Friday, October 9, 2015

The eruption is how big? Deposit volume story

-Alison

It seems obvious to say volcanoes are big, but as with anything in geology size is not immediately obvious. Volcanoes can loom over a landscape, spread ash over large chunks of the planet and even influence our climate-that all sounds big. On the scale of an individual human any eruption is big. They are frequently faster than, slower than (yes both), hotter than and physically larger than a human being. One of the exercises I use when teaching volcanology is focused on understanding just how big, "big" is. We go about this by finding things that we already understand the size and compare them to volcanoes. Like many natural processes (and humans), volcanoes and volcanic eruptions come in all shapes and sizes. If volcanoes only had one size and style of eruption our job studying them and anticipating future eruptions would be much easier.



The only human-sized volcano I have ever seen. It was a volcano costume in Kagoshima Japan in 2013.
We need to be able to put a size to volcanoes because it is one way to wrap our heads around the hazards they pose. One of the first steps to coming up with a hazard plan at any volcano is to study what it did in the past, reconstructing its eruption history. This helps geologist answer questions like how many eruptions has this volcano produced? What type of eruptions? How big were the eruptions? Were they all the same or did the eruptions change with time? We can now measure the changes of a volcano in real time, but the life span of a volcano is far longer than we have been able to measure, and we need context for these modern changes.  Frequently, we want a worst-case scenario, so we want to figure out what was the largest eruption this volcano was, or what a volcano like it, has produced.

Prehistoric deposits from an eruption from Cotopaxi, Ecuador 2013.

So what parameters can we use to define "big" for an eruption? We could measure the height of the eruption cloud, how far it threw rocks and ash into the atmosphere.  We could measure how much material was thrown out during the eruption. We could measure how much energy was released by the eruption. But not all of these measurements are possible for eruptions that happened in the past.  

For past eruptions we might be lucky and have historical observations of the eruption plume, but estimating the height of an eruption plume is a challenging task by eye and is generally imprecise from past records. Energy is even trickier and cannot really be measured from visual or auditory observations without instruments, so we are at a loss for historical or older eruptions. It is much easier to start with eruption volume. For explosive eruptions this means figuring out how far away ash traveled form the volcano and then measuring the thickness of that deposit from that furthest point back to the volcano. The more measurements we make the better the final volume estimate, but this is a tedious and frequently challenging job. To get deposit thickness you need to find both the bottom and top of the deposit. This means digging lots of pits, hunting for road cuts, and making friends with land owners to see if you can dig holes in their property or if they are willing to share any pits they may have recently dug themselves. For modern eruptions citizen scientists and observatory volcanologists set up lots of stations to just collect the ash as it falls.
Geologists digging to find the bottom of an ash layer from a pre-historic eruption. Colorado, USA 2015.

People closer to eruptions generally don’t want to collect ash, they want to remove it. In the northern hemisphere people are more likely to be familiar with how heavy winter snow is, and what a pain it is to shovel. Volcanic ash is pieces of pulverized rock, which is much heavier than snow, and also doesn’t melt in the spring. This makes volcanic ash is a serious hazard for roof tops, crops, and anything that breathes. These are just a few reasons why understanding the potential distribution of ash from a volcanic eruption is important. Even volcanoes that do not produce enough ash to threaten rooftops present constant challenges to those who live near it. I remember visiting Sakurajima in Japan and being told that they could never hang laundry out to dry because it would come back in gritty, ash it gets into everything, and clogs air filters really quickly.
Sakurajima in Kagoshima Bay, Japan produces multiple small ash plumes every day. These images are from a visit in 2013.

Once we have the volume of a deposit calculated we have a number, but what does it mean.  Usually we talk about eruption volumes in cubic kilometers (km3). Mt St Helens 1980 eruption produced 1 km3 of ash during its climatic phase on May 18. So what does that mean? Is that big?
May 18, 1980 Mt St Helens, USGS image.
Mt Pinatubo in the Philippines erupted in 1991 and was larger than Mt St Helens, producing 10 km3 of material. Is that as big as eruptions get?  
Pinatubo erupted in 1991 producing this impressive ash plume, USGS image.
Yellowstone caldera has in the past produced some of the largest eruption deposits ever measured, which is why they can be called super eruptions (See these links: adding super to volcanoes and eruptions and why not to lose sleepover them). These eruption deposits were up to 1,000 km3. THAT is big. But it is still hard really wrap your head around.
Most people do not think in cubic kilometers, and most Americans do not think in any variety of kilometers. The metric system was designed to be able to talk about things at all sorts of different sizes and compare them. It is mostly a matter of keeping track of the number of zeros to tell big from small. So metric is ideal for science as it helps us communicate size more efficiently. I think of the imperial system (inches, miles, pounds) as a more ad hoc system where we only want to compare similar sized things. Miles were for big distances, and feet was fine for around home. Eventually someone wanted to know how many feet were in a mile so they counted it out,  but it was clearly an afterthought.  

Anyway, the best way to get used to thinking about what volume means is to think about objects that we are more familiar with, figure out what their volume is, and then and compare them to eruption deposits. XKCD had an excellent comic that found familiar shapes to compare to metric, including humans. In that spirit, I find it easy to think about 2 meters as a fairly tall person 6 ft 6 inches. You can start to measure things terms of your friend Ted who is over 6 ft tall (Ted is hypothetical, I just made him up, but you can find your own Ted. Most field geologists have a Ted).  

Now that you are ready to measure some things in Teds let us calculate the volume of an office building, to compare volumes instead of just lengths. We’ll start with the building I work in. It has 12 stories and has two rectangular towers. The floors are pretty tall so we’ll say they are 10 ft each, which is about 3 meters or 1.5 Teds. That means the building is roughly 36 meters tall, or 18 Teds. The sides of the rectangle are maybe 15 meters each (that is 7.5 Teds lying down). Each tower of my building then has a volume of 8100 m3, so the whole building is 16200 m3. That is pretty big, right? Well that is only 0.0000162 km3. Well, what about a bigger building? The Empire State Building is still only 0.001047 km3 .

Looking up at the Empire State Building, Wikimedia commons.

Ok, so tall buildings still aren’t big enough to match Mt St Helens, let alone Yellowstone. The largest building on earth is an airplane factory that reaches 0.013 km3. Sticking with manmade constructions, the Hoover Dam blocks the Colorado River to form Lake Mead. The estimated capacity, if we could fill it all the way (the lake is currently at one of its lowest levels), is about 32 km3, but in 2010 it was only at half that volume 16 km3.  So, we jumped up to Pinatubo volumes with a lake. I’ll be honest, I had trouble finding something familiar at 1 km3, so we are going to skip it for now. We have more than enough orders of magnitude to jump. Imagine, Lake Mead filled with ash and pumice and you get an idea of how much material was thrown up (and then had to fall back down) in 1991 by Pinatubo.

Lake Mead, Wikimedia commons. Imagine filling this basin with pulverized rock. Now you see why no one wants to shovel all that.

Let’s go bigger. Lake Erie, the smallest of the great lakes, is large enough that you cannot see the opposite shore and is approximately 500 km3. Now we are talking! So, Lake Erie has a volume on the same order of magnitude as the Tambora 1815 eruption. This eruption produced the "year without a summer". The Eruptions Blog did a great job of discussing the size of this eruption.

So what about those large Yellowstone eruptions? The Great Slave Lake and Lake Ontario are both just over 1,000 km3. Those are some pretty serious lakes. Was that the largest eruption on Earth? Nope! There is an eruption deposit, the Fish Canyon Tuff, that is estimated to be 5,000 km3 which would fill the Grand Canyon (4,000 km3), Lake Michigan (5,000 km3) or even Phobos (one of the moons of Mars: 5700 km3). This deposit covers a good portion of the western United States, and likely came from the Jemez Mountains in Southern Colorado.

Close up view of the Fish Canyon Tuff with a nice biotite crystal from 2014.
Everyone wants to talk about big eruptions, but what about small eruptions? They produce 0.01-10 m3. Stromboli produces small eruptions like this several times a day. These can be watched safely from a distance. The University at Buffalo explosion experiments produce 0.15-0.99 m3 of ejected material, which makes then comparable to natural eruptions! I was pretty excited about that. A big refrigerator has the volume of about 1 m3 for comparison (less than a Ted tall, about half a Ted wide, and maybe a third of a Ted deep). So more flying rock than a human wants to be near, but much smaller than one of the great lakes.

These volumes are of ash and pumice, so they are much bigger than the volume of magma involved. The bubbles that make pumice light take up a LOT of space.  So what about lava flows? How do we compare them? We can still use volume, but if we want to compare lava to ash we'll have to estimate how much of that air space is in the deposit to calculate the Dense Rock Equivalent. It is also important to remember that magma densities are variable as well, depending on composition. The magma that produces pumice, called rhyolite, is 25% less dense than basalt melt that typically produces runny lava flows. I'll save that math for another post.

In 2014 a fissure in Iceland opened up and produced almost 1.5 km3of runny lava.  So that is comparable to the material erupted from Mt St Helens. To get a good idea of the size of this lava flow you can look at satellite images. But is it a big eruption for lava flows? Not even close. In Iceland alone, there were eruptions that produced 14.7-18 km3 of lava. Flood basalts tend to have lava flows on the order of 500 km3, and are composed of hundreds of flows. Because of the differences between an explosive eruption and a lava flow producing eruption, you can begin to see the challenge of describing the size of an eruption.

The Holuhraun fissure eruption in Iceland just before it ended in Feb 2015.

The volume of a deposit also represent how much material came out in total during an eruption, not all at once. We would need to calculate the discharge rate of the eruption, or the mass per time, which is a different challenge.  But these volumes hopefully give you a better idea of how diverse volcanic eruptions are, and part of the reason the jobs of volcanologists are both challenging and interesting.

BUT I do not want these very large eruptions to make you worry that an eruption on that size is a common occurrence or something to lose sleep over! The bigger an eruption is the less frequently they occur. Smaller eruptions occur all the time. In fact, on any give day there are twenty volcanoes actively erupting, but those eruptions involve small lava flows, little explosions (like the ones mentioned above at Stromboli), steam explosions and other burps and bumps that do not even leave a deposit. The Volcano Explosivity Index (VEI) is partially defined on eruption volume and ranges from 1 to 8.  Recently this scale was expanded to include more small eruptions because they are what we observe most of the time from volcanoes (academic paper). Many processes on earth seem impossibly large, that is what makes them fascinating, and in many cases beautiful. Just because we measure something doesn't mean it is less awe inspiring, in fact, for me quantifying geologic processes helps me understand just how impressive our planet is.


 


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