Hello Michael, Regarding Bogoslof - we're not ready to go to yellow yet, especially when we're lacking visual confirmation of what is or isn't happening at the island. For Herbert, we have just barely begun to study this island. The caldera was likely formed as part of a large volcanic eruption - but I can't speculate on its mechanism of formation at this time. For Cleveland - Cleveland is also not well studied yet. There's a continuous gradation between "sub-Plinian" and "Plinian" for eruptions, and many volcanologists don't agree on the same definition for Plinian. But, if we agreed that Plinian meant things like a VEI between 4 and 6, and a column height of at least 20 km, then Cleveland hasn't had a Plinian eruption in historical time. Cleveland's 2001 eruption had a column to 12 km that lasted for several hours, so certainly Cleveland is capable of decent-sized eruptions. I'd be reluctant to state that it couldn't produce a much larger eruption, although we certainly see no signs pointing to one developing anytime soon. Hope this is helpful, Cheryl Cameron -------- Michael Firth wrote: Hi, I have three questions (if you don't mind!) I was wondering how long you'll probably wait during the current pause in activity at Bogoslof before considering lowering the alert level to yellow? Do you how the caldera of Herbert volcano was formed (like a very large eruption or collapse/St. Helens type event)? Is Cleveland capable of Plinian eruptions? -- Cheryl Cameron Geologist, Alaska Volcano Observatory Alaska Division of Geological & Geophysical Surveys voice: 907-451-5012 fax: 907-451-5050