GEOS 470R/570R Volcanology
L03, 23 January 2015

Handing out
 Summary of slides
 Hildreth (1981) (for L04)
 Metz and Mahood, 1985 (for L05)
 McPhie et al., 1993, p. 21-33, 66-71

As preparation for Lab 1 (Monday), read p. 21-33 and 66-69
“If we are facing in the right direction, all we have
to do is keep on walking.”
--Buddhist Proverb
Readings from textbook

For L03 from Lockwood and Hazlett
(2010) Volcanoes—Global Perspectives
Chapter 3

For L04 from Lockwood and Hazlett
(2010) Volcanoes—Global Perspectives
Chapters 2 and 3
Assigned reading

For today L03
 None

For L04, 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.
Study this diagram frequently for
the lab and before first field trip
McPhie et al., 1993, Fig. 33
Photographic context for the laboratory
exercise: Aerial view from the resurgent
dome toward Glass Mountain on the NE
rim of the Long Valley caldera
Glass Mountain
http://lvo.wr.usgs.gov/gallery/30714277-095_caption.html
Photo by C. D. Miller, 1982
http://lvo.wr.usgs.gov/gallery/GalleryMap.html
Photographic context for the
laboratory exercise: Pine Grove, UT
Photo E. Seedorff, May 1979
Superstition Mountains
Last time: Physical and
chemical properties of magmas



Time, length, area, volume, and energy scales
Chemical and mineralogical characterization of
volcanic rocks
Physical properties
 Temperature T°
 Viscosity η
 Density ρ
 Thermal conductivity k
 Crystallization rates
Compositions: 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
Eruptive volumes

DRE = dense-rock equivalent
 Vdre ≈ 0.6 V for tephra
 Vdre ≈ V for lavas

Volumes (DRE) for eruptions of the last century
 Katmai-Novarupta, AK
 Pinatubo, Philippines
 Mount St. Helens, WA

June 1912
June 1991
May 1980
13 km3
5 km3
0.5 km3
Comparison
 Huckleberry Ridge, Yellowstone 2.0 Ma
 Bishop Tuff, Long Valley, CA
0.7 Ma
2500
500
km3
km3
Hildreth, 1981; Wohletz and Heiken, 1992; Wolfe and Hoblitt, 1996
Temperature summary
Composition
Temperature (°C)
Rhyolite-rhyodacite
700-900
Dacite
800-1100
Andesite
950-1170
Mafic (tholeiites)
1050-1250
Alkali basalts and
nephelinites
Ultramafic (komatiites)
900-1100
1400-1700 (est.)
Williams and McBirney, 1979, Table 2-2; Cas and Wright, 1987, Table 2.3;
Kilburn, 2000, Table 2
Viscosity η

Melt viscosity issues
Temperature [η ↓ with ↑ T]
Dissolved volatile content, especially water
content [η ↓ with ↑ H2O]
Chemical composition, especially silica
content [η ↑ with ↑ SiO2]
Crystal content [η ↑ with ↑ volume fraction
solids]
Viscosity: Network formers and
Network modifiers



Network formers contribute to η ↑
Network modifiers contribute to η ↓
Si, Al
 Network formers (strong bonds with O) (η ↑)

Fe, Mg, Ti, others
 Network modifiers (η ↓)

Alkalis: Na, K, Rb, Cs
 Network formers in peraluminous and metaluminous
melts (η ↑)
 Network modifiers in peralkaline rocks (η ↓)

Volatiles: H2O, F, Cl
 Network modifiers (η ↓)
Viscosity comparison

e.g., Hawaiian tholeiite
1200°C
1130°C

By comparison, H2O
25°C

η = 500 poise = 50 Pa s
η = 8000 poise = 800 Pa s
η = 0.01 poise =
0.001 Pa s
If basalts are much more viscous than
water, why, then, do basalts flow fairly
rapidly?
Viscosity changes during flow

Typically increases by 2 to 10X from vent
to toe of flow
Primarily because of loss of volatiles
Minor effect of cooling
Density summary
(at liquidus temperature and anhydrous,
except as noted)
Composition
Granite / rhyolite
Granite / rhyolite
(2 wt% H2O)
Granodiorite /
dacite
Gabbro / basalt
Komatiite
Liquidus Density Density
T° (°C) (kg/m3) (g/cm3)
900
2349
2.35
900
2262
2.26
1100
2344
2.34
1200
1500
2591
2748
2.59
2.75
Spera, 2000, Table 3
Importance of density
Important control on rise of magmas
through crust
 Strong control on fluid dynamics of
magmas

Petrologic implications for mixing of magmas
Transport of magmatic heat

Convection
Heat transported by bulk flow

Conduction (phonon conduction)
Phonon = quantized thermal waves
Heat transported by atomic vibration of lattice

Radiation
Electromagnetic phenomenon involving
photon transfer
Crystallization rates

Rate decreases as viscosity increases
Rate ↓ with ↑ η

Consequences
Rhyolites (high η) crystallize slowly  glassy
groundmass
Basalts (low η) crystallize rapidly fine
crystalline groundmass
Recrystallization of glass

Rhyolitic glass  silica mineral + alkali
feldspar (and/or clay minerals and zeolites
in alkaline lakes)
Hydrate and crack
Nucleate crystals along cracks
Summary: Chem & Phys Properties

The time, length, area, volume, and energy scales of
volcanism and volcanic rocks
 Each vary by many orders of magnitude, but
 Characteristic features vary within fairly narrow ranges

Mineralogy is a function of chemical composition
 Silica content and alkalinity are key compositional variables

The most important physical properties are
 Temperature T°, Viscosity η, Density ρ, Thermal conductivity k,
and Crystallization rates

Impacts on viscosity
 η ↓ with ↑ T; η ↓ with ↑ H2O and most other volatiles; η ↑ with
↑ SiO2; η ↑ with ↑ volume fraction solids (e.g., phenocrysts)

The properties are not independent of one another
 Many can be linked to chemical composition of the magma
 Many observations can be explained in terms of viscosity (e.g.,
shapes of volcanoes, eruptive style)
Lecture 03: Volatiles and sampling
magmatic gases

Volatiles
Undersaturation vs. saturation
Solubility (saturation limit)

Solubility controls
H2O
CO2
S
Halogens: Cl, F
Volatiles

How do we know that volatiles (e.g., H2O
or F) are important constituents of
magmas on Earth?
Volatile

An element (e.g., S) or compound (e.g.,
H2O or CO2) that forms a gas at relatively
low pressure and magmatic temperature
Can be dissolved in silicate melts
Can occur as bubbles of exsolved gas
Can be incorporated in the structure of
phenocrysts
Why are volatiles important?

Volatiles modify the behavior of silicate melts
and magmas (last lecture)
 e.g., decrease viscosity and density

As magmas ascend toward the surface,
decrease in pressure may cause exsolution of
volatiles
 Form bubbles
 Could cause great increase in volume
 Hence, volatiles are widely thought to be important in
governing eruptive phenomena

Important ligands to transport metals to form
certain types of ore deposits
Bursting gas bubbles in lava lake
at Kilauea, Hawaii
Schmincke, 2004, Fig. 4.30
Important magmatic volatiles
Water (H2O)
 Carbon dioxide (CO2)
 Sulfur (SO2, H2S)
 Halogens (Cl, F)
 Other volatiles (He, Ar, B) are minor
components

Nonetheless, my provide clues to sources of
magmas and origin of atmosphere
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,
although many may be saturated prior to eruption
Solution mechanisms for H2O in
silicate melts


Speciation controlled by amount dissolved
At low concentrations of H2O
 Dissolves by forming OH- groups that are structurally
bound to the aluminosilicate network
 Indicated because solubility varied by P(H2O)0.5
 Concentration via this mechanism varies directly with T

As total dissolved H2O increases,
 Relative proportion of molecular H2O increases
 Concentration via this mechanism independent of T

Proportions: OH- groups = molecular H2O
 At 3 wt% dissolved H2O for rhyolites
 At 3.5 wt% dissolved H2O for basalts
Solution mechanisms for H2O as a
function of dissolved water content
Best and Christiansen, 2001, Fig. 4.9; after
Silver et al., 1990
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
Recall: Viscosity η

Melt viscosity issues
Temperature [η ↓ with ↑ T]
Dissolved volatile content, especially water
content [η ↓ with ↑ H2O]
Chemical composition, especially silica
content [η ↑ with ↑ SiO2]
Crystal content [η ↑ with ↑ volume fraction
solids]
Solubility of H2O

Strongly dependent
on pressure [↑
solubility with ↑P)
 Gaseous form has
larger partial molar
volume than does
water when dissolved
in silicate melt
 Typical for many other
volatiles

Dependent on
composition [↑
solubility with ↑ SiO2]
Wallace and Anderson, 2000, Fig. 1
Degassing of Bocca Nova crater,
summit of Mount Etna, Italy

Hot H2O gas is
condensed to
water
At the boundary
between the
rising gas
stream and
The cold
atmosphere at
the summit of
the volcano
Schmincke, 2004, Fig. 4.20
Expansion of H2O
17,000 X expansion from liquid to steam
at sea level
 Drives phreatic eruptions (steam
explosions)

These were the early eruptions in early 1980
at Mount St. Helens (pre-climactic eruption)
Why would they occur early?
Concentration of water as a
function of magma composition
Schmincke, 2004, Table 4.1, after Fisher and Schmincke, 1984
Solubility of CO2

CO2 is less soluble in magmas than H2O
 By 1 to 2 orders of magnitude by weight at same P,
T°

Solubility of CO2 greater in rhyolite than basalt
 As with H2O

Speciation of dissolved CO2 varies according to
bulk melt composition
 Rather than according to volatile concentration, as for
H2O
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
Solution mechanisms for CO2 in
silicate melts


Varies according to bulk silicate melt
composition
Silica-poor melts (basalts, basanites,
nephelinites)
 As structurally bound CO32 Solubility of CO2 as carbonate increases with
decreasing silica content

Silica-rich melts (rhyolites)
 CO2 molecules

Intermediate silica compositions
 As structurally bound CO32- and as CO2 molecules
CO2 bubbles in lake at Laacher See,
Eifel, Germany (erupted 12 900 yr B.P.)
Schmincke, 2004, Fig. 4.22
Multicomponent systems
Likely to have numerous volatile species
present
 If sum of partial pressures of all dissolved
species exceeds confining pressure,
magma will be saturated with a
multicomponent gas phase
 Gas phase will not be “pure”

Because of different solubilities, elements will
partition differently between melt and vapor
H2O and CO2

At saturation and
constant total
pressure, increase
in partial pressure
of P(H2O), and
hence dissolved
H2O in the melt,
will depress
P(CO2) and
dissolved CO2
 Hence negative
slopes
Wallace and Anderson, 2000, Fig. 3
Sulfur—Complex behavior

Occurs in multiple oxidation (valence)
states
Reduced form: sulfide S2- (species HS-, S2-)
Oxidized form: sulfate S6+(species SO42-,
HSO4-)

Maximum S that can be dissolved in
silicate melt is controlled by saturation of
the melt with a sulfur-bearing phase (liquid
or solid)
Sulfur-bearing phases that control
S content of silicate melt

Immiscible Fe-rich sulfide liquid (with ~10% O)
 In high-temperature basaltic magmas at relatively low
f(O2)

Crystalline pyrrhotite (Fe1-xS)
 Common in andesitic to rhyolitic magmas

Pyrrhotite may coexist with a Cu-Fe-sulfide
mineral, intermediate solid solution (iss)
 In andesitic to rhyolitic magmas

At higher f(O2), may crystallize anhydrite
 First reported as a microphenocryst in trachyandesite
tephra from El Chichón, México
 Microphenocrysts dissolve in rain water
Dissolved sulfur



At relatively low
oxidation states,
sulfide is the
dominant form of
dissolved sulfur
At relatively high
f(O2), sulfate is the
dominant form of
dissolved sulfur
Within a few log units
f(O2) of NNO buffer,
both dissolved sulfide
and sulfate significant
Wallace and Anderson, 2000, Fig. 4
Sulfur solubility

S more soluble at
high oxygen
fugacities
 Where dissolved
sulfur occurs as
sulfate
 Anhydrite present
Wallace and Anderson, 2000, Fig. 5
Sulfur solubility

The solubility of
reduced S
(sulfide, S2-)
increases with
Fe
concentration in
the melt
Wallace and Anderson, 2000, Fig. 6
Sulfur solubility


Important
temperature effect
Solubility of sulfur
increases with
increasing
temperature
 Under both
oxidizing and
reducing
conditions
Wallace and Anderson, 2000, Fig. 5
Degassing of S-rich gases from
fumaroles at Vulcano, Italy
Schmincke, 2004, Fig. 4.23
Degassing of bluish S-rich gases
from open central conduit of crater
at Masaya, Nicaragua
Schmincke, 2004, Fig. 4.24
Degassing of CO2 and SO2 from Mount
Etna, Italy—Why different abundances?
Schmincke, 2004, Fig. 4.21, after Allard et al., 1991
Solubility controls for Cl

Cl solubility strongly dependent on silicate
melt composition
Solubility increases as ratio of (Na + K) / Al in
the silicate melt increases

Maximum Cl content of melt is when melt
is saturated with immiscible alkali chloride
melt (molten salt)
Saturation concentration varies with P, T°,
dissolved H2O, silicate melt composition
Solubility of Cl

For silicate melts saturated with both H2O and
CO2
 Maximum Cl solubilities range from X000 ppm to ~2
wt% Cl
 Cl partitions strongly into vapor phase
 Concentrations in vapor can be 5-20 X mass
dissolved in melt


Hence, solubility of Cl is intermediate between
H2O and CO2
Possible for melt to be saturated with a
hydrosaline melt, H2O, and CO2
Solubility of F
Solubility strongly dependent on silicate
melt composition
 Highly soluble in silicate melts (to 10 wt %
F)—similar to H2O

Natural melts generally have much lower
dissolved F
Sn-W rhyolitic magmas as much as 5 wt% F
Ultrapotassic mantle-derived magmas as
much as 2 wt% F
Summary

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)

Next time: Petrologic overview