What Happens Before a Volcano Blows?

A caldera is a very large crater that forms after a very large volcanic eruption. The eruption is explosive and ejects a lot of material. Most of what comes out of the volcano is blown a great distance into the atmosphere and over a large area, so a huge volume of the local landscape is simply gone—thus the large crater.

Many people know about the Yellowstone Caldera because it is the location of a lot of interesting ongoing thermal and volcanic activity, some of which has been in the news lately, and it has even been featured in a recent epic disaster fiction film called 2012 in which the re-explosion of the Yellowstone Caldera is only one problem of many faced by the film’s heroes and heroines.

Somewhat less known but still famous is the Santorini Caldera. It is in the Aegean Sea, in Greece, near the island of Crete. Santorini blew about 1,600 B.C. and seems to have caused the end of the Minoan Civilization; the edge of the volcano’s caldera is now a ring of islands. By comparison with Yellowstone, Santorini is small. The Yellowstone Caldera is about 55 by 72 kilometers in size, while Santorini’s is about 7 by 12 kilometers.

Santorini is the subject of an investigation just reported in the journal Nature. The volcano has blown numerous times in the past. The investigation shows that the last explosion, the one at about 1,600 B.C., was preceded by a stunningly short period of build-up of underground magma. It seems as though the magma, enough for a very large eruption, moved into the zone beneath the caldera in two or more events less than 100 years prior to the explosion, with a significant amount of the magma moving into place just a few years before the blast.

If we go back a decade or so, volcanologists thought that the buildup to a major eruption like this would take more time, perhaps many centuries. Various lines of evidence have caused scientists to start to think that the buildup to blast-time might be shorter than that, and the present report is an excellent direct measurement of the timing which seems to confirm these growing suspicions.

How can scientists tell that it happened this way? Using volcano forensics, of course! Here’s the basic idea:

When shocking events happen, such as the intrusion of a bunch of magma into an area of rock, or associated seismic activities, the various chemicals in magma become “zoned.” Waves of energy passing through the molten rock cause bands of specific types of chemicals to form. During a period of no shocks, if the temperature is high enough, these bands dissipate. Some bands dissipate in very short periods of time, others over very long periods of time. If at any point the magma is released in a volcanic explosion such as the type that forms a caldera, the material suddenly cools and the state of the bands, dissipated to a certain degree, is preserved. Later, sometimes thousands of years later, geologists can study the rocks and estimate the amount of time between shock event and the volcanic explosion by measuring how much dissipation has occurred. It is a sort of magma-based clock.

In the case of Santorini, everything seems to have happened well within a century. This formation of a magma chamber large enough to cause a major eruption occurred after an 18,000-year-long dormant period. So, if we were thinking that the long period of time between caldera eruptions was characterized by a slow and steady buildup of magma, we were probably wrong. The real significance of this is that we can’t look at a caldera that is known to have erupted multiple times and rule out a future eruption simply on the basis of a low level of current activity. And of course, we are left wondering what initiates this rather rapid recharge of the magma underneath a caldera.

It’s a good thing that scientists are studying and monitoring these volcanoes!

Druitt, T., Costa, F., Deloule, E., Dungan, M., & Scaillet, B. (2012). Decadal to monthly timescales of magma transfer and reservoir growth at a caldera volcano Nature, 482 (7383), 77-80 DOI: 10.1038/nature10706

Greg Laden | | READ MORE

Greg Laden is a freelance science writer.