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