GEOS 470R/570R Volcanology
L18, 27 March 2015

Handing out
 PowerPoint slides for today

Summary of background reading due Friday, 27 Mar 15 (25 pts);
Instructions L13, 2 Mar 15





Abstract instructions (GSA, AGU, or a journal)
Abstract with relevant content and in appropriate format
Instructions to authors for a journal
Reference list in format of a journal
Writing assignment due L19, Wednesday, 1 Apr 15 (25 pts)
 Essay

Field trip
 This weekend, e. Arizona; good weather predicted

Note
 No lecture Fri 10 Apr 15 (GeoDaze)
“The great thing in the world is not so much where we stand, as
in what direction we are moving.”
--Oliver Wendell Holmes
Readings from textbook

For L18 from Lockwood and Hazlett
(2010) Volcanoes—Global Perspectives
[None]

For L19 from Lockwood and Hazlett
(2010) Volcanoes—Global Perspectives
Chapters 2 and 9
Assigned reading

For today L18
None

For L24, 20 April 2015
Voight, B., 1990, The 1985 Nevado del Ruiz
volcano catastrophe: Anatomy and
retrospection: Journal of Volcanology and
Geothermal Research, v. 44, p. 349-386.
Last time: Composite intermediate
cones (stratovolcanoes)
Geometries of stratocones
 Examples

Mount Mazama-Crater Lake, OR
Mount Katmai-Novarupta, AK
Mount St. Helens, WA
Mount Lassen, CA
Mount Rainier, WA
Mount Adams, WA
Mount Shasta, CA
Classic cone
(vent stationary
over time)



Mt. Damavand, Iran
Elev. 5671 m
Large composite
volcano in an
intraplate setting
Davidson and De Silva,
2000, Fig. 3c
Compound volcano (vent migrated
over time)


Nonconical, ridgelike--oldest part is to the left
Irruputuncu, Chile/Bolivia
Davidson and De Silva, 2000, Fig. 6c; photo by G. Worner
Twin volcanoes


Nevados de Payachata, central Andes
Twin volcanoes
 Older glaciated Pomerape cone to the N (left)
 Younger, more symmetrical cone is Parinacota
Davidson and De Silva, 2000, Fig. 6e
Compound volcano with numerous
Holocene vents


Tongariro, North Island, New Zealand
Youngest vent is Mount Ngauruho
Davidson and De Silva, 2000, Fig. 2c
Mount Mazama
and
Crater Lake, OR

Llao Rock and
Wizard Island
E. Seedorff
Southeasterly aerial view over Novarupta
toward Trident and Mount Katmai
Katmai
Trident group
Hildreth and Fierstein, 2000, Fig. 3
Caldera at Mt. Katmai
Press and Siever, 1998, Fig. 5.8
Mount St.
Helens, WA
The climactic eruption, Mount St.
Helens, 18 May 1980




Cross section showing
catastrophic unroofing of
magma-hydrothermal
system
Before earthquake-induced
2.3 km3 rockslide, with
bulge above dacite
cryptodome
Failures of three successive
rock masses; movement of
masses I and II caused a
lateral blast, pyroclastic
surge, and plume of ash
and steam
Movement of mass III
beheaded the magma body
Moore and Rice, 1984, Fig. 10.1
The climactic eruption, Mount St.
Helens, 18 May 1980
View from Bear Meadow, 17 km NE of summit
About 14 s after initial detachment of landslide at 0832 PDT
Blast directed through scar left by landslide removal on volcano’s north flank
Christiansen and Peterson, 1981, Fig. 11; photo by Keith Ronholm
Mount Lassen, CA

Lassen volcanic center
contains about 200 km3 of
lava
 All erupted in last 0.73 my
Photo taken July 2006 by Wikipedia User: DanielSchwen
Mt. Lassen: One of only three
Cascade volcanoes with an
associated caldera

Mount Baker
 Kulshan caldera, 4.5 x 8 km
 Ignimbrite of Swift Creek and Lake Tapps tephra,
rhyodacitic,1.15 Ma

Mount Mazama
 Crater Lake caldera, 8 x 10 km
 Wineglass Welded Tuff and overlying ash-flow deposits,
rhyodacitic, 7700 cal. yr B.P.

Mount Lassen
 Unnamed caldera on N flank of Brokeoff volcano, ~6 x 6 km
 Rockland ash and ignimbrite, rhyolitic, 400 ka
Mt. Rainier, WA

Summit at 14,410 ft (4392
m)
 Highest in the Cascade
chain

Total volume 140 km3
 Main cone contributes 86
km3

Most common volcanic
deposit
 Lahar

Volcano was especially
active
 6500-4500 yr and
2500-2000 yr ago
 Sector collapses
created debris
avalanches and lahars
Mt. Adams, WA

Located along axis of the
arc
 50 km east of Mount St. Helens

Second-largest Cascade
volcano in volume
 Mt. Shasta 500 km3--comparable
to Fujiyama
 Mt. Adams 210 km3; 3 km high
Hildreth and Lanphere, 1994, Fig. 1
Mount Shasta, CA
English Wikipedia June 2006

Mt. Shasta’s cone volume is
about 350 km3
 All erupted in last 0.73 my


Debris avalanche dated at 300
ka adds 45 km3
Smaller volcanoes ringing
Shasta on three sides add
another 121 km3
 51 km3 since 1 Ma
Summary: Composite intermediate
cones (stratovolcanoes)
Geometries of stratocones
 Examples

Mount Mazama-Crater Lake, OR
Mount Katmai-Novarupta, AK
Mount St. Helens, WA
Mount Lassen, CA
Mount Rainier, WA
Mount Adams, WA
Mount Shasta, CA
Lecture 18: Petrologic applications of
volcanology to intermediate magmas

Stratovolcanoes, continued






Mount Pinatubo, Philippines
El Misti, Arequipa, Perú
Mount Damavand, Iran
El Chichón, Chiapas, México
Synthesis on stratovolcanoes, built around Mt. Adams
Heterogeneous eruptions with compositional gaps in intermediate magma
chambers
 Two proposed explanations

Petrologic review








High-K calc-alkalic intermediate
Alkalic, silica-undersaturated intermediate rocks (phonolite-trachyte)
Rhyolite / gap / zoned intermediate (“I-type magmas”)
Zoned intermediate (“I-type magmas”)
Monotonous intermediate (“I-type magmas”)
Boninites (high-Mg andesites)
Adakites (sodic andesites and dacites of trondhjemite-tonalite-granodiorite suite)
Igneous charnockites (“C-type magmas”), including pigeonite-bearing
intermediate to silicic rocks
Mount Pinatubo, Philippines
Mount Pinatubo, Philippines

Tectonic setting
Pinatubo web site
Mount Pinatubo,
Philippines

Distribution of
deposits
Eruption of 15
June 1991

Note locations of
Manila
Capital city of
Manila
Clark US Air
Force Base
Subic Bay US
Navy Base
Schmincke, 2004, Fig. 13.29
Prior to climactic eruption in 1991


Pre-eruption view from the NW of Mount Pinatubo on 16 April 1991
Steam from fumaroles created during phreatic explosions of 2 April
1991 visible
Punongbayan et al.,
1996, Fig. 2A; photo by
R. S. Punongbayan
Leading up to
climactic eruption
in 1991
 Phreatic explosion craters
created on 2 April 1991 at east
end of geothermal area
 New fumaroles created, 1 km
long
NOAA Mt Pinatubo-1991 Set, #2;
photo by C.G. Newhall, U.S.
Geological Survey
NOAA Mt Pinatubo-1991 Set, #3; photo
by R. Batalon, U. S. Air Force
Prior to climactic eruption in 1991

Photo taken in late April
 Area affected by explosions on 2 April 1991
NOAA Mt Pinatubo-1991 Set, #1; photo by C.G.
Newhall, U.S. Geological Survey
First major eruption


View to the west from Clark Air Base of the first major
eruption on 12 June 1991
This tephra column rose to an altitude of about 20 km
NOAA Mt Pinatubo-1991
Set, #5; photo by R. S.
Culbreth, U. S. Air Force
First major
eruption,
12 June 1991


View to the west
from Clark Air Base
of the first major
eruption on 12 June
1991
Pyroclastic flows
advanced 5-15 km
down the N, NW,
and SW flanks of
the volcano
NOAA Mt Pinatubo-1991 Set, #6;
photo by K. Jackson, U. S. Air Force
Mt. Pinatubo,
Philippines


Climactic eruption
on 15 June 1991
Typhoon Yunya
struck concurrent
with eruption
Pinatubo web site
Climactic eruption, 15 June 1991


View to the west from Clark Air Base
This tephra column rose to an altitude of about
30-40 km
NOAA Mt Pinatubo-1991
Set, #7; photo by R.
Lapointe, U. S. Air Force
Climactic eruption, 15 June 1991

Eruption began just before 6 am, 15 June 1991
and lasted for >15 hours
 Sent tephra 30-40 km into the atmosphere
 Generated voluminous pyroclastic flows
 Left a caldera (crater) where the former peak was,
but crater is offset to the north
 Highest elevation lowered by 145 m

“Day of darkness” stretched for 36 hrs, featuring
 Black blizzard of coarse sand-sized particles
 >50 earthquakes
 Volcanic thunder and brilliant lightning
 Orange fireballs
Pinatubo

Climactic eruption on 15 June 1991 was
second-largest eruption of the century
(after VTTS, at 12 km3 DRE)
4 to 5 km3 DRE
10 X volume erupted at Mount St. Helens in
1980
Pinatubo eruption was still “just a baby”
Rumbling aftermath

Aerial view of the
northern side of
Pinatubo crater
 Small explosion in
progress, 22 June
1991

Caldera wall
 200 m high at one
point
 Dropped to level of
the crater floor on
the eastern side
NOAA Mt Pinatubo-1991 Set, #8; photo by
R. Batalon, U. S. Air Force
Pyroclastic flow deposits from
climactic eruption

Aerial view of the NE
side of Pinatubo
 22 June 1991

Pyroclastic flow
deposits
 Locally >200 m thick
 Generally occurred in
main valleys
 Flow to distances of
12-18 km from caldera
 Caused surface
drainage diversions
 Accompanied by ash
clouds, whose
deposits were X-X0
cm thick
NOAA Mt Pinatubo-1991 Set, #9; photo by
R.P. Hoblitt, U. S. Geological Survey
Pinatubo crater

Aerial view to the south of Pinatubo crater
 2.5 km wide
 Start of a small explosion 1 August 1991

Declining activity continued to October 1991 and beyond
NOAA Mt Pinatubo-1991 Set,
#10; photo by T.J. Casadevall,
U. S. Geological Survey
Lake Pinatubo

View to the
south
 Crater lake of
Pinatubo on 10
September 1991

Steam
emissions
 Along southern
wall

Fresh landslides
 Along steepsided crater
walls
NOAA Mt Pinatubo-1991 Set, #11; photo
by T.C. Pierson, U. S. Geological Survey
Inside crater of Mt. Pinatubo
Press and Siever, 2001, Fig. 5.31
Summit crater and lake

View from the
NW
 On 5 October
1992
 Mt. Negron in
the background

Partly
submerged
relicts (rocky
isles) of a dome
 Grew from July
to October 1992
Punongbayan et al., 1996, Fig. 2B; photo
by R. S. Punongbayan
Lahar
NOAA Mt Pinatubo-1991 Set, #17; photo by T.J. Casadevall, U. S.
Geological Survey

Scientists observe lahar (hot mudflow) in the
Sacobia Valley below the Mactan Gate of
Clark Air Base on 14 August 1991
Lahars reworking pyroclastic flows

Aerial view of the
Sacobia River
drainage
 Pyroclastic flow
deposits are being
partially reworked
by lahars
 On 15 August

Five days later
 55,000 people
evacuated
because of more
mudflows
NOAA Mt Pinatubo-1991 Set, #16; photo by T.J.
Casadevall, U. S. Geological Survey
Mount Pinatubo lahars

Two hot lahars, marked by trains of steam plumes,
flowing down forks of the Marcella River
 19 September 1991
Newhall and Punongbayan, 1996, p. 894
Mudflows

Aerial view of the Acaban
River channel
 As it passes through
Angeles City near Clark Air
Base on 12 August

Mudflows caused
collapse of main bridges
 Note makeshift bridges for
pedestrians at lower left
NOAA Mt Pinatubo-1991 Set, #16; photo
by T.J. Casadevall, U. S. Geological
Survey
Mount Pinatubo lahars


Looking east across the Central Plain of Luzon, about
20 km northwest of summit of Mount Pinatubo
Lahars have cut across and dammed a stream, forming
a lake that has flooded several farms
Fisher et al., 1997, Fig. 6-5b; photo by G. Heiken
More flooding

Aerial view of flooding
of the village of Santa
Rita de Concepción
 On 23 July 1991

Main flow of Bamban
River has migrated
from its channel
 Causing extensive
flooding downstream
of Pinatubo
NOAA Mt Pinatubo-1991 Set, #19; photo
by T.J. Casadevall, U. S. Geological
Survey
Mount Pinatubo mass balance
Fisher et al., 1997, Fig. 6-4
Setting of El Misti, near Arequipa,
Perú
Thouret et al., 2001, Fig. 1
Pliocene Sillar

Vapor-phase-altered,
unwelded tuff
 Great material for
building stone
E. Seedorff, 1993
E. Seedorff, 1993
Pliocene Sillar

Lithic fragments in
vapor-phase-altered
unwelded tuff
E. Seedorff, 1993
E. Seedorff, 1993
Compound volcanoes near Misti
Volcán Chachani
Volcán Pichupichu
E. Seedorff, 1994
E. Seedorff, 1994
El Misti, Perú
Thouret et al., 2001, Fig. 2A
El Misti, Perú: Debris avalanche

Prehistoric debris avalanche
Note subangular shapes and variety of clast
sizes
Lockwood and Hazlett, 2010, Fig. 11.2
El Misti, Perú


Volcán El Misti in the distance
Río Chili, which drains El Misti, running through the city
of Arequipa
E. Seedorff, 1993
Mt. Damavand,
Iran

Large composite volcano





Elev. 5671 m
Volume >400 km3
Located near Tehran
Currently dormant
Intraplate setting
 Trachyandesite
composition

Present cone (Young
Damavand)
 Has been constructed over
~600 k.y.
 Built of mostly lava flows
 Only one significant
pyroclastic event
recognized
Davidson and de Silva, 2000, Fig. 3c
El Chichón, Chiapas, México

Andesitic dome field, with summit at 1350 m above sea level
 Oval crater partially filled by lava dome and a lake

Eruption on 28 March 1982 had largest impact on climate since
Krakatau in 1883
 Plinian column to 24 km; SO2-rich; new crater 1 km across
 Anhydrite-bearing trachyandesite pyroclastic flow deposits and surges
El Chichón, México
Dunes formed by surge deposits
Fisher, 1999, Fig. 20
El Chichón, México
Lake on floor of new crater, rimmed by
fumaroles and yellow sulfur beach
Fisher, 1999, Fig. 21
A petrologic
return to Mt.
Adams, WA


Located 50 km east of Mount
St. Helens
Second-largest Cascade
volcano in volume
 Mt. Shasta 500 km3-comparable to Fujiyama
 Mt. Adams 200 km3; 3 km high

Volumetric dominance of
central andesite
 Dominantly effusive;
pyroclastic flow deposits are
rare (cf. with dacites of Mount
St. Helens)

Inception of central
stratovolcano at ~520 ka
 Andesites emplaced in three
main cone-building stages at
~500, 450, 30 ka
Mt. Adams, WA

No historical eruptions in the Mount Adams volcanic
field
 Most of the eight Holocene eruptive units mapped are older than
a distinctive layer erupted from Mount St. Helens ~3500 yr ago
 Last main episode of cone construction occurred ca. 40 – 10 ka

A corridor of vents, 6 km wide, extends for 50 km N-S
 Nearly every vent erupted inside corridor is basalt or olivine
andesite
 Within 5 km of the summit, only a single basalt is known to have
penetrated the andesitic focus in 500 ky

Original focus of volcano was 5 km to the SE
 Middle Pleistocene andesite-dacite edifice of the Hellroaring
volcano
Hildreth and Lanphere, 1994
Eruptive style

Glacial ice cap for summit eruptions
 Fragmental deposits: phreatomagmatic and steam-blast
eruptions

Spatter and scoria: lava fountaining and Strombolian
activity
 Scoriaceous lavas that overflowed the summit rim lost
coherence on steep slopes—lava-debris avalanches and blockand-ash flows


Pyroclastic flow deposits are rare
Basalt and olivine andesite erupted peripheral to the
main cone
 Produced ~35 cinder cones and constructed several shields
Hildreth and Lanphere, 1994
Composition

Central vent eruptions
 Phenocryst-rich pyroxene andesite (56-62% SiO2)

Flank eruptions
 Olivine andesite (53-57% SiO2) and pyroxene dacite (63-68.5%
SiO2)

No Quaternary product of the volcanic field has
phenocrysts of qtz, san, or bio
 Amph present in only two peripheral lava flows


All basaltic units have oliv; many have plag and cpx
Originally thought to be monotonously andesitic
 But eruptive products range from 52-68.5% SiO2 on main cone
and 47-61% SiO2 on the periphery
 Basalt ranges from alkalic to low-potassium varieties
 Dacite erupted 18 times, mostly on flank vents, but predating
construction of modern summit cone
 Rhyolite is absent
Hildreth and Lanphere, 1994
Implications for arc volcanic
systems
1) Growth spurts
 Stratovolcanoes commonly grow in spurts
2) Intervening periods
 May stay active between episodes of peak activity
 Never really shut down
 “Dormancy” is not geologically meaningful
3) Stages
 Eruptive or constructional “stages” should be maintained
skeptically without
 Detailed mapping
 Extensive compositional data
 Extensive geochronology
Hildreth and Lanphere, 1994, p. 1426
Implications for arc volcanic
systems, II.
4) Longevity
 Large stratocone systems can remain active for
half a million years
 Documented examples of greater longevity are rare
 Claims require geochronology linked with
stratigraphic verification of integrity of system

Cascade comparisons
 Baker, Rainier, Adams, Hood, Mazama, Shasta
probably lasted >300 ky
 Mount St. Helens dates from ~36-40 k.a.
Hildreth and Lanphere, 1994, p. 1426
Implications for arc volcanic
systems, III.
5) Eruptive patterns
 Stratocone systems display a wide spectrum of
recurrence time scales
 At various times in their lives, a single cone may
exhibit a variety of eruptive frequency patterns
6) Volumetric eruption rates
 Can be misleading, depending on the time and
spatial scales of interest
 Average rates do not have much meaning in light of
variability in eruptive patterns
 Eruptive rates important for some hazards
 Storage rates relevant for geothermal resources
Hildreth and Lanphere, 1994, p. 1426
Implications for arc volcanic
systems, IV.
7) Focal region versus periphery
 Activity in andesite-dacite focus coexisted with peripheral
(“parasitic”) basaltic activity
 Peripheral activity is unrelated to maturity of the
stratocone
 Recommends that parasitic be abandoned; implies that
peripheral eruptions are leaks from central conduit (generally not)
 The opposite view may be more valid: the peripheral basalts are
more fundamental; the andesite-dacite focus may be derivative
8) Relationship to large magma chambers
 Stratovolcanoes need not develop large upper-crustal
magma chambers
 There is no standard sequence of magmatic
compositions
 There is no unidirectional progression
Hildreth and Lanphere, 1994, p. 1426
Compositional gaps

Inverted stratigraphy but with discontinuities in
composition, e.g., zoned rhyolitic compositions,
but skip abruptly to dacite
 In spite of thermal continuity indicated by
geothermometry


e.g., Katmai/Novarupta, AK, Valley of Ten
Thousands Smokes Tuff
Also discussed earlier with respect to rhyolite
domes
 “Quenched blobs” of mafic magma in rhyolitic magma
Eruptive
types vs.
SiO2
content

Note
compositional
gaps,
especially in
IV and V
Hildreth, 1981, Fig. 1
One interpretation for the origin of
compositional gaps

Interpretation by Hildreth (1981) and
others
The zonation developed over time as a result
of protracted fractionation (differentiation) in a
subvolcanic magma chamber
Some mechanism, such as sidewall
crystallization with efficient expulsion and
collection of melt, produces a gravitationally
stable, silicic over mafic layering
Evolution of silicic magma chambers
as a function of tectonic environment

Time 
Tectonic
extension, if any,
is subordinate
and shallow
 Abundant
intermediate
magmatism
 Island arcs,
continental arcs,
continental
interior systems
Wohletz and Heiken, 1992, Fig. 1.10,
adapted from Hildreth, 1981, Fig. 15
Early stage
Intermediate stage
Mount St. Helens
Mazama prior to
formation of Crater
Lake caldera
An alternate view for the origin of
compositional gaps

From John Eichelberger, a long-time proponent of the
importance of magma mingling in the origin of
intermediate magmas
 Eichelberger, J. C., Chertkoff, D. G., Dreher, S. T., and Nye, C.
J., 2000, Magmas in collision: Rethinking chemical zonation in
silicic magmas: Geology, v. 28, p. 603-606.


Proposal: Heterogeneous eruptions represent magma
mixing captured “in the act” of homogenization
Heterogeneous eruptions triggered by either of two
types of mixing events
 1) Mafic magma intrudes an intermediate magma chamber
 2) Silicic magma intrudes an intermediate magma chamber
1) Mafic magma intrudes a magma
chamber of intermediate composition

Dike of mafic magma intrudes a subvolcanic magma
chamber of dacitic or andesitic “slush”
 Accounting for widely recognized case of “quenched blobs”:
Mafic enclaves in silicic host lavas and granitoids
Mount Dutton, Alaska
Magma mingling occurs throughout
a dome lava on the cm-scale
Eichelberger et al., 2000, Fig. 1a, b
Whole-rock and melt (glass)
compositions of silicic and mafic
components at Mount Dutton

Similarity of melt
compositions, and
large difference
between whole-rock
and melt
compositions within
enclaves
 Interpreted to reflect
thermal equilibration
between mafic
enclaves and dacitic
host
Eichelberger et al., 2000, Fig. 1c
Model for the behavior


Dense intruder spreads like lava across the
chamber floor
Ponding mafic magma acts like a rising piston
 Expelling modestly contaminated hybrids from the
top of the chamber

Slow expulsion of the reservoir magma allows it
to reach the surface in a volatile-poor, nonexplosive condition
 Consistent with strong association between enclave
occurrence and effusive eruptive behavior
Eichelberger et al., 2000
Time scales

Preservation of chemical disequilibrium in
phenocrysts
Requires that the time between mixing and
eruptive quenching is short

At Mount Pinatubo in 1991, deep, lowfrequency seismicity beneath the crustal
chamber was followed in a week by
extrusion of lava bearing chilled enclaves
Interpretation: Mafic recharge of crustal
reservoirs triggers eruptions
Pallister et al., 1992; Eichelberger et al., 2000
2) Silicic magma intrudes a magma
chamber of intermediate composition

Dike of silicic magma intrudes a subvolcanic magma chamber of
andesitic “slush”
 Accounting for less commonly observed case in granitoids of
composite dikes
Aniakchak caldera, Alaska, where
ignimbrite is zoned from xl-poor
rhyodacitic base (light) abruptly to
andesite + rhyodacite (dark layer)
Banded pumices with millimeterscale dark and light fluidal bands
that show marble-cake mingling,
rather than discrete blobs
Eichelberger et al., 2000, Fig. 2a, b
Aniakchak caldera, AK


Location 670 km SW of Anchorage
Map of Aniakchak caldera
 Contains small lake today, Lake Surprise
 Drainage to Aniakchak River is via The Gates
Waythomas et al.,
1996, Fig. 1
Aniakchak
caldera,
Alaskan
Peninsula, AK

Caldera diameter
 ~10 km

Caldera-forming
eruption
 3400 yr BP

>50 km3 of tephra
and pyroclastic flows
Oblique aerial photograph
taken 9 May 1943, looking to
the southeast; photo from
the U. S. National Archives
Whole-rock and melt (glass)
compositions of silicic and mafic
components at Aniakchak caldera

Large difference
in melt
compositions
Interpreted to
indicate a lack of
thermal
equilibrium
 Requires that
magmas were not
stored in contact
Eichelberger et al., 2000, Fig. 2c
Time scales

Lack of thermal equilibrium requires that
voluminous magma batches of different
compositions encountered each other
syneruptively
 Limited mingling
 Almost instantaneous quenching


Interpreted that crystal-poor silicic magma was
fed into an intermediate chamber by wide dikes
Each influx accompanied by a counterflow of
denser reservoir magma down the silicic feeder
 Presume that silicic magma would flow rapidly toward
the roof of the chamber (like a lava lamp)
Eichelberger et al., 2000
Drawing of two different types of opensystem behavior of arc magmas
Mount Dutton case
Aniakchak caldera case
Eichelberger et al., 2000, Fig. 3
Compositions of stored versus intruded
magma
Encounters far from Y = X
 Mafic replenishment
 Mount Dutton example
 “Quenched blobs” of
enclave

Silicic replenishment
 Aniakchak example
 Marble-cake mingling
Encounters near Y = X
 Mafic replenishment
 Mid-ocean ridgechambers

Silicic replenishment
 Large silicic chambers
like Bishop and
Bandelier Tuffs?
Eichelberger et al., 2000, Fig. 4
High-K calc-alkalic intermediate

Definition
 K2O vs. SiO2 diagram

Mineralogy
Typical: Plag, Bio, Cpx, Opx, Mt, Ilm, Zir, Ap,
Po
Occasional: Qtz, San, Hbd, Ol, Sph, Anh
(rare?)
High-K calc-alkalic intermediate

Other characteristics
Basalt uncommon
“Bimodal” compositional distribution common:
rhyolite + dacite/andesite common
High-K calc-alkalic intermediate

Examples—volcanic rocks
El Chichón, Chiapas, México
Egan Range lavas and Kalamazoo Tuff, NV
Richmond Mountain andesite, Eureka, NV
Montana, Eocene
SW Pacific
High-K calc-alkalic intermediate

Examples—hypabyssal and plutonic rocks
British Columbia, Triassic-Jurassic
Bingham, UT, late Eocene
Robinson, NV, mid-Cretaceous
High-K calc-alkalic intermediate

Tectonic setting
Continental settings
Most common in more mature arcs,
especially at greater distances from margin
Also commonly erupted concurrent with rapid
crustal extension in eastern Great Basin
Alkalic, silica-undersaturated
intermediate volcanic rocks (phonolitetrachyte)

Definition
Strongly silica-undersatured alkalic centers

Mineralogy
San, Plag, Hauyne, Amph, Cpx (including
aegerine-augite), Sph, Ap, Mt, Bio, Neph,
Cancr, Zir, Leuc
Alkalic, silica-undersaturated
intermediate volcanic rocks (phonolitetrachyte)

Other characteristics
Small systems
Appear to fractionate from basanites (strongly
silica-undersaturated basalts)
Alkalic, silica-undersaturated
intermediate volcanic rocks (phonolitetrachyte)

Examples—volcanic rocks
Laacher See, Eifel, Germany (phonolite)
Tenerife, Canary Islands (phonolite)
Leucite Hills, WY (phonolite)
Guffey volcanic center, Thirtynine Mile
volcanic field, CO (trachybasalt to trachyte)
Fogo A, Sao Miguel, Azores (trachyte)
Campi Flegrei, Naples, Italy (trachyte)
Roccamonfina, Italy (trachyte)
Alkalic, silica-undersaturated
intermediate volcanic rocks (phonolitetrachyte)
Examples—hypabyssal and plutonic rocks
(nepheline syenites and associated fenites)

Shonkin Sag laccolith, Montana (shonkinite:
Ksp, Neph, Cpx)
Alkalic, silica-undersaturated
intermediate volcanic rocks (phonolitetrachyte)

Tectonic setting
Continental settings far inboard from arc with
weak extension
Oceanic islands
Rhyolite / gap / zoned intermediate
(subset of “I-type magmas”)

Definition
Has high-silica rhyolite but with a large
compositional gap between it and the
dominant volume of intermediate magma

Mineralogy
Qtz, Plag, Opx, Mt, Ilm, Ap, Po, Cpx, Ol
Rhyolite / gap / zoned intermediate
(subset of “I-type magmas”)

Other characteristics
Eruption of VTTS displays compositional gap
from high-silica rhyolite to dacite/andesite
High-silica rhyolite filled the Novarupta vent
after eruption was complete
Rhyolite / gap / zoned intermediate
(subset of “I-type magmas”)

Examples—volcanic rocks
Valley of Ten Thousand Smokes Tuff,
Katmai-Novarupta, AK 1912
Chaitén, Chile 9400 yr ago
Rhyolite / gap / zoned intermediate
(subset of “I-type magmas”)

Examples—hypabyssal and plutonic rocks
Dioritic stocks?
Rhyolite / gap / zoned intermediate
(subset of “I-type magmas”)

Tectonic setting
Arcs
Zoned intermediate (subset of
“I-type magmas”)

Definition
Dominant volume is silicic but lacks highsilica rhyolite; thermally continuous
compositional gap generally present between
rhyodacite and andesite

Mineralogy
Typical: Plag, Opx, Hbd, Mt, Ilm, Ap, Po
Occasional: Cpx, Ol
Zoned intermediate (subset of
“I-type magmas”)

Other characteristics
Eruptions from stratovolcanoes or from
calderas in stratovolcano clusters (Mazama)
Zoned intermediate (subset of
“I-type magmas”)

Examples—volcanic rocks
 Shikotsu, Japan
 Mazama (Crater Lake), OR
 Aniakchak, AK
 Aso-4, Japan
 Krakatau, Indonesia 1883
 Quizapu, Chile 1932

Small volume counterparts?
 Pinatubo
 St. Helens
Zoned intermediate (subset of
“I-type magmas”)

Examples—hypabyssal and plutonic rocks
Diorite stocks and small granodiorite plutons
Zoned intermediate (subset of
“I-type magmas”)

Tectonic setting
Continental arcs
Monotonous intermediate
(subset of “I-type magmas”)

Definition
Rhyodacite that is weakly zoned
compositionally

Mineralogy
Typical: Plag, San, Cpx, Opx, Qz, Mt, Ilm
Occasional: Bio, Hbd
Monotonous intermediate
(subset of “I-type magmas”)

Other characteristics
Huge volumes erupted from large calderas
Monotonous intermediate
(subset of “I-type magmas”)

Examples—volcanic rocks
 Fish Canyon Tuff, La Garita caldera, CO
 Snowshoe Mountain Tuff, Creede caldera, CO
 Blue Creek Tuff (concealed caldera), CO
 Cebolla Creek Tuff, San Luis caldera complex, CO
 Cottonwood Wash Tuff (31.0 Ma), NV/UT
 Wah Wah Springs Tuff (30.2 Ma), NV/UT
 Lund Tuff (29.0 Ma), White Rock caldera, NV/UT
 Monotony Tuff, NV
 Tuff of Mount Jefferson, NV
 Loma Seca Tuff, Calabozos caldera, Chile
 Cerro Galan Tuff, Cerro Galan caldera, Argentina
Monotonous intermediate
(subset of “I-type magmas”)

Examples—hypabyssal and plutonic rocks
Large granodioritic batholiths
Monotonous intermediate
(subset of “I-type magmas”)

Tectonic setting
Mature continental arcs on thick crust
Continental settings with thick crust but with
less certain tectonic settings
Boninites (high-Mg andesites)

Definition
Low- or medium-K series, dominantly
andesite (~56% SiO2) but with high
Mg/(Mg+Fe)
Has high MgO (>8 wt %) and low TiO2 (<0.5
wt %) contents
Very depleted in incompatible elements

Mineralogy
Typical: Opx (orthorhombic and monoclinic
enstatites), Ol (Mg-rich), Cpx (calcic)
Boninites (high-Mg andesites)

Other characteristics
Commonly glassy lavas
Clinoenstatite phenocrysts characteristic;
more fractionated magmas have opx
Low-Ca px always >> oliv
Low-Ca px may be clinoenstatite,
orthopyroxene (enstatite-bronzite),
magnesian pigeonite, or all three
Order of crystallization magnesiochromite,
olivine, low-Ca px, calcic px, + amph, ± plag
Boninites (high-Mg andesites)

Examples—volcanic rocks
Chichi-jima, Bonin Island, Japan
Cape Vogel, Papua New Guinea
Various magnesian andesites from México,
Chile, Aleutian Islands
Boninites (high-Mg andesites)

Examples—hypabyssal and plutonic rocks
Uncertain
Boninites (high-Mg andesites)

Tectonic setting
Initiation of arcs
Adakites (sodic andesites and dacites
of trondhjemite-tonalite-granodiorite
suite)

Definition
Silica-oversaturated sodic dacites and
andesites (SiO2 ≥56%)
Associated with high-Nb basalts

Mineralogy
Typical: Plag, Hbd, Bio, Ap, Zir, Sph, Mt
Occasional: Opx, Cpx (rare; only in mafic
andesites)
Adakites (sodic andesites and dacites
of trondhjemite-tonalite-granodiorite
suite)

Other characteristics
 Very low HREE and Y concentrations
 High La/Yb ratios
 High Sr/Y


Interpreted to result from partial melting of
metabasalt during transition from amphibolite to
eclogite (garnet present, plagioclase absent)
Requires T >700°C at shallow depths (75-85
km)
Sajona et al., 1993
Adakites (sodic andesites and dacites
of trondhjemite-tonalite-granodiorite
suite)

Examples—volcanic rocks
Adak Island, central Aleutian arc, AK
Vizcaino Peninsula, southern Baja California,
México
Mindanao, Philippines
Cayambe volcano, Ecuador
Mount St. Helens, WA?
Mount Pinatubo, Philippines?
Adakites (sodic andesites and dacites
of trondhjemite-tonalite-granodiorite
suite)

Examples—hypabyssal and plutonic rocks
San Jacinto intrusive complex, Peninsular
Range batholith, CA
Great Tonalite sill, Ruby Range batholith,
Skagway orthogneiss, Coast Batholith, AK
and BC
Famatinian belt, NW Argentina
Trondhjemite-tonalite-granodiorite suites
worldwide, including in Precambrian
greenstone belts
Adakites (sodic andesites and dacites
of trondhjemite-tonalite-granodiorite
suite)

Tectonic setting
Melting of downgoing slab during subduction
of hot, young crust (e.g., Defant and
Drummond, 1990)
Melting of subducting oceanic crust during
onset of subduction (e.g., Sajona et al., 1993)
Flat subduction (Gutscher et al., 2000)
Igneous charnockites (“C-type
magmas”), including pigeonite-bearing
intermediate to silicic rocks

Definition
Intermediate to silicic rocks with high
crystallization temperatures

Mineralogy
Typical: Plag (unusually potassic), Cpx
(augite, pigeonite, inverted pigeonite), Bio,
Ap, Ilm, Mt, Zir
Occasional: San (unusually calcic), Qtz, Opx
(hypersthene), Fa
Igneous charnockites (“C-type
magmas”), including pigeonitebearing intermediate to silicic rocks

Other characteristics
High abundances of K2O, TiO2, P, LIL
elements and low CaO for a given level of
SiO2
Generally lack hbd
Igneous charnockites (“C-type
magmas”), including pigeonitebearing intermediate to silicic rocks

Examples—volcanic rocks
 “Tholeiitic andesite/ferrolatite” of the eastern Snake
River Plain, ID (small volumes compared to basalts)
 Bruneau-Jarbidge area, NV/ID, esp. 9.5 – 10.5 Ma
(Yellowstone younger)
 Middle Proterozoic Yardea Dacite of Gawler Range
Volcanics, South Australia
 Cretaceous Etendeka quartz latites and Lebombo?
rhyolites of Karoo Province, South Africa?
 Lower Cretaceous volcanics, Paraná Province, South
America?
Igneous charnockites (“C-type
magmas”), including pigeonitebearing intermediate to silicic rocks

Examples—hypabyssal and plutonic rocks
Ardery charnockitic intrusions, Windmill
Islands, Antarctica
Intrusions in various other ArcheanProterozoic granulite terrains, e.g., Limpopo
belt (Africa), Musgrave Ranges (Australia)
Igneous charnockites (“C-type
magmas”), including pigeonitebearing intermediate to silicic rocks

Tectonic setting
Tend to occur in areas of flood basalts and
plateau basalts
Thought to form by partial fusion of relatively
dry (hbd-poor) crustal lithologies (granulite)
Summary

Stratovolcanoes, continued





Mount Pinatubo, Philippines
El Misti, Arequipa, Perú
Mount Damavand, Iran
El Chichón, Chiapas, México
Synthesis on stratovolcanoes, built
around Mt. Adams









Growth spurts
Intervening periods
Stages
Longevity
Eruptive patterns
Volumetric eruption rates
Focal region versus periphery
Relationship to magma chambers
and plutons
Heterogeneous eruptions with
compositional gaps—two proposed
explanations
 Developed over time as a result of
protracted fractionation
 Represent magma mixing captured “in
the act” of homogenization

Petrologic review
 High-K calc-alkalic intermediate
 Alkalic, silica-undersaturated
intermediate rocks (phonolite-trachyte)
 Rhyolite / gap / zoned intermediate (“Itype magmas”)
 Zoned intermediate (“I-type magmas”)
 Monotonous intermediate (“I-type
magmas”)
 Boninites (high-Mg andesites)
 Adakites (sodic andesites and dacites
of trondhjemite-tonalite-granodiorite
suite)
 Igneous charnockites (“C-type
magmas”), including pigeonite-bearing
intermediate to silicic rocks
Next time: Contrasting settings of mafic magmatism (flood basalts; oceanic
islands; mid-ocean ridges); mafic shield volcanoes