado del Ruiz, which erupted on the 13th of November, 1985.
And this is a photograph taken, I think, a few weeks after the eruption. We're looking
at the summit of the volcano. That's a bit of degassing you can see going on at the
summit. And how high is the summit here? The summit is more than 5,000 meters
tall. It's covered by snow and ice, which is very, very important in terms of the
hazard and the impact from the volcano.
And Nevado del Ruiz is part of the Indian Chain. And so it's a classic subduction
related volcano. Andesitic in composition, explosive eruptions, and so forth. OK? So
a hazardous volcano.
So what makes it particularly hazard is the fact that it does have snow and ice on
the surface. So you've seen this map already, this hazards map, of the volcano. So
the proximal hazards located in this area right here. And again, let's look at the
scale bar right here. Here's 15 kilometers. So for a diameter of maybe 25 or 30
kilometers one might be affected by such phenomena such as lavas, ash fall,
pyroclastic flows, lahars and so forth.
But as you get away from the volcano, in this area located right here, essentially the
main hazard is lahars. There can be ash fall, and there was ash fall during the 13th
of November eruption. But it was very, very minor and really had no impact relative
to the lahars.
So the take-home message here is that this map was available before the eruption
to people. And the key area to look at now is the town of Armero. This area right
here. And the area shown in gray was mapped out as high lahar hazard and mud
flow hazard. And the areas shown in red are the actual mud flows that came
through Armero and were deposited in Armero.
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So it was known beforehand that the town of Armero and vicinity was at very high
risk from mud flows. But this information was not properly communicated to the
people living in the town. So now we're going to look at the passage of lahars. First
through the mountain valleys, through these squiggles here. There. And then what
happened to the lahars as they arrived at Armero.
So this is a very nice photograph showing the lahar deposits that flowed through
one of these mountain valleys. And remember the material is flowing from near the
summit of the volcano at around 5,000 meters all the way down to the town of
Armero, which is at roughly 1,000 meters altitude. So a change of 4,000 meters in
elevation. And that's an incredible, incredible vertical drop. And this material was
being transported very rapidly.
And how fast were the lahars moving in this river valley? Along this river valley?
They were moving at tens of kilometers per hour. I can't tell you exactly, but they
were probably moving at 30, 40, 50 kilometers per hour. Very fast.
But if we backtrack just a minute, look at where Armero is relative to the volcano. So
here's the volcano right here. And here's Armero right here. And you can see-Here's our scale bar here. Our crucial scale bar here. --you can see that Armero is
easily 50 kilometers away from the volcano. And even farther if you follow the track
of the river valleys.
So it takes a few hours for the mud flows to flow from the volcano to the town. So
there was sufficient time to tell people to get out of town before the lahars arrived in
town. So these flows were coming down. They were moving fast, but they needed to
travel a very large distance before arriving in the town.
So if you were in this valley as a lahar flowed down you would obviously be in
trouble if you were at the bottom of the valley. But if you had just hiked up, maybe
from the river valley if you had been there-- See where the red dot is? --and you
had just maybe climbed through the forest to up around here you'd be absolutely
fine. You would see a spectacular sight. You would probably see nothing actually,
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because it was occurring at night. Lahars were occurring at night. But if it had been
day, it would've been a spectacular sight.
But if you had been there, you would have felt the rumbling of the lahars as they
pulsed through the valley. It would be a very, very impressive and quite frightening
thing. But you would be fine. Just a little above the lahars. OK? In the river valley.
But when the lahars arrived in town, the lahars essentially spread out.
So why did they spread out? They spread out because the mountain front ended at
Armero. So the mountain front basically ends right about here. OK? So the lahars
are coming through this river valley. Through there and then essentially spreading
out. And spreading out because there's no channel, there's no river valley, anymore
to contain the lahars. OK?
So let me just go back to get rid of lines. Because we want to look at some details
here in this amazing photograph here. So this is what was once Armero. And you
can see some of the town located right here partially inundated, or mostly
inundated, by lahar deposits.
And then of course this part of town is completely wiped out. There is nothing left.
This was the main lahar channel through which the material came rushing down
and was deposited. And then if you look over here, there's some fields over here.
And there's some wooded areas on this side of the valley. These are slightly higher
ground, which were safe zones.
So if the information had been correctly transmitted to people, to the townspeople
even though it was the dead of night and it was raining-- They would have needed
to have woken up, gotten out of their houses, left their possessions behind. But took
themselves and moved a few hundred meters away from the main area of lahar
deposition to safe areas, such as over here and over here. So it would have been
an inconvenience, but it would have saved lives.
So this did not happen. People stayed in their houses. And as a result 25,000
people lost their lives. This was one of the worst volcanic disasters ever. And it
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really made people, scientists, say to themselves. we must be able to better
communicate information to people so that something like this does not happen
again.
The hazard map was in place. People knew that Armero was a highly hazardous
area, at a very high risk from lahars. And this is a photograph of the destroyed
buildings and the lahar deposits left by the flows.
So let's maybe just look at this building right here. You can see that the top of the
building is sticking through the lahar deposit. And how thick is the lahar deposit
there? The lahar deposit is maybe one to three meters thick.
And so many people were just engulfed and inundated in the lahar deposit. And the
lahar deposit is something that sets like concrete. So if you were buried in this, you
would have lost your life very quickly. And if you were partially stuck into this-- If you
had been stuck into this, buried up to your waist in the lahar deposit, you might still
be alive. But you would not be able to extract yourself. You would need somebody
else to extract yourself.
So what were some of the lessons learned from this event? One key lesson was
that it doesn't require a large eruption to create a large disaster. This was a very
small eruption. Much smaller than the eruption of Mount St. Helens in May 1980.
And yet the eruption melted lots of snow and ice and created these large lahars,
which flowed a very great distance away from the volcano.
Secondly so these lahars may travel very, very great distances. And that's why
hazard maps are so important to show the area of hazard to people who might be
living very far from the volcano. And who might think that they're not affected at all
by the volcano. And who sometimes might not even know there's a volcano nearby.
So a key point is that this information that is available needs to be transmitted in an
effective manner and exchanged among scientists, politicians, officials, the public to
warn people and to prepare people beforehand. And so if people had been
prepared beforehand-- If people had been told in schools-- For example kids had
4
been told in schools, they would be able to come home and tell their parents here's
the hazard map we were given today in school.
And our house is in an area of high hazard. What are we going to do if a lahar
comes down? Where do we go? What do we do? What do we take with us? What
do we leave behind? And so forth. People need to think about and plan for these
types of events, even though they may not occur very frequently.
5
McGillX | MCGATOCXT314-G001300_TCPT
PROF. JOHN
We're going to look now at impacts and mitigation from volcanoes. So this is a very
STIX:
large subject, and what I'd like to do is focus on one aspect of impacts and
mitigation, namely, that of volcanic ash in the atmosphere and its impact on
aviation. It's a very important issue, because volcanic ash is very bad for jets and jet
engines.
The photograph of-- a spectacular photograph of-- the eruption of Mt. Pinatubo in
June 1991. So, this was one of the largest eruptions of the 20th century. And this
eruption column went to very, very high altitudes-- went up to about, maybe 30, 35
kilometers in altitude. And to put that in perspective, modern jet aircraft fly at an
altitude of around 10 kilometers or so.
This was a large eruption, but even a moderate-sized eruption will have potential
impacts to aviation. And so, there needs to again be very good communication
amongst scientists, pilots, people involved in the aviation industry, so that aircraft
can avoid volcanic ash clouds. Very, very important thing.
This is the eruption plume of Mt. Pinatubo as it was blown to the southwest during
the day of June 15. Each black line shows the leading edge of the eruption column
as it was blown and dispersed to the southwest.
So, for example, after a few hours, the eruption column here, at the beginning of-the front of the eruption column was here, three hours later it was over here, six
hours later it was here, and so forth and so on, 'til it got to Singapore, for example.
And over this area, when this event was occurring, there were aircraft flying around
in the area.
And the next slide shows a red circle, and all the black dots in this red circle show
encounters between the ash from the Pinatubo eruption and jet aircraft. So, there
were 16-- at least 16-- aircraft-ash encounters. There might have been some that
weren't recognized. Ten engines needed to be replaced, and there were several
airports that were also closed. No aircraft fell from the sky. Nobody died. There
1
were no fatalities. But there were significant losses from this very large eruption.
And so the question is, how can we reduce and minimize the impact of such
eruptions to aviation? And this is a real challenge, because, modern aviation is
growing in its scale. It's becoming more and more important to us as a global
society. So there will inevitably be encounters in the future between eruptions-volcanic ash-- and aircraft in the sky.
So, this is actually an aircraft on the ground, showing a DC-10 near Pinatubo that is
covered in ash. And actually, during the Pinatubo eruption, there was a typhoon
occurring at the same time. So, there was a lot of water falling down from the sky.
So, here we have this eruption going on-- this very large eruption-- and, at the same
time, a huge amount of rainfall is occurring. So, the ash that is falling in many places
is very, very wet ash, and very wet ash is very heavy ash. And so, in this case here,
it actually tipped the plane over, caused by the weight of this very waterlogged ash.
And this is a fantastic image of the global air roots that exist today across the globe.
And the areas that concern us, in terms of volcanism, are mainly the Pacific-- the
Pacific Ring of Fire. And so, here we have two circles showing the Eastern Pacific
and the Western Pacific, and this is the area where there is lots and lots of
subduction-related volcanoes. And you can see that in these circled areas there's a
huge number of volcanic-- of aviation routes. And so, these are areas where there
might be an increased probability of encounters of aircraft with volcanic ash.
So, in terms of aviation safety, there are four points consider. Firstly, many of the
world's civil air routes cross active volcanoes. So, this is an issue, and will continue
to be an issue, and will continue to be even more of an issue as civil aviation grows.
Secondly, ash erupted from a volcano easily gets to the heights of where aircraft fly- and above. It doesn't take a large eruption to get ash up to around 10 kilometers,
where jets fly. And so, the potential to get ash into this level in the atmosphere is
fairly high from a small eruption, or a moderate eruption, or a large eruption, which
will put ash up much higher, of course-- 20 or 30 kilometers, even.
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So, aircraft need to avoid ash at all costs. Ingestion of ash into engines is very, very
dangerous-- and potentially fatal-- for the aircraft. The engines may fail. And so, if
the engines fail, then the aircraft could crash.
There have been a couple of near misses. There have been two cases, at least, of
all four engines failing, but they were able-- the pilots were able-- to restart them.
So, there were some very close calls. And the loss of a single 747, or a big jumbo
Airbus aircraft, would have huge, huge losses in terms of lives lost and insurance
claims. Billions of dollars, probably.
From Chaiten volcano in May 2008. So, this eruption was occurring on the west
coast of Chile. So, the volcano is located right here-- and then the eruption column,
as you can see, is being dispersed by relatively strong winds to the north-- to the
southeast. And so, any area of this eruption column is hazardous for aircraft. This
needs to be communicated to pilots, and pilots need to avoid this area and fly in
alternative airspace.
And so, scientists need to monitor volcanoes, and understand volcanoes, and need
to try to forecast and predict volcanic activity. So, they do different types of
measurements on volcanoes. So, this scientist is taking a gas sample from a
fumarole on an active volcano to see if there are changes in the composition of gas
over time to assess the activity level of the volcano.
Here's a seismic station on Mt. Spurr volcano in the Aleutian Range in Alaska. And
this seismic station monitors earthquakes that are generated by the volcano. And
this is another way to monitor the activity state of a volcano, to assess its potential
for different types of activity, including small, medium, and large eruptions.
We need to study the geology of a volcano to unravel its past history. If we can
study the rocks from a volcano and understand that this volcano in the past 1,000
years has erupted, on average, 10 times, then we can say with some confidence
that this level of activity will be repeated in the future 1,000 years. On average, the
volcano will probably erupt 10 times during that period.
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So, this is very important information. And not only can we get some information on
the relative frequency of eruptions from different volcanoes-- we can also assess
the nature of the eruptions. Are they mainly large, explosive eruptions, or are they
mainly dominated by growth of lava domes with small pyroclastic flows? This is also
very crucial to assess the nature of a volcano. And when the volcano reactivates,
we will have some idea as to what to expect from the volcano.
Many volcanoes-- especially volcanoes in subduction zones-- slumber for long
periods of time-- hundreds of years, commonly. And then they reawaken,
sometimes very suddenly, and we need to be prepared to say, this volcano, we
think it's reawakening, it seems to be reawakening very fast. There's heightened
seismicity, more earthquakes, the gas emissions are increasing. What can we
expect from this volcano? If it's been studied, and we have some idea of its
geological past, we have some measure of what it might do in the next days,
months, and years.
But if a volcano has not been studied at all-- and there's many volcanoes around
the world which have not been studied at all-- then we are at a disadvantage.
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McGillX | MCGATOCXT314-G001400_TCPT
PROF. JOHN
I'd like to summarize, then, some of the important points I think that are worthwhile
STIX:
remembering for volcanoes and volcanic hazards. Firstly, it's important to assess a
volcano in terms of its explosivity and its potential for violent activity. Some
volcanoes and notably those associated with subduction zones-- the photograph on
the left-- can be very violent and very explosive.
While other volcanoes produce mainly lava flows. And so the hazards from each are
quite different. So it's very important to assess and understand why some volcanoes
are explosive, some volcanoes are non-explosive, and some volcanoes show both
forms of activity.
Some of the main controls on explosivity are firstly the composition of the magma. Is
the magma poor in silica, or is the magma very rich in silica? And the composition is
related to how sticky or viscous the magma is, and that's directly related to how
explosive the magma is and also the amount of gas in the magma. Basaltic
magmas are relatively gas poor, and andesitic magmas and rhyolitic magmas are
relatively gas rich. So these are important things to assess, and we can measure
these. We can measure these parameters and assess the potential for explosivity
for one type of magma versus another type of magma.
Plate tectonics is very important in controlling where volcanic activity occurs. And as
you can see from the map, the great majority of volcanoes occur on plate
boundaries, and relatively few number of volcanoes occur in the middle of tectonic
plates. Of course, those are interesting volcanoes, because they cannot be easily
explained by plate tectonic theory. So they have to be explained by other
mechanisms. But in terms of hazards and risks, the volcanoes that are occurring
inside a plate are far outnumbered by the number of volcanoes that are found along
subduction zones, along divergent boundaries, and so forth.
Different types of volcanoes pose different types of hazards. So if we look at a
caldera here, a caldera volcano can have local, regional, and even global impacts if
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the eruption is large enough. Whereas a cinder cone poses mainly local hazards.
The volcano will erupt only once, and the size of the eruption will be quite small. So
it's important to assess what type of volcano are we dealing with.
And finally, it's very important to create and use hazard maps. These are key pieces
of information which can be used by many different stakeholders. And so scientists
create these maps, make these maps based on their knowledge of a volcano. And
they can delineate zones on and near the volcano which are hazardous from certain
phenomena and other zones which are not hazardous from certain phenomena.
And these maps can be used in many different ways, not least in terms of sighting
where communities might grow, or sighting where factories might be established,
and so forth. So these are very important pieces of information.
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