03 February 2012

Volcanoes, Earthquakes, and Plate Tectonics

Yes, they ARE connected.

With two notable exceptions, volcanoes are associated with (a) tectonic plates splitting apart (Iceland and east central Africa come to mind) or (b) tectonic plates that are coming together (the Pacific Ring of Fire comes to mind). In the former case, magma is simply rising into an opening gap between crustal plates that are being pulled apart - like the mid-Atlantic Ridge. In the latter case, an over-ridden oceanic plate, loaded with water and chemical sediments, heats up as it goes deeper into an increasingly-hotter-with-depth mantle. Something called partial melting takes place: the lighter materials like silica and water and CO2 segregate from the down-going slab and float up - Mount St Helens in the Pacific Cascades, Sheveluch in Russian Kamchatka, and Mount Fuji in Japan are examples of these.

The notable exceptions are the volcanoes of the Hawai'ian Islands in the middle of the Pacific oceanic plate, and Reunion Island in the Indian Ocean. The generally accepted understanding for their existence is that a "hot spot" in the Mantle feeds up through a moving crust (the Pacific plate) and creates a string of volcanoes. In the Hawai'ian chain, the oldest are in the northwest, and the youngest are in the southeast on the Big Island. There's even a new one, called Loihi, that is forming on the ocean floor even farther southeast of the Big Island.

When we talk about moving tectonic plates, it's hard to come up with a reference point that everything is moving with respect to... Certainly the North American continent is moving westward over the Pacific and subsidiary plates, but Kamchatka is moving southeast over the same plate(s). If in fact there IS a "hot spot" in the middle of the Pacific plate - perhaps that is the one non-moving reference point on this entire planet.


With the increased recent activity around the "ring of fire", New Zealand, Japan and Gulf of California, is there an increased risk for earthquake in other areas of the ring of fire?
Thank you
David H

Geologic events never happen according to a regular clock - sometimes things are quiet around the Ring of Fire, sometimes several events happen in relatively close succession. There is no recognized relation between the huge Tohoku earthquake in Japan and the much earlier Christchurch, New Zealand event - they are too far apart in both space and time. THAT said, there have been several cases observed where a large earthquake has "lit up" distant volcanic or earthquake-prone areas.  The large Denali fault earthquake of November 2002 apparently triggered swarms of small earthquake in Yellowstone, for instance. Nothing big happened, but there were a cluster of small earthquakes that correlate closely with the p-wave of the Denali event passing through.

The likelihood of other earthquakes around the Ring of Fire correlates much more closely with the rate of subduction - how fast the continental plate is over-riding and "smothering" the oceanic plate. This rate is much higher off the coast of Kamchatka, in eastern Russia for instance (about 8 cm/year), than the collision rate of the Pacific Northwest (moving only about 2.5 cm/year). For this reason the volcanoes in Kamchatka are historically much more active than those in the Cascades. In the 10 years that I've been receiving daily volcanic notices about Kamchatka, I'm at a loss to think of a time when a volcano in Kamchatka was not erupting. Whereas in the last century, here in the Pacific Northwest, we've only had Mount Lassen erupt (1915-17), then Mount St Helens in (1980-86).

Any plate motion will translate into earthquakes - the plates are scraping past each other - and the subduction (over-riding plate) earthquakes can be real doozies.

Slower tectonics translates to a quieter life: fewer earthquakes, fewer volcanoes.


01 February 2012

Nuke it!

While I was serving as the chief scientist for volcano hazards of the US Geological Survey, Mount St Helens chose that time to erupt (October 1, 2004). At the time I was also still volunteering to answer questions for Ask-a-Geologist. Perhaps because of my calling at the time, I received not one but two AAG queries that went something like this (I couldn't find them in the archives or I would quote directly):

Why can't you drop an atom bomb on <Mount St Helens> to stop it from erupting?

A variant on this suggestion is to use a nuclear device to trigger a pending eruption at a time of your choosing.

There are several problems with this approach:
A. Highly radioactive debris scattered widely over a populated area.
B. You would need to get the device under the ground to open the ground.
C. The inherent energy of most volcanoes is far larger than any nuclear devices built by man.

"A" is, I hope, obvious. Nearly as many people died of radiation poisoning after the Hiroshima uranium bomb was dropped than died of the immediate blast itself. Half-lives for things like the unstable isotopes of strontium and cesium are looooong - thousands of years - and they are poisonous the whole time they are decaying. Plutonium is, gram for gram, far more deadly than botulinum toxin.

"B" is basic physics. A small stick of dynamite will blow OPEN a standing safe by over-pressuring it, but a cluster of dynamite sticks taped to the outside and detonated may or may not crush a safe door down onto the inner contents of the safe. Despite what you may have seen on Butch Cassidy and the Sundance Kid, safes don't blow up nicely.

Translation: you will need a very big, very expensive drill to place the nuclear device at a strategic place. Assuming it was powerful enough, that is.

When you come down to the many trade-offs, it's far easier to just (1) monitor the volcano, and (2) evacuate people when it's restive behavior starts accelerating and the seismometers start going ape on you.

"C" is just a numbers game. The Hiroshima uranium bomb and the Nagasaki plutonium bomb had estimated explosive yields between 12,000 and 20,000 tons of TNT. For you metric nerds out there, a metric ton of TNT equivalent is a bit over 4 gigajoules.  Mount St Helens' 1980 eruption was a VEI = 5 level blast. That's short for Volcano Explosivity Index, and a VEI 5 is about 10 times bigger than a VEI = 4; the values are approximate, and approximately logarithmic. The 1980 eruption of Mount St Helens released the equivalent of 20 million tons of TNT. That's between 1,000 and 30,000 times more energy released than the Hiroshima atom bomb.

The eruption of Yellowstone supervolcano about 640,000 years ago has been estimated as a VEI = 8 event, or 1000 times larger than the 1980 Mount St Helens eruption. That's between 1,000,000 and 30,000,000 times the power of a Hiroshima bomb.

Translation: a nuclear device is to a VEI 5 volcanic eruption, as a fly doing push-ups is to you doing push-ups. I may be exaggerating a bit with the fly, but you get the point. Volcanoes are BIG. That's why no one has ever seriously considered engineering around a volcanic eruption. Just get out of the way if you can.

If you want to open a can of spinach, ya gots ta squeeze it, to quote Popeye. No sissy atom bombs.