GEOS 470R/570R Volcanology
L02, 16 January 2015

Handing out
Copies of slides
Review of minerals
One-page questionnaire
“Write it on your heart that every day is the best
day of the year.”
--Ralph Waldo Emerson
Who are you?

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Why are you here?
Class
Major
E-mail
Questionnaire
Photo courtesy of M. D. Barton
“Did you arrive with a particular
career in mind?”
“Goodness, no. I not only didn’t feel prepared, I
had no idea what course of study I should
follow. . . . You needed some language credits
and some math credits; some science. So your
registration sheet filled up with required areas
pretty fast.
“I took a geology course and absolutely adored it,
and I really thought, gosh, maybe that’s what I
should study. But I ended up majoring in
economics.”
--Who said this?
“Did you arrive with a particular
career in mind?”
“Goodness, no. I not only didn’t feel prepared, I
had no idea what course of study I should
follow. . . . You needed some language credits
and some math credits; some science. So your
registration sheet filled up with required areas
pretty fast.
“I took a geology course and absolutely adored it,
and I really thought, gosh, maybe that’s what I
should study. But I ended up majoring in
economics.”
--Sandra Day O’Connor, Stanford BA ’50, JD ’52
Interview in Stanford magazine, 2006
Readings from textbook

For L02 from Lockwood and Hazlett
(2010) Volcanoes—Global Perspectives
Chapters 3 and 4

For L03 from Lockwood and Hazlett
(2010) Volcanoes—Global Perspectives
Chapter 3
Assigned reading

For today
None
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First assignment due
26 January 2015
Hildreth (1981)
Last time: The volcanic center
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Course overview
Tectonic settings of volcanism
Definitions
 Igneous and volcanic materials
 Lavas and pyroclastic rocks
 Pyroclastic depositional processes
 Volcano


Volcanic landforms
The volcanic center
Summary: The volcanic center

Course is designed to provide perspectives on
 Volcanologic processes and active volcanoes
 Working with partially eroded, altered, and deformed volcanic rocks
 Applications to petrology, mineral resources, extraterrestrial volcanism,
hazards, climate change, geothermal energy
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Volume of volcanism: ridges > arc > intraplate settings
Igneous materials: melt, magma, lava, pyroclast
Flows (coherent mass movements): lava flows, pyroclastic flows
Pyroclastic falls, flows, and surges
Lahars: volcanic debris, transitional, and hyperconcentrated flows
Shapes and main types of volcanoes mainly reflect
 Lava composition or chemistry (viscosity) and eruptive style


The volcanic center: fundamental mapping and stratigraphic unit
Volcanic stratigraphy depends on
 Geologic mapping, chemical characterization, radiometric dating
Next time: Physical and chemical properties of magmas
Lecture 02: 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
Time scales

Quenching of a pyroclast during ejection from a vent
 10-7 – 10-6 yr (seconds)

Cooling of a single lava flow
 10-2 – 100 yr (weeks to months)

Lifetime of single cinder cone (crystallization of small
gabbroic stock)
 100 – 101 yr (a few years)

Lifetime of a composite volcano (crystallization of a
dioritic intrusive complex)
 105 – 106 yr (~500 ka)

Lifetime of silicic caldera complex (crystallization of
large granitic composite pluton)
 105 – 106 yr

Lifetime of a volcanic field
 107 yr (10 m.y.)
Length scales

Diffusion distance for components near
interface of growing crystal or bubble
10-4 – 10-2 m (millimeters or less)

Height of a composite volcano
(stratovolcano)
103 – 104 m (1 – 3 km)

Height of a volcanic plume
104 – 105 m (10 - 40 km)
Area scales

Area occupied by a typical rhyolite dome
 100 - 101 km2

Area occupied by a composite volcano
 ~102 km2
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Area occupied by silicic caldera
 101 - 103.5 km2 (25 - 2500 km2)
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Area of plinian fall deposits at 1-m isopach
 101 - 104 km2
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Area of flood basalt provinces
 105 – 106.5 km2 (160,000 – 3,000,000 km2)
Volume
scales and
frequencies

Sizes of
eruptions and
their frequency
anywhere on
Earth
Fisher et al., 1997, Fig. 2-5
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
600
km3
km3
Hildreth, 1981; Wohletz and Heiken, 1992; Wolfe and Hoblitt, 1996
Eruptive
volumes

Volumes (DRE)
of erupted
magma
 Historic,
prehistoric, and
Pleistocene
eruptions
 Basalts in gray
 All were
explosive except
for Laki,
Lanzarote, and
Nyiragongo
Schmincke, 2004, Fig. 4.17
Energy released in an eruption

Heat (main component for Hawaiian
eruptions)
Radiation of heat
Conduction away from surface by convecting
air
Conduction into surrounding rocks
Transport outward by gases
Explosive energy (main component for
Krakatau)
 Earthquakes

Energy scales
Press and Siever, 2001, 18.11
Magma
Completely or partly molten natural
substance that, on cooling, solidifies as a
crystalline or glassy igneous rock
 Melt ± crystals ± vapor

Constituents of magma

Liquid
 Generally silicate: modified Si - O framework
 Rarely carbonate, sulfur, etc.
 Lacks long-range periodicity and symmetry (as in
crystalline solids) but has short-range order

Solid
 Crystals, glass
 Phenocrysts, microphenocrysts, microlites

Gas
 Dissolved
 Exsolved separate phase
Definitions

Phyric
 Contains
phenocrysts
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Aphyric
 Lacks phenocrysts

Vitrophyric
 Contains
phenocrysts in a
glassy matrix
Le Maitre, 2002, Table 2.1
Role of elements in silicate liquids

Si, Al
 Network formers (strong bonds with O)

Fe, Mg, Ti, others
 Network modifiers

Alkalis: Na, K, Rb, Cs
 Network formers in peraluminous and metaluminous
melts
 Network modifiers in peralkaline rocks

Volatiles: H2O, F, Cl
 Network modifiers
We will see these groupings reflected in
classification schemes for rocks
Silica content

Ultramafic
 <45 wt% SiO2

Mafic
 45 - ~ 52 wt% SiO2
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Intermediate
 ~52- ~63 wt% SiO2

Silicic
 >~63 wt% SiO2
Some prefer to use a
higher division between
intermediate and silicic
rocks (e.g., 65 to 68),
instead of 63 wt% SiO2
Analogy with crystalline solids

Increasing polymerization from
Orthosilicates—isolated Si - O tetrahedra
Single chain structures
Double chain structures
Sheet structures
Framework structures

Melts can also display varying degrees of
polymerization of Si - O tetrahedra
Review of petrology
Rogers and Hawkesworth, 2000, Fig. 2
Classification of volcanic rocks by
modal phenocryst content
Q-A-F-P diagram
Quartz (Q)
Alkali feldspar (A)
Feldspathoid (F)
Plagioclase (P)
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
What is a limitation
on the usefulness of
this classification
scheme?
Wohletz and Heiken,
1992, Fig. 1.3
Chemical classification of volcanic
rocks

TAS (total
alkalis vs.
silica)
diagram
Rogers and Hawkesworth, 2000, Fig. 1
Chemical classification
of volcanic rocks
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TAS (total alkalis vs. silica)
diagram
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Covariation
of other
components
Wohletz and Heiken, 1992, Fig. 1.2,
adapted from Cox et al., 1979
Silica content

Ultramafic

IUGS divisions commonly
followed for ultramafic to
andesite

No agreement on terms for
silicic rocks
 <45 wt% SiO2
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Basalt
 45 – 52%
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Basaltic andesite
 52 – 57%
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Andesite
 57 – 63%
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Dacite
 63 – 68%
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Rhyodacite (quartz latite)
 68 – 72%
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Rhyolite
 72 – 75%
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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
Silica content

Ultramafic
 <45 wt% SiO2

Basalt
 45 – 52%

Basaltic andesite
 52 – 57%

Andesite
 57 – 63%
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Dacite
 63 – 68%
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Rhyodacite (quartz latite)
 68 – 72%
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Rhyolite
 72 – 75%
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High-silica rhyolite
 75 – 77.5%
Rogers and Hawkesworth, 2000, Fig. 1
Characterizing volcanic rocks
Reminder about handout on minerals
 Begin Lecture 04 with further discussion
of petrologic classification schemes
(especially chemical)
 Now we will move on to physical
properties

Physical factors that influence
volcanic processes
Sigurdsson, 2000, Table 1
Physical properties of lava flows
Kilburn, 2000, Table 2
Temperature T°

Importance
 Influences magma viscosity (more later)
 Affects energy available for rise of eruption plume

Units
 Kelvin (K)
 Celsius (°C)

Measure directly with
 Optical pyrometer (mafic lavas only)
 Color when viewed with unaided eye
 Thermocouple

Lockwood and Hazlett
 “Red” vs. “gray” volcanoes
Optical pyrometer
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Essentially a telescope in which a wire filament
is visible at same time as the glowing object
(e.g., lava)
Pass current through filament, causing it to glow
Color of filament varies with strength of current
Various corrections/calibrations required
 Have significant uncertainty
 Problems with smoke/haze

Are not measuring T° of interior—only exterior
crust
Macdonald, 1972
Color viewed with unaided eye

Use old principle
Blacksmiths
Operators of steel furnaces

Temperatures related to color
Valid when seen in dark (e.g., at night)
Valid only if clear line of sight (not any
intervening brownish fume clouds)
Macdonald, 1972
Visual calibration at night
Kilburn, 2000, Table 2
Thermocouple


Method subject to the least error
Pair of metallic wires of different composition
welded together at both ends
 One end immersed in hot material
 Generates an electrical current in the circuit

Strength of current depends on difference in T
between hot and cold ends
 Cold end kept at 0° C with ice water bath
 Current measured with ammeter near cold end


Can calculate T of hot end
Practical limitations
 Lava too viscous to insert
 Thermocouple can be damaged by movement/flow
Thermocouple


Temperature
measurements
in an active
lava flow at Mt.
Etna, Italy
Obtained with a
thermocouple
during an
eruption in
1991
Stix and Gaonac'h, 2000, Fig. 11
Actual field measurements for
eruption temperatures

Tholeiitic basalt, Kilauea, HI
 1050-1190°C

Hawaiite, Mt. Etna, Italy
 1050-1125°C

Basaltic andesite, Parícutin, México
 943-1057°C

Dacite, Mount St. Helens, WA
 850°C

No data on rhyolites
 No eruptions measured or even viewed since Vulcan,
Italy, in 1700’s
Cas and Wright, 1987, Table 2.2
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
The high-T° end: Availability of
any “superheat”?
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

Aphyric rocks are unusual
In other words, few, if any, lavas are hotter than the
temperature at which they first begin to crystallize
Exceptions
 Rare glassy basalts
 Some aphyric rhyolites (volatile-rich; heated by coeval
hotter, underplating basalt?)

“Superheat” generally not available, especially in
silicic magmas (e.g., for melting rocks at or near the
surface)
 Cannot cool without nucleating and growing crystals
The low-T° end

Erupted rocks are only partly crystalline
Uncommon for volcanic rocks to have much
>50% phenocrysts

Why don’t we see cooler lavas with
greater phenocryst contents?
Limitations on the low-T° end

Low-T end observed for a given
composition probably corresponds to
upper limit on viscosity for magmas of
those compositions to remain mobile
Migrate in crust
Flow on surface

Implies a “gap” in time between last
extrusion from a magma chamber and its
final crystallization as a pluton
No geologic record for this interval
Eruptive temperatures of
prehistoric volcanic rocks
No way to directly measure temperature
 Mineral geothermometers

Return to in L04
Viscosity η

Definition
 Resistance to flow, or
 Ratio of shear stress (σ) applied to a layer of
thickness z to the rate at which it is permanently
deformed in a direction x parallel to the stress
Williams and
McBirney, 1979,
Fig. 2-1
Importance of viscosity

Viscosity affects
Fluidity of magmas and mobility of lavas
Geometry and morphology of lavas and
associated volcanoes
Exsolution and nucleation of bubbles
(vesiculation)
Growth of bubbles
Rise and escape of bubbles from magmas
Fluid flow state: Laminar vs.
turbulent

Turbulent behavior of magmas (or pyroclastic
and epiclastic aggregates) during flow is
promoted by
 Increasing velocity
 Increasing irregularity of channel bottom and walls
 Decreasing viscosity  more turbulent (i.e., more
viscous  less turbulent)

We will return to this when we discuss lava
flows, pyroclastic flows, pyroclastic surges, and
lahars
Classification of fluids on basis of
rheology (viscosity, yield strength)

Newtonian fluid (linear
relationship)
 Zero yield strength (σ0=0)
 Linear relationship of shear
stress to strain rate
 Good approximation for
silicate melts (but not for
multiphase suspensions or
glasses)

Bingham fluid (one of many
non-Newtonian fluids)
 Finite yield strength (σ0>0)
 Linear relationship of shear
stress to strain rate
 Good approximation for
magmas
Cas and Wright, 1987, Fig 2.3
Shear stress vs. strain rate
Note: slopes = viscosity η
Bingham fluids (magmas)

If a stress less than the yield strength is
applied (σ> σ0), resulting strain is
Elastic (recoverable)

If a stress greater than the yield strength
is applied (σ> σ0), resulting strain has two
components
Elastic (recoverable)
Viscous (non-recoverable)
Viscosity η

Units
 kg / m s = Pa s
 1 poise = 1 g / cm / s = 0.1 Pa s (pascal second)



Measure directly with penetrometers (data only
for basalts)
Estimate from velocities down channels
(underestimates)
Calculate from partial molar viscosities
 Pioneered by Bottinga and Weill and Shaw
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]

Additional issues for magmas
Rheological properties of magmatic
suspensions (crystals, vapor bubbles) [η ↑
with ↑ volume fraction solids]
Viscosity vs. temperature


Log viscosity vs.
temperature, as a
function of composition
(volatile-free)
Rhyolites—more Si - O
bonds to break
 Greater resistance to
flow (higher viscosity)

Basalts—fewer Si – O
bonds to break
 Less resistance to flow
(lower viscosity)
Spera, 2000, Fig. 4
Viscosity vs. dissolved water
content

Log viscosity
vs. dissolved
water content,
as a function
of composition
Spera, 2000, Fig. 5
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: 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 ↓)
Why do basalts flow fairly
rapidly?

Aided by gravity
Flow down slopes of a shield volcano

High density
Have a density considerably greater than
water
Viscosity vs. dissolved water
content

Log viscosity
vs. dissolved
water content
for rhyolitic /
granitic
melts, as a
function of
temperature
Wallace and Anderson, 2000, Fig. 14
Viscosity vs. volume fraction solids

Log viscosity vs.
volume fraction
solids
Spera, 2000, Fig. 6
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 ρ


Definition: mass per unit volume
Units
 kg / m3
 g / cm3

Melt density is a function of
 Temperature [ρ ↓ with ↑ T]
 Pressure [ρ ↑ with ↑ P]
 Dissolved water content [ρ ↓ with ↑ H2O]

Density decreases (volume increases) on
melting
Changes in density ρ

Temperature dependence of density
Coefficient of thermal expansion
Similar for most compositions:
~2 – 3 X 105 deg-1

Pressure dependence of density
Compressibility
Increases sharply in the melting range
Density vs. temperature

Density of melt
vs. temperature,
as a function of
composition
Spera, 2000, Fig. 1
Density vs. pressure

Density of model
basaltic melt vs.
pressure, for
temperatures of
1800 and
2800°C
Spera, 2000, Fig. 3
Density vs. dissolved water content

Density of melt
vs. dissolved
water content, as
a function of
composition
Spera, 2000, Fig. 2
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
Thermal conductivity k

If




k = thermal conductivity
κ = thermal diffusivity
ρ = density
Cp = specific heat,

Then the thermal conductivity k = ρ Cp κ

Units
 J / (m K s) = W / (m K)

Most melts, rocks, and minerals are characterized by
low thermal diffusivity and thermal conductivity
Specific enthalpy of fusion Δhf

Definition
Heat per unit mass needed at constant
pressure to transform a crystal or crystalline
assemblage to the liquid state

Units
kJ / kg

Very high for magmas, with wide variation
100 – 300 kJ / kg for crustal phases
~1000 kJ / kg for refractory phases that are
components of mafic and ultramafic melts
Enthalpy of fusion--Implications
Anatexis of crust by heat exchange
between mafic magma and crust is
thermally efficient
 Heat required to completely melt Earth’s
mantle, 3 x 1030 J

Is <10% of the kinetic energy delivered to
Earth by impact of a Mars-sized body (15% of
mass of Earth) with an impact velocity equal
to Earth’s escape velocity of 11.2 km/s
Molar isobaric heat capacity Cp

Definition
 Heat needed at constant pressure to raise
temperature of one mole by one Kelvin

Units
 J / kg K

Low for magmas (< half that of water)
 Silicic anhydrous melts 1300 – 1400 J / kg K
 Mafic – ultramafic anhydrous melts 1600 – 1700 J /
kg K

Implies mafic and ultramafic magmas are better
transporters of magmatic heat
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

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)
Next time: Volatiles