GEOS 470R/570R Volcanology
L04, 26 January 2015

Handing out
Today’s PowerPoint slides
“It is the mind that rules the body.”
--Sojourner Truth
More on careers and geology
“At the age of twenty, with a great deal of luck, I stumbled
upon the profession of geology, and for nearly fifty years
I have courted the earth, roaming its deserts and
forests, its mountains and volcanoes.
“I was often alone with the elements, extracting stories
from the rocks, making new discoveries and building on
those of my contemporaries and predecessors.
“It has been the most enjoyable of occupations, solving
unsolved mysteries of the earth and thinking of new
ideas—the lifeblood of science.”
--Richard V. Fisher, 1999, Out of the Crater: Chronicles of
a volcanologist
Study this diagram frequently for
the lab, first field trip, and exams
McPhie et al., 1993, Fig. 33
Superstition Mountains
Readings from textbook

For L04 from Lockwood and Hazlett
(2010) Volcanoes—Global Perspectives
Chapters 2 and 3

For L05 from Lockwood and Hazlett
(2010) Volcanoes—Global Perspectives
Chapters 6 and 9
Assigned reading

For L04 today, 26 January 2015
 Hildreth, W., 1981, Gradients in silicic magma
chambers: Implications for lithospheric magmatism:
Journal of Geophysical Research, v. 86, p. 10,15310,192.

For L05, 30 January 2015
 Metz, J. M., and Mahood, G. A., 1985, Precursors to
the Bishop Tuff eruption: Glass Mountain, Long
Valley, California: Journal of Geophysical Research,
v. 90, p. 11,121-11,126.
Last time

Volatiles
Undersaturation vs. saturation
Solubility (saturation limit)

Solubility controls
H2O
CO2
S
Cl
F
Differences in proportions of volatiles
as a function of magma composition
Basaltic
CO2
H2O
Rhyolitic
H2O
SO2
F
Cl
Schmincke, 2004, Fig. 4.18
CO2
SO2
Solubility

Maximum amount of a species or component
that can be dissolved under a given set of
conditions
P
 T°
 X (melt composition)

If melt contains less than maximum amount
possible at given conditions, then the melt is
undersaturated with respect to that volatile
component
 Magmas are not necessarily volatile-saturated
Dissolution of H2O forming OHgroups




Dissolved oxygen forming hydroxyl ions
Breaking O-Si-O polymers
Reducing degree of polymerization of melt
Predicted effect on viscosity?
Best and Christiansen, 2001, Fig. 4.8
Concentration of water as a
function of magma composition
Schmincke, 2004, Table 4.1, after Fisher and Schmincke, 1984
Solubility of CO2

Bulk melt
compositional effect
for solution
mechanism
 CO2 dissolves in
rhyolitic melt as CO2
molecules
 CO2 dissolves in
basaltic melt as a
dissolved carbonate
ion
 Intermediate
compositions have
both species
Wallace and Anderson, 2000, Fig. 2
Sulfur solubility

S more soluble at high
oxygen fugacities
 Where dissolved sulfur
occurs as sulfate
 Anhydrite present

Solubility of sulfur
increases with
increasing temperature
 Under both oxidizing
and reducing conditions

Important temperature
effect
 What is relationship to
composition of
magmas?
Wallace and Anderson, 2000, Fig. 5
Summary: Volatiles

Volatile: Element or compound that forms a gas at low P
and T°
 Typically a multicomponent system in nature (mixed volatiles)



Conditions: Undersaturated vs. saturated
Solubility: Saturation limit
Solution mechanisms differ widely; can be a function of
 Amount of volatile dissolved (H2O)
 Bulk silicate melt composition (CO2, S, Cl, F)
 Oxidation state / oxygen fugacity (S)

Solubility of pure volatiles is a function of many factors
 Pressure (H2O)
 Bulk silicate melt composition, including other volatiles (H2O,
CO2, S)
 Oxidation state / oxygen fugacity (S)
 Presence of separate sulfide phase (S)
Lecture 04: Petrologic overview

Chemical characterization and classification of
volcanic rocks
 Individual rocks




Normative mineralogy
Silica content
Silica saturation
Alumina saturation
 Rock suites





Volcanic rocks as samples of magma chambers
Physical sorting in pyroclastic eruptions
Types of zonation
Compositional gaps
Enrichment factors
Silica content

Ultramafic

IUGS divisions commonly
followed for ultramafic to
andesite

No agreement on terms for
silicic rocks
 <45 wt% SiO2

Basalt
 45 – 52%

Basaltic andesite
 52 – 57%

Andesite
 57 – 63%

Dacite
 63 – 68%

Rhyodacite (quartz latite)
 68 – 72%

Rhyolite
 72 – 75%

High-silica rhyolite
 75 – 77.5%
 IUGS has only two terms for
SiO2 > 63 wt% (dacite and
rhyolite)
 Many people who work on
non-alkalic silicic rocks use a
subdivision similar to what is
at left
Normative minerals
Convert the chemical composition of a
rock into hypothetical assemblage of
water-free, standard minerals using a
standard calculation scheme
 Normative composition, or CIPW norm

Cross, Iddings, Pirsson, Washington

Purpose
Facilitate comparison of rocks using their
chemical analyses
Norms

Hypothetical minerals based on simple molecular end
members of complex solid solutions, e.g.,








Hy for hypersthene
Di for diopside
Q for quartz
An for anorthite
The minerals that would be present if there were no
solid solutions
Normative calculation emphasizes concentration of SiO2
relative to oxides of K, Na, Ca, Mg, and Fe
You can use norms to classify rocks
What solid solution series (minerals) contain the above
normative minerals?
Concept of degree of silica
saturation


SiO2 is most abundant component of igneous rocks
Looking at system with only O, Si, Al, and Na
Best and Christiansen, 2001, Fig. 2.14
Degree of silica saturation

Silica-oversaturated (e.g., rhyolite, granite)
Contain Q in norm

Silica-saturated (e.g., andesite, diorite)
Contain Hy but lack Q, Ne, or Ol

Silica-undersaturated (e.g., phonolite,
nepheline syenite)
Contain Ol and possibly Ne
These normative definitions have analogs in
other types of classification schemes
Concept of degree of alumina
saturation
Alumina (Al2O3) is the second most
abundant component of igneous rocks
 Alumina saturation index

Molecular ratio of
Al2O3 / (K2O + Na2O + CaO)
Ratio = 1 in feldspars and feldspathoids
Any excess or deficiency must be
accommodated in mafic or accessory
minerals

Important for classification of silicic rocks
Alumina saturation
Best and Christiansen, 2001, Fig. 2.15
Peraluminous rocks

Alumina-oversaturated rocks
Al2O3 / (K2O + Na2O + CaO) > 1

Excess alumina is accommodated in
Micas (Al-rich biotite and muscovite)
Other aluminous minerals, e.g., andalusite,
cordierite, almandine-spessartine garnet,
tourmaline

After allocation of CaO for apatite, contain
normative corundum, C
Metaluminous rocks

Alumina-undersaturated rocks that do not have
excess alkalis
Al2O3 / (K2O + Na2O + CaO) < 1
and
Al2O3 / (K2O + Na2O) > 1


Deficiency in alumina is accommodated in
minerals such as hornblende, Al-poor biotite,
titanite
After allocation of CaO for apatite, contain
normative anorthite, An, and diopside, Di (or
wollastonite, Wo)
Peralkaline rocks

Alumina-undersaturated rocks that have excess
alkalis
Al2O3 / (K2O + Na2O + CaO) < 1
and
Al2O3 / (K2O + Na2O) < 1

Deficiency in alumina and excess in alkalis is
accommodated in alkali mafic minerals such as
aegirine, riebeckite, richterite, aenigmatite
 Mafic minerals with alkalis, Fe2O3, and TiO2
substitute for Al2O3

After allocation of CaO for apatite, contain
normative acmite or sodium metasilicate, Ac or
Ns, and lack normative anorthite, An
Subdivision of peralkaline rocks
Comenditic
Al2O3 > 1.33*FeO + 4.4

 Rhyolitic  Comendite
 Trachytic 
Comenditic trachyte
Pantelleritic (Fe-rich)
Al2O3 < 1.33*FeO + 4.4

Le Maitre, 2002, Fig. 2.18; after
Macdonald, 1974
 Rhyolitic 
Pantellerite
 Trachytic 
Pantelleritic trachyte
Rogers and Hawkesworth, 2000, Fig. 1
Alumina saturation
Best and Christiansen, 2001, Fig. 2.15
Problems in practice
Hard to analyze alkalis precisely
 Alkalis are easily removed by

Hydration
Vapor phase alteration
Hydrothermal alteration
TAS
diagram

Central
portion
divided into
sodic and
potassic
suites
sodic
Le Maitre, 2002, Fig. 2.14
potassic
TAS diagram


Analyses of
rocks from
three suites
of rocks
Alkaline
 Tristan da
Cunha
oceanic
island

Subalkaline
 Andean
continental
arc
 Tongan
island arc
Best and Christiansen, 2001, Fig. 2.16
Suites of rocks
Previous diagrams are used to classify
individual rocks based on their chemical
composition
 In contrast, the next few diagrams are
used to

To display compositions of suites
To classify suites of rocks
Harker variation diagram



Plot of abundance of one chemical component against another
Here, major elements shown for Crater Lake
Danger: Plotting genetically unrelated rocks but interpreting them as if
they were related
Carmichael et al.,
1974, Fig. 2-1
Harker variation diagram with trace
elements

Ta vs. Th of several large
eruptive units
 P = Pantelleria
 DC = Tuff of Devine
Canyon
 TT = Tala Tuff,
Primavera
 LCT = Lava Creek Tuff,
Yellowstone
 HRT = Huckleberry
Ridge Tuff, Yellowstone
 BT = Bishop Tuff
 M = Mazama (Wineglass
Tuff), Crater Lake
 K = Katmai (Valley of
Ten Thousand Smokes)
Hildreth, 1981, Fig. 2
AFM diagram
F

Presence or absence
of iron enrichment
trend

Tholeiitic suite =
Low-K suites (early
iron enrichment)

Calc-alkalic suite =
High- and medium-K
suites (no Fe
enrichment)
A
M
Best and Christiansen, 2001, Fig. 2.17
Alkali-lime index of Peacock (1931)


Harker diagram, Na2O + K2O and CaO vs. SiO2
Vary inversely; crossing points





<51% SiO2: Alkalic
51-56% SiO2: Alkali-calcic
56-61% SiO2: Calc-alkalic
>61% SiO2: Calcic
Should be used only for cogenetic rocks
Hyndman, 1985,
Fig. 43
TAS diagram


Analyses of
rocks from three
suites of rocks
Alkaline
 Tristan da
Cunha
oceanic
island

Subalkaline
 Andean
continental
arc
 Tongan island
arc

Would need to
superimpose a
plot with CaO
for an alkalilime index
 Add to right
axis
Best and Christiansen, 2001, Fig. 2.16
Eruptive temperatures of
prehistoric volcanic rocks: Mineral
geothermometers

Fe-Ti oxide geothermobarometer
 e.g., Ghiorso and Sack, 1991

Two pyroxenes
 e.g., Lindsley, 1983

Clinopyroxene
 e.g., Kretz, 1982

Olivine-ilmenite
 e.g., Andersen and Lindsley, 1981
Oxygen fugacity


Concentration
Adjusted concentrations to account for non-ideal
behaviors
 Liquids: activity
 Gases: fugacity


Fugacity: The partial pressure value needed to
make a real (non-ideal) gas behave as an ideal
gas
Oxygen fugacity: A measure of the abundance
of oxygen (adjusted “concentration”)
 May be present in exceptionally low concentrations,
e.g., 10-20
 The species O2 may not even be present, although
reactions can be written as if it were
Fe-Ti oxide geothermometer /
oxygen barometer

Uses compositions of
two coexisting Fe-Ti
oxide phases
 Cubic (spinel) phase:
Magnetite-ulvöspinel
solid solution
 Rhombohedral phase:
Ilmenite-hematite solid
solution

Method is insensitive
to total pressure
Carmichael et al., 1974, Fig. 3-5,
after Buddington and Lindsley, 1964
Applying Fe-Ti oxide geothermometer /
oxygen barometer
Highly useful, though many complexities
(e.g., Ghiorso and Sack, 1991)
 Application requires

Experimental data
Means of extrapolation to experimentally
inaccessible conditions (thermodynamic
treatment)
Accounting for “impurities” in the two phases

No agreement yet on uniform method
Oxygen fugacity and buffers

Oxygen fugacity of
geologically pertinent
buffer assemblages
varies with
temperature
 Absolute value of f(O2),
therefore, is not
meaningful without
corresponding T

Buffer assemblages
 HM (MH): hematitemagnetite
 NNO: nickel-nickel
oxide (bunsenite)
 FMQ (QFM): quartzfayalite-magnetite
 MW: magnetite-wüstite
 IW: Iron-wüstite
Hildreth, 1981, Fig. 3
Oxidation state


Oxidation state
(relative oxygen
fugacity):
Position
referenced to
buffer
assemblages at
the same
temperature
Example: ΔNNO
Hildreth, 1981, Fig. 3
Oxidation state

Higher oxidation state—between NNO and MH
 Lamprophyric lavas
 Hornblende andesite lavas
 Basaltic andesites

Lower oxidation state—between FMQ and IW
 Oceanic basalts
 Kilauea
Reduced
Oxidized
Carmichael and Ghiorso, 1990, Fig. 2
Natural assemblages from
intermediate to silicic volcanic
rocks
Assemblages with quartz, oxides, and
ferromagnesian silicates
Highest oxidation state

 Sphene (titanite)-quartz-clinopyroxene
 Hornblende-quartz-clinopyroxene
 Biotite-feldspar
 Orthopyroxene-quartz
 Olivine-quartz
Lowest oxidation state
 Why might we care about the temperature and
oxidation state?
Why might we care?

Probably telling us something about the
source(s) of the magmas and processes
that led to their generation
Compositions and crustal history of the
source(s)


Alkaline vs. subalkaline basalts
Assimilation of hematite-bearing redbeds and
hydrothermally altered rocks vs. organic-rich or
graphite-bearing shales and schists
Tectonic setting of magmas
Phenocryst species (and
abundance) and oxidation state


Note presence of fayalite (Fa), orthopyroxene (Op), ilmenite (I),
magnetite (M), sphene/titanite (Sp)
Also anorthoclase (Anor), chevkinite (Ch), aenigmatite (Aen),
pyrrhotite (Po)
Hildreth, 1981, Table 2
Volcanic rocks as samples of
magma chambers


Capturing products of a magma chamber at an
instant in time
Some characteristics probably are “quenched”
in the bulk rock by the eruption
 Phenocryst content

Others characteristics probably not preserved in
the bulk rock
 Escape of volatiles from magma
 For these, need to look at compositions of fluid
inclusions, melt inclusions, S content of apatite, etc.
Chamber geometry versus eruptive
sequence

Inverted stratigraphy
 Material that is first out of vent
winds up at stratigraphic base
 Material that is last out of vent
winds up at stratigraphic top

Initial assumption
 Single eruptive vent (or
simultaneous vents tapping
chamber at same level)
 “Bath tub like drawdown” of
magma chamber

Lots of complications
imaginable
Cashman et al., 2000, Fig. 1
Zonation of chamber (thermal)

Total phenocryst
abundance vs. SiO2
content for various
eruptive units
Late (bottom)
 SiO2 correlates with
eruptive temperature


Decreasing SiO2
corresponds to
increasing temperature
Note maximum
phenocryst
abundances
 Recall discussions of
viscosity
Early (top)
Temperature 
Wohletz and Heiken, 1992, Fig. 1.7, adapted from Hildreth, 1981, Fig. 6
Zonation of chamber (chemical)

Chemical zonation, Early/Late, for Bishop Tuff, Long
Valley caldera, CA
 Compared to average granitic rocks
Hildreth, 1981, Fig. 10
Fragmentation during eruption
Cashman et al., 2000, Fig. 1
Physical sorting in pyroclastic
eruptions

During eruption, rock is fragmented
 “Pyro” + “clastic”

Finest clasts/least dense materials most likely to go high
in eruptive cloud and wind up in ash fall and be carried
farthest away
 Fine glass shards from inflated melt

Coarsest clasts/most dense materials most likely to wind
up in pyroclastic flow
 Accidental lithic fragments, crystals (phenocrysts)

Likelihood of fractionation of components based on size,
density, etc.
 No longer in same proportions as in magma

When sampling pyroclastic rocks
 Avoid sampling the whole rock--fractionated clasts
 If possible, sample pumice (but still have issue of volatiles!)
Types and degrees of zonation

Essentially unzoned
Tapping chamber of uniform composition
“Monotonous intermediates”

Normal zonation
Most silicic, least dense material generally at
bottom of stratigraphy/top of magma chamber
Most mafic, most dense material generally at
top of stratigraphy/deeper in magma chamber

Reverse zonation
Opposite of normal
Enrichment factors—A measure
of the degree of zonation in the
magma chamber

Ratio of abundances of elements in two rocks
 Typically rocks from the same eruption, inferred to be from top
of magma chamber and bottom of magma chamber




Commonly ratio of most silicic rocks to least silicic rocks
In simple case, corresponds to earliest rocks to latest
rocks (Early/Late)
Early/Late = Shallow/Deep (assumption)
Different elements behave differently in one eruption
 Some enriched, others depleted toward roof

Same elements may behave differently in different
volcanoes
 Largely a function of different magma compositions
Enrichment factors


Bishop Tuff, Long
Valley, CA
 Many others, too
Ratio of abundances
of elements in two
rocks (especially
trace elements)
 Typically rocks from
the same eruption,
inferred to be from top
of magma chamber
and bottom of magma
chamber
Wohletz and Heiken, Fig. 1.6, after
Hildreth, 1981, Fig. 7
Hildreth, 1981, Fig. 10
Chemical analyses, Early/Late
Hildreth, 1981, Table 1
Enrichment factors

Enrichment factor diagram for Bishop Tuff, Long
Valley caldera, CA: Example of strong zonation
Enriched upward
toward roof of
magma chamber
(No enrichment /
depletion)
Depleted upward
(enriched downward
toward floor of
magma chamber)
Wohletz and Heiken, Fig. 1.6, after Hildreth, 1981, Fig. 7
Plot of Ta vs. Th of several large
eruptive units








P = Pantelleria
DC = Tuff of Devine
Canyon
TT = Tala Tuff, Primavera
LCT = Lava Creek Tuff,
Yellowstone
HRT = Huckleberry Ridge
Tuff, Yellowstone
BT = Bishop Tuff
M = Mazama (Wineglass
Tuff), Crater Lake
K = Katmai (Valley of Ten
Thousand Smokes)
Hildreth, 1981, Fig. 2
Laacher See
Schmincke, 2004, Fig. 3.18
Laacher See
Schmincke, 2004, Fig. 3.17
Compositional gaps

Compositional gaps
Inverted stratigraphy but with discontinuities
in composition, e.g., zoned rhyolite, but skip
to dacite
In spite of thermal continuity indicated by
geothermometry
e.g., Katmai/Novarupta, AK, Valley of Ten
Thousands Smokes Tuff
Silica content

Ultramafic

IUGS divisions commonly
followed for ultramafic to
andesite

No agreement on terms for
silicic rocks
 <45 wt% SiO2

Basalt
 45 – 52%

Basaltic andesite
 52 – 57%

Andesite
 57 – 63%

Dacite
 63 – 68%

Rhyodacite (quartz latite)
 68 – 72%

Rhyolite
 72 – 75%

High-silica rhyolite
 75 – 77.5%
 IUGS has only two terms for
SiO2 > 63 wt% (dacite and
rhyolite)
 Many people who work on
non-alkalic silicic rocks use a
subdivision similar to what is
at left

Ultramafic
 <45 wt% SiO2

Basalt
 45 – 52%

Basaltic andesite
 52 – 57%

Andesite
 57 – 63%

Dacite
 63 – 68%

Rhyodacite
 68 – 72%

Rhyolite
 72 – 75%

High- SiO2 rhyolite
 75 – 77.5%
Hildreth, 1981,
Fig. 1
Eruptive
types vs.
SiO2
content

Gaps
Note


Ranges of
SiO2 contents
Compositional
gaps
Gap
Gaps
Gap
Hildreth, 1981, Fig. 1
Evolution of silicic magma chambers
as a function of tectonic environment

Time 
Basalt-rhyolite
magmatism
under crustal
extension
 Bimodal
magmatism
 Bimodal
magmatism in
areas of
continental
extension
Early stage
Wohletz and Heiken, 1992, Fig. 1.10,
adapted from Hildreth, 1981, Fig. 15
Advanced stage
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
Evolution of silicic magma chambers
as a function of tectonic environment

Evolutionary trend with time
Time 
Hildreth, 1981, Fig. 15
Early stage
Intermediate stage
Hildreth, 1981, Fig. 12
Late stage
Dynamic model of large, mature
Cordilleran magmatic system



Bishop Tuff, Long Valley
caldera, CA
Yellowstone, WY
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
Magma
supply vs.
percolation
rate model
Hildreth, 1981, Fig. 16
Ia
Ib
Ib
IIb
IIa
IIa
IIa
IIa
IIa
IIa
IIa
Hildreth, 1981, Fig. 1
Magma
supply vs.
percolation
rate model


Note that “magma
supply” to the
lithosphere and
eruption rate may
differ by 1 to 3
orders of
magnitude
* = rapid transient
injection of basalt
at restricted crustal
levels, resulting in
generation and
separation of
rhyolite but few
intermediates
VIII
VI
IV
V
VII
Ib
Ia
IIa
IIb
Hildreth, 1981, Fig. 16
III
Multi-dimensional continuum of
magma compositions

Earth’s petrologic universe
We will be exploring it the rest of the
semester!
Somewhat arbitrary subdivisions
 Given multiplicity of factors, might not
expect there to be a perfect correlation of
magma composition to tectonic setting

Summary

Chemical characterization of individual volcanic rocks
 Rock names (norms, TAS diagrams)
 Silica-saturation and alumina-saturation indices

Chemical characterization of rock suites
 Variation diagrams
 Tholeiitic, calc-alkalic, shoshonitic suites
 Low-K, medium- and high-K, potassic suites

Volcanic rocks represent nearly instantaneous samples
of magma chambers (prior to their late crystallization
histories)
 Beware of physical sorting during pyroclastic eruptions because
of size and density differences (sample pumice, not whole rock)
 Types and degree of zonation measured by enrichment factors
(magma chamber and inverted stratigraphy)
 Compositional gaps are common

Next time: Silicic lava domes and flows