A little known, potentially dangerous, volcanic system at Laguna del Maule, Chile

Maule Lake image
from http://earthobservatory.nasa.gov/IOTD/view.php?id=76827
Note the grey lava flow at the bottom center edge of the Lake

Where in the world is the earth moving up at about 1' per year? Not Yellowstone, but at a relatively little known volcanic field that straddles the crest of the Andes at 36 S latitude.  A recent article by Brad Singer et al. entitled "Dynamics of a large, restless, rhyolitic magma system at Laguna del Maule, Southern Andes, Chile" in GSA Today, v. 24(12) pp. 4-10, 2014 describes this field and it's potential danger. The Lake lies within a 15x25 km caldera. The volcanic complex covers about 300 square kilometers, and contains a cluster of stratovolcanoes, lava domes and cinder cones. The volcanoes sit about 90 km over the subducting slab of the Nazca plate.

The field has 13 cubic kilometers of rhyolite erupted during the past 20,000 years. There have been a dozen crystal-poor, glassy rhyolitic lavas during the Holocene (the past 11,700 years).

In March 2013, the Observatorio Volcanologico de los Andes del Sur (OVDAS) issued a yellow alert, indicating a potential eruption within months to years based on an alarming surface uplift over the last 7 years and swarms of shallow earthquakes.  (In 2010 there was a M8.8 earthquake 230 km to the east.) Early activity in the Pleistocene culminated in "a spectacular concentric ring of 36 separate post-glacial silicic eruptions" between about 25,000-2,000 years ago. The most recent eruptions "were from 24 vents and produced 15 rhyodacite and 21 rhyolite coulees and lava domes." The vents encircle the lake basin. Pumice and ash fall deposits in Argentina may equal these flows in volume.  The only comparable Holocene rhyolite flareup, the authors point out, is along the Mono Craters chain in California.

According to Fournier et al. (2010)*, the rate of surface deformation was negligible from January 2003 to February 2004, but then accelerated between 2004-2007. Feigel et al. (2014)^ have found uplift rates exceeding 280 mm/year (28 cm/year; 11 inches per year). In comparison, this is 2-5 times the greatest rates measured for Yellowstone or Santorini.

Electrical resistivity data suggest a magma body with a hydrothermal system at about 5 km depth, at a location that agrees well with the source of inflation inferred from the geodetic data. 69% of recorded earthquakes between 2011 and 2014 are shallower than 5 km, and most occur under rhyolite vents along the periphery of the uplifting region.

Figure 5 in the referenced paper. Hypothesized
cross section of the Laguana del Maule complex.

The current observations are interpreted in terms of the magmatic mush model of Hildreth (2004) and Hildreth and Wilson (2007), a model that was originally developed to explain the integrated observations of the Long Valley system that erupted 650 cubic kilometers of the Bishop Tuff 767,000 years ago. The magma system is inferred to contain a thin boundary layer of granitoid that is solidified against country rocks.  "Inboard" of this is a rigid "sponge" consisting of crystals with some minor interstitial melt, and inside of this is a crystal rich mush. The mush is maintained in its partial molten state by fluxing of heat and magic magma through the deeper parts of the crustal reservoir. Melt-rch lenses develop near the roof, creating a low-density barrier through which the denser mafic magma cannot rise to the surface. This mush near the top can be tapped to provide the recent/future rhyolitic eruptions.

The proposed setting under the volcanic complex is shown in the figure to the right/above. It includes inferences consistent with the rapid uplift, shallow earthquakes, active intrusion of magic magma at 5 km depth, and normal faulting and geodetic data that record radial extension to form the circumference of vents.

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*Fournier, T.J., et al., Duration, magnitude and frequency of subaerial volcano deformation events: Nw results from Latin America using InSAR and global synthesis, Geochemistry, Geophysics, Geosystems, 11, doi: 10.1029/2009GC002558

^Feigl, K.I., et al., Geophysical Journal International, v. 196, 885-901, doi:10.1093/gji/ggt438

Steamboat geyser erupts!

AP Photo/Robb Long

One of Yellowstone's most spectacular geysers erupted today, making headlines in major newspapers and radio news shows. Why such a fuss? Steamboat erupts to hundreds of feet, but only rarely and never predictably. Having worked in Yellowstone studying the geysers, I can imagine the excitement of the tourists as they hear the roar of the geyser erupting, see the towering plume, and perhaps get showered by its steam and droplets (hopefully not by any entrained rocks or sand). I wonder how many have glasses or camera lenses that now bear witness to the chemicals in Steamboat's water with fine deposits of minerals precipitated when that water evaporates?

What is a geyser? Is Steamboat really a geyser or should it be called a "hydrothermal explosion?" Geysers are usually defined as systems of hot water that intermittently ejecta some or all of their water into plumes consisting of water and steam. Geysers lie along a spectrum of thermal features that range from hot springs (warm to scalding) on one end and fumaroles on the other. A fumarole spouts dry or wet steam, usually continuously, but sometimes intermittently. Geysers can be thought of as hot springs that intermittently eject the hot water from their reservoirs.

The activity of a particular thermal feature in an area like Yellowstone depends on a delicate balance of heat and water (heat is supplied by hot water, vapor, or gases that rise from magma at depth; most of the water is circulating cool groundwater that is heated by the magmatic hot water or gases, or by contact with surrounding hot rocks). Too much water and you have a hot spring. Too much heat and you have a fumarole.  Just a right balance and you get a geyser that discharges and recharges its heat and water in cycles. (There are even cold "geysers" that charge and recharge carbon dioxide, but that's another story!)

Conduit of Old Faithful

What does the reservoir of a geyser look like? We don't know for most geysers as it is difficult and dangerous to try to find out. However, in the 1990's, with the permission and assistance of the National Park Service, we lowered an ice-cooled miniature video camera into Old Faithful. This was in the days before truly miniature videos were available, and our system had to be less than 4" diameter to pass through a known constriction, had to contain its own lighting system, had to be cooled with ice because it was in an environment of 92 C steam, and had to have a heated lens to prevent condensation on the otherwise cool unit.  My colleague Jim Westphal at Caltech cleverly designed the system. From this work, we were able to constrict the cross-section of Old Faithful shown to the right. The reservoir is a tortuous series of narrow and wide spaces.  Researchers in Russia have probed geysers there and found similarly complex conduits, and it is likely that the reservoir of Steamboat Geyser is a similar complex of caverns and constrictions.

That word "intermittently"in the definition of a geyser is taken by many to mean "regularly" and somewhat "predictably." So, tourists get used to the fact that the National Park Service can tell them within about 10 minutes or so when Old Faithful is going to erupt; and within several tens of minutes or several hours for a number of the other geysers around Yellowstone.

So, what is Steamboat? It is a geyser if the word "intermittently" is used properly. But, the eruptions are so rare that they do resemble those rather nasty events called "hydrothermal explosions." The term "hydrothermal explosion" is usually used to describe a new feature--one that has popped up where no known feature, such as a geyser, existed before.  The vent of Steamboat is so large that it is well-known, and the boardwalk for tourists passes right by it so one can look down on its vent. But, just as some tourists probably got taken by surprise if they were close to Steamboat when this mighty eruption (reported to have been 200-300 feet high for 9 minutes) took off, tourists in the thermal basins are always at risk of a true hydrothermal explosion. That's one reason for staying on the boardwalks constructed for walking through these basins--they are built in areas deemed as safe as possible from future surprises.

**Reference: Hutchinson, R., Westphal, J., and Kieffer, S., In situ observations of Old Faithful Geyser, Geology, 25(10), 875-878, 1997.