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GEOS 470R/570R Volcanology
L02, 16 January 2015 Handing out
Copies of slidesReview of mineralsOne-page questionnaire
“Write it on your heart that every day is the best day of the year.”
--Ralph Waldo Emerson
Who are you? Why are you here? Class Major E-mail
QuestionnairePhoto 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 ’52Interview in Stanford magazine, 2006
Readings from textbook
For L02 from Lockwood and Hazlett (2010) Volcanoes—Global PerspectivesChapters 3 and 4
For L03 from Lockwood and Hazlett (2010) Volcanoes—Global PerspectivesChapter 3
Assigned reading
For todayNone
First assignment due26 January 2015Hildreth (1981)
Last time: The volcanic center
Course overview Tectonic settings of volcanism Definitions
Igneous and volcanic materialsLavas and pyroclastic rocksPyroclastic depositional processesVolcano
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 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 ηDensity ρThermal conductivity kCrystallization 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 bubble10-4 – 10-2 m (millimeters or less)
Height of a composite volcano (stratovolcano)103 – 104 m (1 – 3 km)
Height of a volcanic plume104 – 105 m (10 - 40 km)
Area scales
Area occupied by a typical rhyolite dome100 - 101 km2
Area occupied by a composite volcano~102 km2
Area occupied by silicic caldera101 - 103.5 km2 (25 - 2500 km2)
Area of plinian fall deposits at 1-m isopach101 - 104 km2
Area of flood basalt provinces105 – 106.5 km2 (160,000 – 3,000,000 km2)
Volume scales and frequencies
Fisher et al., 1997, Fig. 2-5
Sizes of eruptions and their frequency anywhere on Earth
Eruptive volumes
DRE = dense-rock equivalentVdre ≈ 0.6 V for tephra
Vdre ≈ V for lavas
Volumes (DRE) for eruptions of the last centuryKatmai-Novarupta, AK June 1912 13 km3
Pinatubo, Philippines June 1991 5 km3
Mount St. Helens, WA May 1980 0.5 km3
ComparisonHuckleberry Ridge, Yellowstone 2.0 Ma 2500 km3
Bishop Tuff, Long Valley, CA 0.7 Ma 600 km3
Hildreth, 1981; Wohletz and Heiken, 1992; Wolfe and Hoblitt, 1996
Eruptive volumes Volumes (DRE)
of erupted magmaHistoric,
prehistoric, and Pleistocene eruptions
Basalts in grayAll 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 heatConduction away from surface by convecting
airConduction into surrounding rocksTransport 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 frameworkRarely carbonate, sulfur, etc.Lacks long-range periodicity and symmetry (as in
crystalline solids) but has short-range order Solid
Crystals, glassPhenocrysts, microphenocrysts, microlites
GasDissolvedExsolved separate phase
Definitions
PhyricContains
phenocrysts
AphyricLacks phenocrysts
VitrophyricContains
phenocrysts in a glassy matrix Le Maitre, 2002, Table 2.1
Role of elements in silicate liquids
Si, AlNetwork formers (strong bonds with O)
Fe, Mg, Ti, othersNetwork modifiers
Alkalis: Na, K, Rb, CsNetwork formers in peraluminous and metaluminous
meltsNetwork modifiers in peralkaline rocks
Volatiles: H2O, F, ClNetwork modifiers
We will see these groupings reflected in classification schemes for rocks
Silica content
Ultramafic<45 wt% SiO2
Mafic45 - ~ 52 wt% SiO2
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 fromOrthosilicates—isolated Si - O tetrahedraSingle chain structuresDouble chain structuresSheet structuresFramework 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)
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 TAS (total alkalis vs. silica)
diagram
Covariation of other
components
Wohletz and Heiken, 1992, Fig. 1.2, adapted from Cox et al., 1979
Silica content Ultramafic
<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 divisions commonly followed for ultramafic to andesite
No agreement on terms for silicic rocks 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%
Dacite 63 – 68%
Rhyodacite (quartz latite) 68 – 72%
Rhyolite 72 – 75%
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
UnitsKelvin (K)Celsius (°C)
Measure directly with Optical pyrometer (mafic lavas only)Color when viewed with unaided eyeThermocouple
Lockwood and Hazlett “Red” vs. “gray” volcanoes
Optical pyrometer
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 uncertaintyProblems with smoke/haze
Are not measuring T° of interior—only exterior crust
Macdonald, 1972
Color viewed with unaided eye
Use old principleBlacksmithsOperators of steel furnaces
Temperatures related to colorValid 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 endsOne end immersed in hot materialGenerates an electrical current in the circuit
Strength of current depends on difference in T between hot and cold endsCold end kept at 0° C with ice water bathCurrent measured with ammeter near cold end
Can calculate T of hot end Practical limitations
Lava too viscous to insertThermocouple 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 summaryComposition 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”? 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 crystallineUncommon 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 mobileMigrate in crustFlow on surface
Implies a “gap” in time between last extrusion from a magma chamber and its final crystallization as a plutonNo 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, orRatio 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 affectsFluidity of magmas and mobility of lavasGeometry and morphology of lavas and
associated volcanoesExsolution and nucleation of bubbles
(vesiculation) Growth of bubblesRise 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 wallsDecreasing 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 rateNote: slopes = viscosity η
Bingham fluids (magmas)
If a stress less than the yield strength is applied (σ> σ0), resulting strain isElastic (recoverable)
If a stress greater than the yield strength is applied (σ> σ0), resulting strain has two componentsElastic (recoverable)Viscous (non-recoverable)
Viscosity η
Units kg / m s = Pa s1 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 viscositiesPioneered by Bottinga and Weill and Shaw
Viscosity η
Melt viscosity issuesTemperature [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 tholeiite1200°C η = 500 poise = 50 Pa s 1130°C η = 8000 poise = 800 Pa s
By comparison, H2O25°C 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 flowPrimarily because of loss of volatilesMinor 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 densityCoefficient of thermal expansionSimilar for most compositions:~2 – 3 X 105 deg-1
Pressure dependence of densityCompressibilityIncreases 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)
CompositionLiquidus T° (°C)
Density (kg/m3)
Density (g/cm3)
Granite / rhyolite 900 2349 2.35
Granite / rhyolite (2 wt% H2O) 900 2262 2.26
Granodiorite / dacite
1100 2344 2.34
Gabbro / basalt 1200 2591 2.59
Komatiite 1500 2748 2.75Spera, 2000, Table 3
Importance of density
Important control on rise of magmas through crust
Strong control on fluid dynamics of magmasPetrologic implications for mixing of magmas
Transport of magmatic heat
ConvectionHeat transported by bulk flow
Conduction (phonon conduction)Phonon = quantized thermal wavesHeat transported by atomic vibration of lattice
RadiationElectromagnetic 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
DefinitionHeat per unit mass needed at constant
pressure to transform a crystal or crystalline assemblage to the liquid state
UnitskJ / kg
Very high for magmas, with wide variation100 – 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 JIs <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
DefinitionHeat 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 KMafic – 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 increasesRate ↓ with ↑ η
ConsequencesRhyolites (high η) crystallize slowly glassy
groundmassBasalts (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 crackNucleate 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