Wilbert Lecture
Sponsored by the Department of Geology and Geophysics
The Life and Times of Supervolcanoes: Inferences from Long Valley
Caldera
Dr. Justin Simon
NASA, Clearlake Texas
Abstract
Cataclysmic eruptions of silicic magma from “supervolcanoes” are among the most aweinspiring natural phenomena found in the geologic record, in terms of size, power, and potential
hazard. Based on the repose intervals between eruptions of this magnitude, the magmas
responsible for them could accumulate gradually in the shallow crust over time scales that may
be in excess of a million years (Smith, 1979; Spera and Crisp, 1981; Shaw, 1985). Pre-eruption
magma residence time scales can also be inferred from the age difference between eruption (i.e.,
using 40Ar/39Ar dating to determine the time when hot erupted material cools to below its Ar
closure temperature, ∼200 to 600 °C) and early pre-eruption crystallization (i.e., zircon saturation
temperatures; Reid et al., 1997). I will discuss observations from Long Valley a Quaternary
volcanic center in California. Long Valley is a voluminous, dominantly silicic caldera system.
Based on extensive dating of accessory minerals (e.g., U-Th-Pb dating of zircon and allanite)
along with geochemical and isotopic data we find that silicic magmas begin to crystallize 10's to
100's of thousands of years prior to their eruption and that rhyolites record episodes of
punctuated and independent evolution rather than the periodic tapping of a long-lived magma.
The more punctuated versus more gradual magma accumulation rates required by the absolute
and model ages, respectively, imply important differences in the mass and heat fluxes associated
with the generation, differentiation, and storage of voluminous rhyolites and emphasize the need
to reconcile the magmatic age differences.
Day: Friday 24, 2014
Room: E137 Howe-Russell Bldg - Time: 3:30 pm
Refreshments: 3:20pm H-R Atrium
SW
~2500 ka
NE
1.
~2400-1700 ka Older Glass Mt.
2.
Intrusions
into shallow
crust
Some granitoids
re-melted
by magma
recharge
Some magma
solidifies
~1700-1200 ka little magma
3.
Crystal-poor
melts
coalesce
~1200-1100 ka expansion of GM
4.
More early
granitoids
melted
by magma
recharge
Unerupted magma
solidifies
~1100-800 ka Younger Glass Mt.
5.
Re-melting
and transient
magma
bodies
~850-780 ka “initiation” of BT
6.
Crystal-poor
melts
coalesce
Massive
melting
event
~780 ka eruption of Bishop Tuff
~700 to 400 ka Early Postcaldera
7.
8.
Bishop
Tuff
Large
shallow
magma
chamber
~400 to 100 ka Late Postcaldera
9.
New melt
components
~100 ka to Present day
10.
Moat
rhyolites
&
basalts
Mammoth Mt.
&
Inyo domes