Closed And Vent Type Eurptions

[The PIT]

Been chatting to USGS about this and how the type of vent effects the eurptions heres what Steve Reiter had to say.

Closed-vent behaviour makes eruptions relatively easy to forecast. Fresh magma working its way to the surface must break through the plug or surrounding rock, generating earthquakes and swelling of the ground. Gas leaks may or may not be detected at the surface. The most easily measured gas, sulphur dioxide, may be absorbed into and hidden in groundwater and thus not reach gas instruments on the surface. Fortunately, most VEI 3 and larger eruptions are going to follow closed-vent behaviour and thus will give at least some warning of reawakening.

Open-vent behaviour tends to bleed off the gas and thus reduce explosive potential. Thus, most eruptions during this behavior will be VEI 2 and smaller. However, be careful, because there are some cases in which either the convection speeds up (increasing explosive potential) or is temporarily stopped (trapping gas and thus also increasing explosive potential). Eruptions from volcanoes in open-vent behavior are generally difficult to forecast because there is virtually no plug to break through. Seismic and ground deformation precursors will be minimal.

Emission of CO2, SO2, and other volcanic gases may increase notably, but these don't indicate likely onset time very precisely. Ground deformation (e.g., tilt) measurements right on crater rims can warn of fresh arriving slugs of magma and thus of explosions or dome collapse to follow within hours to a few days, but very few volcanoes have instruments close enough to their vents to detect such changes. So, in general, eruptions during open-vent behaviour will be difficult to forecast. As an example, after the vent of Mount Spurr, Alaska, was opened in June 1992, a second eruption in August began without clear seismic precursors.

The full paper is here http://www.ofcm.gov/ICVAAS/Proceedings2004...11-session2.pdf

Now some say Galeras displays open vent type style of eruptions so I asked the question since this suggestion was meant with harsh criticism when I tried to discuss it earlier.

This was the following response.

http://vulcan.wr.usgs.gov/Volcanoes/Colombia/framework.html The first link provides a brief description of Galeras. It's erupted many times during the Holocene and is described as possessing a "breached caldera", which supports your categorization. You're probably aware there's no "line of demarcation" separating an open-vent volcano from a closed-vent volcano - it's subjective. Using an Internet search engine, it's easy to find the eruptive history of Galeras (ex: http://en.wikipedia.org/wiki/Galeras) - not much time recently for lava dome creation, as you'll see.

So it's all subjective in one sense.

However it's clear the closed vent style will bring the largest type of eruption followed by smaller eruptions. Open vent the eruptions will be smaller however events like slope failure can take place at any time producing a fairly large eruption than otherwise would be produced.

I also wonder if ST Helens recent eruption was an open vent style of eruption as there wasn't very much in the way warning before hand. It was releatively gas poor indicating that the magma had time to degas hence very large explosions bare the collapse of the dome and the spines that thrust through the dome.