Monday, February 1, 2016

How fast is volcano-fast?

- Janine and Alison

This morning I (Janine) was researching the May 18, 1980 Mount Saint Helens eruption to begin the next phase of my research. Reading through the descriptions of the start of the eruption - when massive blocks of the volcano slid to the north, I enthusiastically jumped out of my chair surprising my office mates. Marking out 1 meter with my feet I looked up at them and told them "this is one meter, now imagine 50 of these. Now imagine a massive chunk of rock moving 50 of these in one second! 50 m per second! This is nuts!" The joys of sharing an office with an over-enthusiastic volcanologist...

50 m/s is how fast the side of the mountain began to travel down and away from the volcano, taking chunks of rock the size of 30 story buildings northward. As the slide evolved into a debris avalanche, the sediment mass began flowing, the blocks reached speeds of up to 80 m/s [1]. So this got us talking about the insane speeds involved in volcanic processes and how we can relate them to something a little easier to grasp than a 2.3 km3 block of volcano racing away from it's former host.


GIF created using the famous Rosenquist photos that recorded the beginning of the Mount St. Helens May 18, 1980 eruption. You can see the first block sliding towards the right of the images, followed by the explosion of the cryptodome caused by the rapid reduction in pressure after losing all of that overlying rock. GIF created by Reddit user PGwojowski.

Janine then messaged me (Alison) to share this revelation, and we immediately began playing the game of what else is fast. Can humans go that fast? In a previous post I found ways to express deposit volumes in more relatable dimensions, now it is time to try with speed.

Out-driving the Yellowstone eruption in the movie '2012'. No. On so many levels, no. (Image courtesy of worldmoviedb.com).

Let us start with 50 m/s. Most humans are familiar with how fast cars go, so let us compare a car moving at 50 m/s with a chunk of rock (there were chunks of volcano the size of buildings, but we'll work with a car for now). 50 m/s is the same as 111 miles per hour or 180 kilometers per hour. Safe drivers don't go this fast. Even on the famed autobahn most drivers don't go that fast. NASCAR drivers go between 40 m/s and 89 m/s. But, before you think that anyone can out-drive these flows, remember, NASCAR tracks are flat. Volcanic landslides other volcanic flows do not. In the case of Mount Saint Helens the debris avalanche traveled through a forest UPHILL!

Hummocks of debris that traveled from the flanks of Mt St Helens uphill over the ridge in the center of the photo to the location on the arrow. When I finally got to see this in 2015 it really made an impression on me. It is one thing to know what a volcano can do, but when you get a good idea of the actual scale it is very different. Photograph by Alison.

To help calibrate your brain for meters per second the fastest animal on land, the cheetah, goes up to 30 m/s in short bursts, which is 75 mph / 120 kph. For comparison, the fastest human is Usain Bolt, who goes 12 m/s which is 27 mph / 42 kph. He would get a speeding ticket in many school zones, but he still isn't close to fast enough to out run a cheetah, let alone a debris avalanche. So even on your best day, even if you are the fastest human, you are only going 20% of the speed of the flows we are talking about.

Underwater the fastest animal is the marlin, though some websites gave the honor to the sailfish. They are on par with cheetahs reaching speeds of 35-36 m/s. Although there is some debate on this matter as measuring the speeds of water dwelling animals and being sure you measured the fastest one is quite a challenge. What we do know from this is that animals finally best the speed of the Mount St. Helen's landslide only when you include animals in the air. Diving birds of prey like the Peregrine Falcon reach 200 mph, or 89 m/s when in a dive. So in a way they are cheating, you know the whole falling thing. But man is that fast!

Damage in Cancun from Hurricane Wilma (Courtesy of telegraph.co.uk)

The bullet trains of Japan, the Shinkansen, have operating speeds of up to 88 m/s. Similarly 80 m/s was the fastest sustained wind speed of a Category 5 Hurricane (note, the scale doesn't go any higher). Hurricane Wilma in 2005 was estimated to have produced winds of 185 mph or 297 kph at its peak.

The eruption plume of May 18 lasted for over 9 hours, with an average speed of 60 miles per hour. The eruption began at 8:32 am and by noon the eruption plume had reached Idao (courtesy of USGS).

What else did Mount St Helens produce that we can discuss in terms of speed? As the eruption began in earnest on My 18, large pyroclastic flows, mixtures of hot rock, ash and gas were expelled from the newly formed crater and traveled 90 m/s [2]. That faster than the debris avalanche and even faster than the peregrine falcon in full dive! But wait, Mount St Helens was even faster than that. The plume of ash and debris that rocketed straight up out of the crater after the pressure release caused by the landslide was estimate to travel at 111 m/s [2]. You can imagine Mount St Helens as a soda bottle that is under pressure because someone shook it up, then when the cap is removed the pressure release means soda escapes rapidly and makes a mess. Only the volcano makes a much larger, faster, and rockier mess.

1980 Mt St Helens eruption throwing rock upwards into the atmosphere at speeds up to 111 m/s that is the top speed for a helicopter.  Image from USGS archive.

At this point we need new comparisons to keep up with this eruption. Helicopters can travel up to 250 mph or 400 kph, which is 111 m/s! But helicopters only travel this fast horizontally, and after they accelerate. The vertical velocity of the Mount St Helens plume started at 111 m/s.

The May 18 eruption was powerful enough to devastate an area of 600 km2 (230 mi2). This image shows the forest trees lying on the ground with some still standing at the top, courtesy of USGS.
Trees still floating on Spirit Lake today. Photograph by Janine.
Volcanic explosions come in all sorts of shapes and sizes and sometimes bigger doesn't always mean the fastest. Hydrothermal explosions, caused by pressure changes in existing hydrothermal systems on volcanoes, can vary from 20 m/s (or twice as fast as fast humans) to 200 m/s [3-4]! For comparison the Japanese have an even faster train, a maglev train that travels over 600 kph or 374 mph, which is 167 m/s, faster than the explosive start to the Mount St Helens eruption, but the ride is a lot smoother. Your other options are planes and rockets, but if I ever get on a rocket it may be one of the few ways to get me to stop thinking about volcanoes.

Landsat false color image of Mt St Helens deposits. Material first moved north at high speeds knocking down trees and traveling over ridges. Then the flows became channelized and traveled along river valleys as lahars, still reaching speed of 45 m/s which is over 100 mph!

Other volcanic explosions, like Strombolian bursts and fire fountains have been estimated to carry molten rocks at speeds between a few meters per second up to 100 m/s [5]. For comparison, my experimental explosions used to model volcanic explosions throw rocks at speeds of 25-60 m/s.

Strombolian eruption in December 1969 from USGS archives. These flying bits of hot rock can go as fast as 100 m/s. Photograph by B. Chouet. 

Experimental explosions to model volcanic eruptions throws rocks up to 60 m/s at the University at Buffalo Center for Geohazards Studies Field Station.  
So what about all the other things volcanoes do to move rock from one place to another?
Before all this violence Mount St Helens had been covered with snow and ice and Spirit lake was nestled on its flanks. All this water mixed with the rapidly moving debris and formed debris flows. Volcanic debris flows, also known as lahars, have been measured to travel at speeds ranging from 10-30 m/s, with the 1980 Mount St Helens lahars reaching 45 m/s. If you want your body to go that fast and not be in an airplane you can travel to Brazil to the Insano water slide where brave bathers can travel up to 29 m/s. That is 105 kph, or 60 mph! All I can think of is after your heart slows down you will have a heck of a wedgie. To go faster you would have to jump out of a plane. Most skydivers go about 50 m/s which is how fast the landslide blocks went at Mt St Helens. Janine and I have both done this, and it is pretty impressive.


Janine in a tandem freefall, experiencing (awesome) acceleration that feels like your stomach is still on the plane!

The fastest a human body (not in a vehicle) has ever traveled was 843 mph, which is 377 m/s, which is how fast Felix Baumgartner fell in 2012. This took falling from 38,000 m (Mt Everest is only 8,000 m tall for reference) in a pressurized suit and lots of training.  Alan Eustace jumped from the Stratosphere, and has the record for highest jump, but his parachute system was different and thus didn't get to fall as fast (only 367 m/s). Either way, that is impressive and gets to be the fastest thing listed in this blog post. But it gives you a good idea of just how impressive things are moving during a volcanic eruption.

Spot the scientist below the May 1980 lahar mudline (Courtesy of USGS).
Volcanoes are a beautiful balance of catastrophically fast processes mixed with slow processes. If you look at a pyroclastic rock you see evidence of the crystals that were produced from patient deep earth processes of melting and crystallization right next to bubbles and glassy margins made by the explosive rise to the surface.

A piece of the Fish Canyon Tuff filled with biotite crystals.  A mix of fast and slow processes that make volcanoes what they are.
So, contrary to what Hollywood would have you believe, we strongly recommend against having driving away from a pyroclastic flow as a back up plan.

The eruption scene in the movie Dante's Peak where they drive away from the pyroclastic flow and safely smash into an old mine to survive the eruption. I wouldn't count on this as a safety plan (Image from www.jimusnr.com).


References
[1] Voight, B., Glicken, H., Janda, R.J., Douglass, P.M.. Catastrophic Rockslide Avalanche of May 18. In: Lipman, P.W., and Mullineaux, D.R., 1981. The 1980 eruptions of Mount St. Helens, Washington. Professional Paper 1250.

[2] Sparks, R.S.J.; Moore, J.G., Rice, C.J. 1986. The Initial giant umbrella cloud of the May 18th, 1980, Explosive eruption of Mount St. Helens. Journal of Volcanology and Geothermal Research, 28: 257-274.

[3] Montanaro, C., Scheu, B., Gudmundsson, M. T., Vogfjord, K., Reynolds, H. I., Dürig, T., Strehlow, K., Rott, S., Reuschle, T., and Dingwell, D. B., 2016, Multidisciplinary constraints of hydrothermal explosions based on the 2013 Gengissig lake events, Kverkfjöll volcano, Iceland: Earth and Planetary Science Letters, v. 434, p. 308-319.
[4] Breard, E. C. P., Lube, G., Cronin, S. J., Fitzgerald, R., Kennedy, B., Scheu, B., Montanaaro, C., White, J. D. L., Tost, M., Procter, J. N., and Moebis, A., 2014, Using the spatial distribution and lithology of ballistic blocks to interpret the eruption sequence and dynamics: August 6, 2012 Upper Te Maari eruption, New Zealand: Journal of Volcanology and Geothermal Research, v. 286, p. 373-386.

[5] Houghton, B. F., Taddeucci, J., Andronico, D., Gonnerman, H. M., Pistolesi, M., Patrick, M. R., Orr, T. R., Swanson, D., Edwards, M., Gaudin, D., Carey, R. J., and Scarlato, P., 2016, Stronger or longer: Discriminating between Hawaiian and Strombolian eruption styles: Geology.


3 comments:

  1. it's Great Janine...
    also my appreciation for Alison.

    ReplyDelete
  2. Cool write up. You mixed up miles and kms in the cheetah comparison, though.

    ReplyDelete