20 September 2009

Accretionary Wedge #20: Geologic discoveries for the future

Posted by Jessica Ball

David Bressan at cryology and co. posed this question for the latest edition of the Accretionary Wedge:

What remains to be discovered for future earth scientists what we (still) don’t know about earth? What are the geological riddles that still lack answers – all questions are allowed – it could be a local anomaly, or a global phenomena, or something strange…(Naturally you can also include a possible answer to your problem).
One question I’d like to see answered in the future is something that many volcanologists have been (and are still) trying to figure out: What does the inside of a volcano really look like?

Early scientists thought that volcanoes were the homes of gods, or passages to Hell, or conduits for a vast network of subterranean fires. (At left is Athanasius Kirchner’s 1664 engraving of the interior of Mount Vesuvius, from his book Mundus Subterraneus. Image from Art.com.) These ideas have all fallen by the wayside, but volcanologists today are still trying to work out the details of volcano ‘plumbing’, as it’s commonly called.

This is no easy task. For one thing, there are many different kinds of volcano; it stands to reason that they’re not going to have similar plumbing systems or structures. For another, if you’re interested in what’s going on inside an active volcano, it’s understandably a little difficult to get close enough to the object in question to even begin to work on the question.

So how do volcanologists know anything at all about the interior of volcanoes – and how can they find out more?

One of the easiest ways is to look for volcanoes that are no longer active, and have been worn down by time and erosive forces. There are actually some great examples of these: The Summer Coon volcano in southern Colorado, which shows a spectacular radial pattern of dikes (see the Google Earth image at right); the cinder cones in the Southwestern Nevada Volcanic Field, which one of my professors studies (Keating et al., 2008); even my own study area, at the Santa Maria volcano in Guatemala, has a spectacular view of the internal structure of a stratovolcano (revealed by the 1902 eruption that blew a huge crater in the side of the mountain).

Another way is to use geophysical or remote sensing methods. Volcano seismology (including 3D seismic tomography, sort of a CAT-scan for volcanoes) has been used for years to locate and define the shapes of magma chambers under volcanoes, as well as the dikes and conduits which feed eruptions. For smaller edifices, methods such as ground-penetrating radar can give some idea of the shallow structure. Some researchers in Japan (Miyamachi et al., 1987) have even used explosions from fireworks to image the interior of lava domes (using the shock of the fireworks explosions to take the place of seismic shocks). GPS, ground tilt, and other deformation measurements give volcanologists an idea of how a volcano changes shape and volume, and from that some idea of the internal workings of the volcano can be discerned.

One interesting study by researchers in Japan (Sakuma et al., 2008) details a project where volcanologists actually drilled into an active volcano – Mount Unzen, which last erupted in 1996 and produced a summit lava dome. The Japanese volcanologists found a conduit with several feeder dikes, showing that Unzen’s eruption wasn’t fed by a single tube but several, and possibly from many sources. Still, this is only a small part of the volcano, and it’s a very expensive and tricky way of finding things out.

Because of these methods, we know something about the interior of volcanoes. For example, the old chamber-conduit-summit eruption model for stratovolcanoes is, we now know, an extremely simplistic and not entirely correct view; many stratovolcanoes are riddled with dikes and smaller pockets of magma and other interesting things like cryptodomes. And they’re hardly ever as nicely layered as this image suggests. (From the USGS Eastern Geographic Science Center Volcanoes! teacher resource.)

But we’ve still got a long way to go. You can’t just X-ray a volcano, and it’s very difficult to image an entire mountain with other methods, especially if it’s in a remote area, large, and (as they tend to do), erupting. Knowing the plumbing of a volcano is, however, important for forecasting how the volcano will behave: Will it erupt vertically or laterally? Is the magma chamber shallow or deep, large or small, and is there more than one? Could there be outbreaks of lava on the flanks as well as the summit? Is the volcano structurally stable or collapsing? Does it contain a pressurized cryptodome that could explode, or lava that will ooze out of vents? These are all questions that depend on what we can find out about a volcano’s insides, and that’s something that volcanologists are always working on.

And hey, maybe I’m being pessimistic and someone will invent a whole-body-scan for a volcano. It could happen…

Additional reading:

Keating, G.N. et al., 2008, Shallow plumbing systems for small-volume basaltic volcanoes. Bulletin of Volcanology, v. 70, p. 563-582.

Miyamachi, H. et al., 1987, Seismic experiments on Showa-Shinzan lava dome using firework shots. Pure and Applied Geophysics, v. 125, no. 6, p. 1025-1037.

Poland, M.P. et al., 2004, Patterns of magma flow in segmented silicic dikes at Summer Coon Voclano,Colorado; AMS and thin section analysis. Earth and Planetary Science Letters, v. 219, p. 155-169.

Sakuma, S. et al., 2008, Drilling and logging results of USDP-4; penetration into the volcanic conduit of Unzenvolcano, Japan. Journal of Volcanology and Geothermal Research, v. 175, p. 1-12.