|AP Photo/Robb Long|
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.