GEOS 470R/570R Volcanology
L11, 23 February 2015
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Handing out
Today’s PowerPoint slides
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Returning graded midterm
“Our plans miscarry because they have no
aim. When a man does not know what
harbor he is making for, no wind is the
right wind.”
--Seneca
MT Scores (100 possible + 10 EC)
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470R Undergraduate Students
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570R Graduate Students
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95 + 10 = 105
86 + 8 = 94
79 + 3 = 82
74 + 6 = 80
73 + 5 = 78
73 + 6 = 79
73 + 4 = 77
69 + 3 = 72
62 + 8 = 70
63 + 6 = 69
96 + 8 = 104
95 + 8 = 103
93 + 10 = 103
65 + 7 = 102
92 + 10 = 102
87 + 8 = 95
87 + 3 = 90
73 + 8 = 81
73 + 8 = 81
75 + 4 = 79
31 + 4 = 35
Statistics 2015
 n = 10
 mean = 74.7 + 5.9 = 80.6
 85 ~ A/B ; 70 ~ B/C
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Statistics 2013
 n=7
 mean = 71 + 7.3 = 78.3
 85 ~ A/B ; 70 ~ B/C
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Statistics 2015
 n = 11
 mean = 81.5 + 7.1 = 88.6
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Statistics 2013
 n=4
 mean = 89.5 + 8.5 = 98
Readings from textbook
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For L11 from Lockwood and Hazlett
(2010) Volcanoes—Global Perspectives
[None]
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For L12 from Lockwood and Hazlett
(2010) Volcanoes—Global Perspectives
Chapter 12
Assigned reading
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For today L11
None
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For L17, 25 March 2015
Hildreth, W., and Lanphere, M. A., 1994,
Potassium-argon geochronology of a basaltandesite-dacite arc system: The Mount
Adams volcanic field, Cascade Range of
southern Washington: Geological Society of
America Bulletin, v. 106, p. 1413-1429.
Last time: Calderas
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History and nomenclature
 Calderas
 Cauldrons
 Volcano-tectonic depressions
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Subsidence mechanisms
Origin of megabreccias and structural versus topographic margins
Resurgence
Caldera cycles and repose times
Diameters and area-volume relationships
Examples of calderas and their associated ignimbrites
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Long Valley, CA
Valles-Toledo, NM
Yellowstone, WY-ID-MT
San Juan volcanic field, including Creede caldera, CO
Southwest Nevada volcanic field, with numerous calderas
Sierra La Primavera, Mexico
Pantelleria, Strait of Sicily, Italy
Crater Lake, OR
Mount Pinatubo, Philippines
Caldera vs. cauldron
Original definitions
 Caldera
 Large volcanic depression ~circular in plan that results from
collapse
 Collapse probably a function of several variables but favored by
high eruption rate, resulting in collapse of roof of magma
chamber
 A morphological (surface) term
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Cauldron
 Volcanic subsidence structure without regard to shape, degree
of erosion, connection to surface volcanism
Current usage
 Virtually synonymous
 Caldera has taken precedence
Caldera formation preceded or
accompanied by voluminous eruptions
of pumice and block collapses
coherently along ring fractures
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Found in continental
areas, associated
with the
largest, silicic
eruptions
 e.g., Long
Valley,
Yellowstone
Wohletz and Heiken, 1992, Fig. 1.10,
adapted from Hildreth, 1981, Fig. 15
Caldera formation preceded or
accompanied by voluminous eruptions
of pumice and block collapses
chaotically or piecemeal
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Found in central vent
arc volcanoes,
especially
andesitic
 e.g., Krakatau
Wohletz and Heiken,
1992, Fig. 1.10, adapted
from Hildreth, 1981, Fig.
15
Caldera formation independent of
pyroclastic eruptions
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Magma lost by lava flows and lateral
removal (drainage of chamber)
Not by pyroclastic explosion from a vent in
the center of the caldera or from vents along
the ring fracture zone
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Examples
Craters in shield volcanoes of Hawaii
Mt. Katmai during eruption of Valley of Ten
Thousand Smokes from Novarupta vent
Volcano-tectonic depressions
Collapse occurs along regional (tectonic)
faults
 Resulting depression commonly rectilinear
 Caution: Some “volcano-tectonic
depressions” are “conventional” calderas
that have been dismembered by postmineral faults
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Original caldera geometry obscured by
younger deformation, such as multiple sets of
normal faults (e.g., Caetano caldera, Nevada,
John et al., 2008; Colgan et al., 2008)
Stages of resurgent calderas
Three stages (Lipman, 1984)
A.
Precollapse volcanism
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B.
Ash-flow eruption and
concurrent collapse
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C.
Clustered intermediate
stratocones (or domes)
Broad uplift
Intracaldera tuff ponds
during subsidence
(asymmetric);
megabreccias; order of
magnitude thicker than
outflow facies
Resurgence and postcaldera deposition
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Resurgence also
asymmetrical
Caldera floor nearly
obliterated
Lake sedimentation
Hydrothermal activity
Schmincke, 2004, Fig. 9.49, after Lipman, 2000, Fig. 2; from Lipman, 1984
Geometries of caldera
subsidence
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Plate (piston)
 Single, large-volume eruption
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Piecemeal
 Multicyclic?
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Trap-door
 Asymmetrical pluton?
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Downsag
 Small volume/deep pluton?
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Funnel
 Small/deep pluton?
Schmincke, 2004, Fig. 9.48, after
Lipman, 2000, Fig. 6; from Lipman, 1997
Topographic wall vs. structural
margin of calderas
Moat
Resurgent
Dome
Lipman, 1976, Fig. 1
Resurgent doming
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Produces structural doming
 May have subsequent keystone graben collapse
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Related to rapid re-establishment of isostatic equilibrium
 Occurs shortly after collapse of caldera
 Conventional view: Buoyant upwarp of cauldron block by reinflation (e.g., Smith
and Bailey, 1968)
 Alternate view: Central uplift by injection of shallow sills or laccoliths into thick
intracaldera fill (McConnell et al., 1995)
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Relationship to magmatic compositions
 Observed for rhyolitic to rhyodacitic systems
 Not observed for andesitic and basaltic systems
 Not observed for strongly peralkaline systems (rheologically similar to andesites)
Keystone graben
Moat
Lipman, 2000, Fig. 2c; from Lipman, 1984
Proportion of chamber erupted
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Evacuated portion of magma chamber
erupted
Large continental calderas perhaps ~10%
Smaller arc calderas perhaps more (10-25%?)
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Remainder of magma in chamber buoyantly
readjusts following eruption
Leads to resurgent doming (structural) and
resurgent magmatism (e.g., eruption of lava
domes)
May be followed by recharge from lower crust
Valles and Toledo calderas (Otowi and
Tshirege Members of Bandelier Tuff),
Jemez Mountains, NM
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Open-system behavior
Lifespan of magmatic system
 Volcanic field: >3.6 m.y. (perhaps 16.5 m.y.)
 One chamber: >1.79 m.y.
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Volume of cataclysmic eruptions
 300 km3, 1.23 Ma, Valles caldera
 300 km3, 1.61 Ma, Toledo caldera
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Repose time for cataclysmic eruptions
 0.38 m.y.
Greater Long Valley region
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Six spatially discrete, successive foci of silicic
magmatism
1) 3.5-2.5 Ma: Pre-caldera dacite field NW of future
caldera at Long Valley
2) 2.2-0.79 Ma: >60 pre-caldera high-silica rhyolite
vents centered at Glass Mtn, NE of Long Valley (total
~100 km3)
3) 0.76-0.10 Ma: Central Long Valley system
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760 ka: Caldera-forming eruption of compositionally
zoned rhyolitic Bishop Tuff (~600 km3)
760-650 ka: Crystal-poor Early Rhyolite on resurgent
dome (total ~100 km3)
527-101 ka: Three clusters of varied Moat Rhyolites
(<10 km3)
4) 160-8 ka: Trachydacite-rhyodacite and peripheral
mafic vents at Mammoth Mountain, W of Long Valley
5) <50 ka: Mono-Inyo craters, NW of Long Valley
Hildreth, 2004, Fig. 6
6) <14 ka: Mono Lake, N of Mono Craters and NW of
Long Valley
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Each silicic focus driven by locally concentrated
basaltic intrusion in deep crust, a response to
 Extensional unloading and decompression melting of
mantle
Hildreth, 2004
What is the state of the Long Valley
magma reservoir?
Earlier view, with large
contemporary magma
chamber
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Near death, based on evidence that
 100-fold decline in eruption rate after 650
ka
 Lack of crystal-poor rhyolite since 300 ka
 Limited volumes of moat rhyolite (most of
it crystal-rich)
 Absence of post-caldera mafic volcanism
inside the structural caldera or to the N
and S
 Paleo-hydrothermal system in resurgent
dome died out soon after 300 ka
 Hydrothermal underflow at Hot Creek,
active since 40 ka, is sourced from outside
the ring fault zone to the west, possibly
driven by intrusion of hybrid dacites
 Low thermal gradients inside the caldera
P. Kresan collection
More recent view, with
chamber largely crystallized
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System has been dying for last 300 ka
 A nearly crystalline pluton
Bachmann and Bergantz, 2008, Fig. 4
Hildreth, 2004
Long Valley caldera
(Bishop Tuff), CA
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Open-system behavior
Six spatially discrete, successive foci of
silicic magmatism in greater Long Valley
Wilson and Hildreth, 2007, Fig. 18
 Abandoned silicic reservoirs have
crystallized
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Mammoth Mtn and Mono-Inyo chains
invaded caldera’s moat
 But outside its western ring-fracture zone
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Lifespan of magmatic system
 Volcanic field: 3.5 my
 One chamber: Vigorous for ~0.5 my
(~790-~288 ka)
 Central Long Valley system near death
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Volume, age of cataclysmic eruptions
 750 km3, 0.76 Ma
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Repose time for cataclysmic eruptions
 ???
Hildreth, 2004, Fig. 6
Yellowstone, WY-ID-MT
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Open-system behavior
Lifespan of magmatic
systems
 Along hotspot trace: >16.6
m.y.
 Volcanic field: >2.2 m.y.
 One chamber: >2.2? m.y.
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Volume of cataclysmic
eruptions
 1,000 km3, 0.64 Ma
 280 km3, 1.29 Ma
 2,500 km3, 2.06 Ma
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Repose times for
cataclysmic eruptions
 0.774, 0.646 m.y.
Lowenstern and Hurwitz, 2008, Fig. 3
Pantelleria, Strait of Sicily, Italy
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Open-system behavior
Lifespan of magmatic system
 One chamber: >0.093 my?
 Volcanic field: >0.25 my
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Volume of cataclysmic eruptions--small
 3 km3, 55 ka
 ? km3, 93 ka
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Repose times for cataclysmic eruptions
 38 ka
Summary: Calderas
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Calderas no longer solely a morphological term
Volcano-tectonic depressions
 Some are structurally-controlled collapse features; others are calderas modified by faulting
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Variety of subsidence mechanisms
Collapse
 Syneruptive failure of caldera walls produces megabreccias
 Collapse of structural walls topographic wall beyond structural margin of caldera
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Resurgence
 Occurs shortly after collapse of caldera
 Produces structural doming
 Related to rapid re-establishment of isostatic equilibrium
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Caldera cycles and repose times
Diameters and area-volume relationships
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Large calderas: ~10% of chamber volume is erupted
Examples of calderas and their associated ignimbrites
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Long Valley, CA
Valles-Toledo, NM
Yellowstone, WY-ID-MT
San Juan volcanic field, including Creede caldera, CO
Southwest Nevada volcanic field, with numerous calderas
Sierra La Primavera, Mexico
Pantelleria, Strait of Sicily, Italy
Crater Lake, OR
Mount Pinatubo, Philippines
Lecture 11: Dynamics of magma
chambers
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Yellowstone hot spot controversy
 Follow up to caldera discussion
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Dynamics of magma chambers
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Emphasis on large silicic systems
Open-system behavior
Basaltic underplating and shadow zones for basaltic volcanism
Construction of batholithic magma chmabers and their geometry
Lifespans of magmatic systems
Repose times for caldera-forming eruptions
Evolution of silicic magma chambers with time
Crystal mush model
 Successor to thermogravitational diffusion model
 As developed for greater Long Valley region
Yellowstone, WY-ID-MT: Present site
of a subcontinental hotspot track?
Perkins and Nash, 2002, Fig. 1
Yellowstone hotspot: History
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Yellowstone is the archetype of a continental hotspot
 First interpreted as being related to a convective plume
from the deep mantle by Morgan (1972)
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“Hotspot” coined by Wilson (1963)
 Linear chains of oceanic volcanoes that appear to record
the motions of the overlying plates
 Long-lived centers of enhanced magma production
 Commonly associated with voluminous surface volcanism
that has propagated linearly w.r.t. plates on which they
occur
Christiansen et al., 2002
Grotzinger and Jordan, 2010, Fig. 12.27
The Yellowstone controversy
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What is the nature of the “melting
anomaly” (hotspot)?
Is it the near-surface expression of
convective plumes that rise through the entire
mantle from a thermal boundary layer at the
base of the mantle?
Are there alternative explanations that relate
to an upper mantle origin?
Ignimbrites that occur regionally
Perkins and Nash, 2002, Table 1
Ash-fall tuffs that occur regionally
Perkins and Nash,
2002, Table 3
Time-space plot of volcanic rocks
related to Yellowstone “hotspot track”
Perkins and Nash, 2002,
Fig. 2
Yellowstone, WY-ID-MT: Present site of
a subcontinental hotspot track?
Christiansen et al., 2002, Fig. 1
The Yellowstone controversy
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Evidence cited by Christiansen et al. (2002) to argue that
melting anomaly is not consistent with a simple deep-mantle
plume hypothesis:
 There is a Miocene to Recent northwestward migration of magmatism,
concurrent with the northeastward “hotspot” migration
 Claim: Seismic imaging provides no evidence for, and several
contraindications of,
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Vertically extensive plume-like structure beneath Yellowstone, or
Broad trailing plume head beneath eastern Snake R. Plain
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And some evidence that it does not.
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 Claim: No evidence that the system extends deeper than ~200 km
Christiansen et al., 2002,
Fig. 1 (detail)
Alternative model
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Christiansen et al. (2002) argue that
 The melting anomaly is a product of feedback
between upper-mantle convection and regional
lithospheric tectonics
 Stay tuned for new developments!
Dynamics of magma chambers
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Emphasis here
 On large silicic systems
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Open-system behavior
Basaltic underplating
 Significance of shadow zones for basaltic volcanism
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Construction of batholith-sized magma chambers
 Relationship to why batholiths have shoulders or floors
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Lifespans of magmatic systems
Repose times for caldera-forming eruptions
Evolution of silicic magma chambers with time
 Crystallization stages of magma chambers
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Crystal mush model
 As developed for greater Long Valley region
 Successor to thermogravitational diffusion model
Magma
supply vs.
percolation
rate model
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VIII
VI
Magmatism
 Fundamentally
basaltic
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IV
Note that “magma
supply” to the
lithosphere and
eruption rate
V
 May differ by 1 to 3
orders of magnitude
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* = rapid transient
injection of basalt at
restricted crustal
levels
 Resulting in
generation and
separation of rhyolite
but few intermediates
VII
Ia
IIa
IIb
Hildreth, 1981, Fig. 16
Ib
III
Underplating and shadow zone
Wohletz and Heiken, 1992, Fig. 1.10, adapted from Hildreth, 1981, Fig. 15
Dynamic model of large, mature
Cordilleran magmatic system
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Based on
 Bishop Tuff, Long Valley
caldera, CA, and
Yellowstone, WY
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System can be
reestablished over time
 Different volcanic centers
in same volcanic field
 Multiple eruptions in same
caldera complex, with
repose times correlated
directly to volumes of
eruptions
Hildreth, 1981, Fig. 12
Why batholiths
have shoulders or
floors
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Field and geophysical evidence
that batholiths have flat bottoms in
mid-crust
 Luhr Hill Granite phase of Yerington
batholith has a shoulder or floor at
Luhr Hill
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Silicic melt arrived in shallow crust
only in small batches, building
reservoir incrementally
 Large silicic magma bodies never
rose en masse from below
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Yellowstone: Lowenstern and Hurwitz, 2008, Fig. 3
Magma chamber, once formed,
may migrate more locally en
masse
 During resurgence following caldera
eruption
 Yerington: Luhr Hill granite (PG)
intrudes Bear quartz monzonite
(QM)
Yerington batholith: Dilles, 1987, Fig. 3
Another view of batholithic
magma chamber construction
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Based on study of characteristics of
accessory zircon from tuffs from a
Yellowstone-type chamber
 Heise caldera cycle, eastern Idaho
 Kilgore Tuff, 1800 km3, 4.5 Ma
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Location of Heise
caldera and Yellowstone
Plateau; Yellowstone Pwave seismic tomography
High-precision U-Pb geochronology
 By isotope dilution-thermal ionization mass
spectrometry (ID-TIMS)
 Precision 0.1% = several thousand years
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Coupled with oxygen and bulk crystal Hf
isotope analyses of the same zircon crystals
 By in situ secondary ion mass spectrometry
(SIMS)
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U-Pb crystallization age 4.4876 ± 0.0023
Ma
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Indistinguishable from 40Ar/39Ar sanidine
ages
Wotzlaw et al., 2014, Fig. 1
Results
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Zircons in Kilgore Tuff display
 Significant intercrystalline and intracrystalline O isotopic
heterogeneity
 Vast majority are 18O depleted
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Interpretation
 Zircons crystallized from isotopically distinct magma batches
 That were generated by remelting of subcaldera silicic rocks
previously altered to low-δ18O meteoric-hydrothermal fluids
 Magma batches were assembled and homogenized into single big
chamber because zircons have indistinguishable U-Pb ages
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This requires that
 Shallow crustal melting, assembly of isolated batches into unified
magma reservoir, homogenization, and eruption occurred extremely
rapidly
 Within resolution of dating: thousands to tens of thousands of years
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Connection of magma batches vertically distributed over several
km would cause a substantial increase of buoyancy overpressure
 Providing an eruption trigger mechanism
 That is a direct consequence of the reservoir assembly process
Wotzlaw et al., 2014
Model for assembly over time: t1
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Middle crustal basaltic sill
complex forms hot, dry
differentiates (low-Si
rhyolites or dacites)
 By fractionation and crustal
melting
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Their intrusion triggers
remelting of
 Variably hydrothermally
altered subcaldera tuffs and
intrusions
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Colors of magma batches
 Refer to equilibrium oxygen
isotopic composition
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Note:
 Vertical exaggeration of
~10:1
Wotzlaw et al., 2014, Fig. 3a
Model for assembly over time: t2, 3
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Progressive amalgamation of magma batches by
Thermal erosion/reactive bulk assimilation and/or
Mechanical failure of intra-reservoir crust and
Convective homogenization prior to eruption
Wotzlaw et al., 2014, Fig. 3b, c
Model for assembly over time: t13
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Colors of magma batches
 Refer to equilibrium oxygen isotopic composition
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Crystal of same color as host magma
 Are in oxygen isotopic equilibrium
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Note increasing zircon oxygen isotopic
diversity with progressive assembly prior to
eruption
 While other phenocrysts equilibrate
Wotzlaw et al., 2014, Figs. 1B, 3
Lifespans of magmatic systems
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Yellowstone
Along hotspot trace: >16.6 my
Volcanic field: >2.2 my
One chamber: >2.2? my
Valles
 Volcanic field: >3.6 m.y. (perhaps 16.5 m.y.)
 One chamber: >1.79 m.y.
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Long Valley
 Volcanic field 3.5 m.y.
 One chamber: vigorous for ~0.5 m.y.
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Pantelleria
 One chamber: >0.093 my?
 Volcanic field: >0.25 my
Repose times: Proportional to
volume of magma chamber
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Yellowstone (volumes 2500, 280, 1,000 km3)
0.646 m.y. Henry’s Fork (Island Park) to
Yellowstone
0.774 m.y. Big Bend Ridge to Henry’s Fork
(Island Park)
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Valles (volumes 300, 300 km3)
0.38 m.y. Toledo to Valles
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Pantelleria (volumes ?, 3 km3)
0.038 m.y. La Vecchia to Cinque Denti
Crystallization stages of magma
chambers (which can be reversed)
A. Low-crystallinity phase
(<45 vol% xls)—Magma
 Convection
B. Medium-crystallinity phase
(~45-60 vol% xls)—Mush
 Absence of convection
 High permeability
provides favorable
window for xl-melt
separation
C. High-crystallinity phase
(>60 vol% xls)—Rigid
sponge
 Permeability too low for
efficient extraction of
high-viscosity melt
Bachmann and Bergantz, 2008, Fig. 6
Context for crystal mush model

Voluminous eruptions of zoned rhyolite like the
Bishop Tuff
 Generally start with high-silica magma poor in crystals
 Tap progressively or stepwise into less evolved
magma richer in crystals
 Many such eruptions terminate with withdrawal of
extremely crystal-rich mush (>50 % phenocrysts)

General inference follows
 Large rhyolite reservoirs do not crystallize from top
down
 Instead, water-enriched, highly evolved melt
accumulates at roof
Hildreth, 2004, Fig. 7 (detail)
 Water-rich melt progressively escapes from the
mushy, qtz-fsp (anhydrous) cumulate plutonic
reservoir below

Water enrichment
 Inhibits crystallization of high-silica roof zone, in spite
of its lower temperature
 Escape of water-rich melt possible because viscosity
range of hydrous rhyolitic melt at 700–800°C (Scaillet
et al., 1998) is far lower than formerly supposed
Crystal mush model of rhyolite melt
extraction from plutonic crystal
mush of intermediate
hybrid composition
xp = crystal-poor (0 –5%), xm = intermediate
crystal content; xr = crystal-rich (15– 55%)
Pre-climactic Long
Valley reservoir
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xp: Crystal-poor (0-5%) magma
xm: Intermediate crystal content
magma
xr: Crystal-rich (15-55%) magma
Root zones
 Depict mush columns consisting of
quartz + feldspar-rich (meltdepleted) cumulates and
 Migmatized protoliths extending to
zones of partial melting in lower
crust

Rejuvenation
 Prior to death, systems can be
rejuvenated by new arrivals of
mantle basalt
Hildreth, 2004, Fig. 7
New , central
Long Valley
focus
Dying
Glass Mtn
focus
Glass Mtn Stage, while
separating two xl-poor batches
Evidence for crystal mush model

Many zoned ignimbrites represent
volumes of high-silica melt so great that
 A proximal source volume of cumulate
mush many kilometers thick must have
underlain the segregated melt

That such mush reservoirs can persist
in granitoid plutons without solidifying
 Demonstrated by great eruptions of
‘‘monotonous intermediate’’ ignimbrite
containing 40–60% crystals and volumes
of 500–5000 km3

Also provides explanation for
uncommon but real ‘‘reversely zoned’’
ignimbrites
 Eruptions that tap marginal mush first
Hildreth, 2004, Fig. 7 (detail)
Crystal mush model and Long Valley

Evolution in time from
 Glass Mountain stage (bottom) to
 Long Valley stage (top)

Characteristics of many other stages
xp = crystal-poor (0 –5%), xm = intermediate
crystal content; xr = crystal-rich (15– 55%)
Pre-climactic Long
Valley reservoir
 Can be envisioned

Note shifting mantle-driven thermal
focus
 From beneath Glass Mountain
 To beneath central Long Valley

Upper-crustal reservoir of Glass
Mountain
 Merged with dying Glass Mountain
focus
 Which ceased being mantle-sustained
and crystallized after the calderaforming eruption

Elliptical shape of caldera
 Caused by eruption during shift
merging of upper crustal reservoirs?
 Recall “unzipping” ring fracture
Hildreth, 2004, Fig. 7
New , central
Long Valley
focus
Dying
Glass Mtn
focus
Glass Mtn Stage, while
separating two xl-poor batches
The next “supervolcano”?
Singer et al., 2014, Cover photo, GSA Today
Laguna del Maule,
Chile

Unusually large and recent
concentration of silicic eruptions
 Encircle Laguna del Maule
 230 km E of epicenter of MW 8.8
Maule earthquake of 27 Feb 2010

Since 2007, the crust at Laguna del
Maule has been inflating
 At astonishing rate of 25 cm/yr
(InSAR, GPS data)

Apparent inception of magma
system in late Pleistocene
 350 km3 of lavas and tuffs of
basaltic to rhyolitic composition
erupted during Pleistocene
 13 km3 of rhyolite erupted in last 20
k.y.

Most recent “super-eruption”
 Oruanui, New Zealand 26,500 yr
Singer et al., 2014, Fig. 1a
Geologic map

A dozen xl-poor glassy
rhyolitic lava flows
 Erupted in Holocene
(<10.k.y.)

Greatest concentration
of post-glacial rhyolite in
the Andes
 36 silicic lava flows
comprising 6.4 km3, with
a likely equivalent
volume of tuff preserved
in Argentina
 Only comparable volume
of rhyolite is >4 km3 of
rhyolite lava and tephra
at Mono Craters,
California
Singer et al., 2014, Fig. 1b, adapted
from Hildreth et al., 2010
Interpreted magma chamber

Interpretation of
magmatic system
 Feeding the crystal-poor
rhyolitic eruptions
encircling Laguna del
Maule
 Along bent cross
section of Fig. 1b

Supports observations
of
 Rapid uplift
 Shallow earthquakes
 Active intrusion of mafic
magma at depth of 5 km
 Normal faulting and
geodetic data that
record extension
Singer et al., 2014, Fig. 6,
adapted from Hildreth, 2004
Summary

One can argue that all magmatism is “fundamentally basaltic”

Magmatic system are dynamic, open systems for much of lives

Batholiths have shoulders or floors
 Silicic chambers act as a shadow zone for basaltic magmatism
 Assembling quickly, leaking out top, being replenished from below
Because they are built by arrival of silicic melt in small batches
 Large silicic magma reservoirs never rose en masse from below
 Once formed, magma may migrate en masse locally and homogenize


Repose times are roughly proportional to

Lifespans of magmatic systems

Crystal mush model

Next time: Silicic magmas interacting with water: Subaqueous
and subglacial eruptions
 The size of the magma chamber (caldera), size of major eruptions
 Volcanic fields typically ~10 m.y.; single chambers, at most a few m.y.
 Means of assembly and homogenization could trigger major eruption
 As developed for greater Long Valley region
 Successor to thermogravitational diffusion model
 Crystallization stages of magma chambers (which can be reversed)