You bring up interesting questions.... your next assignment: come up with interesting answers to them. ;-p
On Tue, Feb 7, 2012 at 2:54 PM, Diorite Gabbro <diorite@yahoo.com> wrote:
--- On Tue, 2/7/12, Lin Kerns <linkerns@gmail.com> wrote:
>Caldera Eruption "Early Warning System"? Not so Fast.
>By Erik Klemetti
February 2, 2012 |
[snip]
>The Druitt et al. (2012) study used these two petrologic
characteristics of minerals and melt to determine two main conclusions:
(1) the magma erupted from Santorini during the Minoan eruption in ~1600 B.C.
was a mixed magma and (2) the intrusion that "got the ball rolling"
toward the Minoan eruption and the subsequent mixing happened
geologically quickly — in the the timescales of a century to a few
months.
One of the frequent triggering events for volcanic eruptions seems to be a mixing event. As for the rapidity, as I recall for Mt. St. Helems the mixing event was apparently within months of the eruption. However, magma recharge to a magma chamber may or may not trigger an eruption. Things that influence the response are differences in temperature, volatile content, and the compositions of the magmas resulting in differences in viscosity. Mafic recharge into a mafic magma chamber doesn't make as much difference. The worst case seems to be recharge of a mafic magma into a chamber that has had time to differentiate a felsic magma (by assimilation or fractional crystallization).
> [snip]The authors do discuss some of the
ways that this recharge/mixing might be manifested once the events have
begun — interestingly not as "bulging" but rather "sagging" of the
bottom of the magmatic system as the magma fills in, so uplift at
Santorini might have been minimal.
Or the reverse - maybe they are inferring sagging of the bottom of the magma chamber because they don't see evidence of uplift.
>[snip] However, what I see as the biggest problem in this "early warning"
claim is that it might still not be easily detectable — what if their
timescale is off by even a factor of 2, so it takes 2 centuries to lead
to an eruption? Human monitoring of an event 200 years in the making
might be very problematic.
This is a very important point. With the lack of direct observation of large rhyolitic eruptions, this is one of few constraints on timing we have. That's what's important here. How much warning will we have?
Personally, I don't think the magma chamber at Santorini has had long enough to differentiate so that a large-scale felsic eruption could take place.
The volcanic system that worries me is Iwo Jima. The island is the resurgence dome on a large caldera. Uplift on the resurgence dome is significant. The shoreline from the late 1770s when Captain Cook visited is just over 140 ft above sea level. I couldn't find it quickly online, but the landing craft abandoned at sea level in 1945 are, as I recall, already over 40 ft above sea level. Recent eruptions have been mafic, but I don't think I've seen anything on how long ago the caldera-forming event took place.
The question is: Is the lag time between mixing events and eruption the same for 1600 BC Santorini-sized eruptions and the even larger events? Is there a "scaling" factor involved? Is the lag time larger form bigger systems or is it a result of mixing volumes instead? It would be nice to know.
Diorite
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