21 November 2012, Mount Tongariro lofted a 6.5 km column of material from the Te Māri Craters. Analysis turned up that it was a phreatic event, meaning that groundwater had flashed to steam and pulverized some rock, and launched that along with re mobilized ash skyward.
Recently, Kverkfjöll had a similar episode. Both events were driven by a steam explosion.
Time to put on yer thinkin’ caps.
The critical pressure of water/steam is 221.2 bar. Above this pressure, it doesn’t matter how hot you make it, you can not distinguish between the liquid or gaseous phase of water/steam. You could heat it to 1200°C and it would not flash to steam.
However… if you decrease the pressure it to below 221.2 bar, and it is still at that 1200°C temp…. it’s going to instantly turn into it’s gas phase. With that sort of temperature, gas law says that it will expand to about 5000 times it’s volume. (give or take). Now, bear with me. That is going to send a pressure pulse through the surrounding rock.. both upwards and downwards… as well as horizontally. In a shockwave, after the pressure pulse passes, the material will relax … possibly overshooting the rest pressure that it was naturally at. If the negative side of the pressure wave, the low pressure relaxation component, drops other fluid below the 221.2 bar critical point, that high temperature fluid will also have the opportunity to turn to gas and join in on the expanding vapor fun. In my opinion, this is how phreatic detonations can be so powerful. In Tongariro’s case, tossing Hobbits as high as 6.5 km.
“Hydrostatic” pressure is the pressure at depth that a column of water open to the surface is at. For a crack filled with water extending deep into the crust, you cross the 221.2 bar pressure level at about 2260 m depth. A bit deeper if the crack has ice as part of it’s makeup. Water constrained by solid rock will feel the pressure of the overburden is. At 2700 kg/m³, the 221.2 bar pressure is achieved at 834 to 840 m depth.
Now… suppose that somewhere between 2260 meters and 840 meters, a pocket of water became exposed to high temperature magma and became superheated. Since the confining pressure was above the supercritical point, nothing happened. The fluid just sat that there, fat dumb and happy, and hotter that all get out. Now supposed that a small tectonic quake opened up a crack to this superheated water pocket, and exposed it to the surface. The fluid instantaneously drops below the supercritical point, and since it was at such a high temperature, expands to about 5000 times it’s initial volume.
Note: an alternate scenario could have the superheated fluid (water) migrate above the supercritical pressure and then go off, but I would think that it would be a bit nosier than a single blast/detonation. More like a slightly constipated geyser.
If I am correct, then we have magma somewhere above 2260 m depth, but not of a make-up to where gases entrained in it will froth and cause an eruption…. if that were the case, we wouldn’t be sitting here ruminating on it, we would be watching cameras.
Alternate scenario #2: The country rock, heated from below, transfered the heat to a pocket of water… then the first scenario idea takes back over. Tectonic quake opens up the pocket to the atmosphere, water flashes to steam… bang. Does not require magma above 2260 meters, though the event would be less energetic than I postulated. (somewhere between 1700 x and 5000 x expansion rate.) But, you still have magma closer to the surface than normal. (not gonna heat that much water with a Bic™ Lighter)
In the regards of the miniscule Kverkfjöll event there is also the possibility that the magma actually moved upwards after the phreatic detonation. This was such a small event that the magma would then have been quenched by the caldera lake and almost instantly sollidified into a plug capping the vent. This is though just to be seen as more of a theoretical nitpicking. /Carl