If the upper chamber is the one filling it will be an explosive event. Its a matter of which magma is causing the pressures. If its filling with rhyolitic magma it will be explosive. One need only look at the geological record to see these events.
Posts 30 and 31 have the information you seek.
Why would the upper chamber be "filling with rhyolitic magma"? ... The rhyolite is what the Rocky Mountains are made of, as you pointed out above, it's this material that is melting and mixing with the basaltic magma coming up from below ... we won't see a purely rhyolite eruption nor the explosiveness associated with this material ... granted, it won't run out like Hawai'i does ... rather somewhere in between ... something contained within the park most likely ...
I have to agree with
Westwall above ... today we stand on the collapsed roof of the magma chamber, if this is filling up and then this roof will be being pushed upwards, not inches but hundreds of feet, perhaps thousands of feet ... and the explosiveness of the eruption depends on how big this chamber gets while it fills ...
We'll have 10,000 years warning ...
ETA: Mt St Helens grew at a rate of 3 to 10 feet an hour during the 1980-86 dome building eruption ...
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