ablock_usgs_gov14 karma2020-05-12 19:41:00 UTC
Good eye. The flows from a collapsing dome are scary, man. Is the one you saw of Unzen in 1991? Blocks fall off the dome , hit the talus slope below and just explode into dust, fragments, steam and gas. The whole mass races down the slope. My simple understanding - and I speak under correction from my learned colleagues with petrological expertise on this thread - is that the dome rock is hot, fragile and charged with hot gas under great pressure. When it falls and hits, it blows itself apart. The hot fragments surge downhill propelled by the explosion, aided by gravity and gliding on a cushion of trapped air, gas and steam. Note - heres an open invitation to my colleagues to comment. My understanding of this is just enough to keep me out of the way when it happens.
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ablock_usgs_gov10 karma2020-05-12 18:06:18 UTC
Hola back to Bob. Steve is retired and loving it; we don't see him at the office too often. Barb changed up Murrays trouseau years ago. Brian will address your more technical query...
ablock_usgs_gov5 karma2020-05-12 17:52:55 UTC
I was at Pinatubo and hope I never see anything as big as that again. I've seen a bunch of smaller ones, or the aftermath of smaller ones. Any eruption that kills people is too big for my taste. I'm afraid that for me volcanoes are the enemy, not the source of breathtaking majesty and wonder they are for others. - Andy
ablock_usgs_gov5 karma2020-05-12 18:27:38 UTC
Thats a good question that occupies a lot of our time. Think of it this way: magma 'floats' up to the surface, propelled by bouyancy and gas. As it rises, it forces rock out of the way, deforming the surface. If the rock is brittle and the force of the rising magma is great enough, the rock breaks. If heat gets close enough to the surface, it boils groundwater. So to know if a volcano might erupt we look for gases, deformation of the ground and earthquakes.
ablock_usgs_gov3 karma2020-05-12 22:06:40 UTC
Ha! Yeah, that was a stressful moment. General Studer's version of the story is funny but I think he already had a pretty firm grasp of the situation without the benefit of my insight. (Insert winky icon here…)
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