AH1 - Magma Properties

7/23/2019 AH1 - Magma Properties

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Volcanology

Key Reading

Volcanoes(2ndEdition) (2004)

P. Francis & C. Oppenheimer, Oxford Univ. Press, 521 p.

Volcanoes: Global Perspectives(2010)

J.P. Lockwood &. R.W. Hazlett, Wiley-Blackwell, 541 p.

Le Volcanisme(1994)

J.L. Bourdier, BRGM dition, Manuels et Mthodes n25, 420 p.

Volcans, source de vie, cause de mor t(2003)

C. Jaupart et al., 2003, Vuibert, 328 p.


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Outline

Basic definitions and concepts, classification

Magma and Volcanic rock properties

Melt properties I: goechemistry, SiO2 Melt properties II: temperature

Vesicles, bubbles and control on viscosity

Magma properties III: crystals

After A. Harris et rfrences cites


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So we need to consider the properties of:

(1) Melt (chemistry, temperature)

(2) Crystals

(3) Volatiles, gas and bubbles (vesicles)

Magma at depth is a mixture of:

= Melt (liquid) + crystals (solids) + volatiles

Magma (upon reaching the surface) is a mix of:

= Melt (liquid) + crystals (solids) + bubbles

Together these define the magma rheological (e.g., viscosity) &

physical (e.g., density) properties

Magma and Volcanic Rock Properties


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Magma and Volcanic Rock Properties:

Basic Definitions & Concepts

(1) Magma:

Molten rock containing volatiles + crystals;

Volatiles will, however, exsolve at shallow depths to

create bubbles (vesicles);

The term is maintained as long as the molten rock

remains below the Earths surface.

(2) Lava and pyroclasts:

The term lava (or pyroclast) is applied to molten rock thathas been erupted onto the Earths surface.

Molten rock from which the volatiles have largely exsolved;

Contains bubbles (otherwise termed vesicles in lava);

Lava = effusive product; pyro-clast = explosive product


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Melt Properties I, Chemistry:

SiO2

Term SiO2(wt%) (Note)

Acid (felsic/salic) >63 (High silica)

Intermediate 52-63 (Intermediate silica)

Basic (mafic) 45-52 (Low silica)

Ultrabasic


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Felsic Rock Type: Properties

End Member Type I : - FELSIC (or Salic / ACID):

Magmas composed of minerals that melt (faire fondre)

at the lowest temperatures.

Felsic magma is K- and Na-rich, and contain plagioclase

feldspars.

While K- and Na-rich plagioclase feldspars are rich in

aluminum (Al), feldspars and quartz are rich in silicon (Si).

The presence of these elements give Felsic rocks a light(clair) color.

The presence of the feldspar and silica gives rise to the

name of these magmas: Felsic (fels for feldspar; sic for silica)


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Mafic Rock Type: Properties

End Member Type I I : - MAF IC (BASIC):

Magmas composed of minerals that melt (faire fondre)

at the highest temperatures.

Mafic magma is Ca-rich, and contains olivine and

pyroxene.

Olivine and pyroxene are rich in iron (Fe) and/or

magnesium (Mg).

These elements give Mafic rocks a dark (fonce/sombre) color.

The high Mg and Fe contents give rise to the name of these

magmas: Mafic [ma for magnesium; fic for ferric (2+)

iron].


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Classification of MacDonald (1972):

Table A2-2, Page 462

Rhyolite

Rhyo-

dacite Dacite Trachyte Phonolite Andesite

Tholeiitic

Basalt Pyroxenite

Alkalic

Basalt Peridotite Serpentine Nephelinite Dunite

SiO2 73.6 66.3 63.6 58.3 56.9 54.2 50.8 50.5 45.8 43.5 40.4 40.3 40.2

TiO2 0.2 0.7 0.6 0.7 0.6 1.3 2.0 0.5 2.6 0.8 tr 2.9 0.2

Al2O3 13.4 15.4 16.7 18.0 20.2 17.2 14.1 4.1 14.6 4.0 1.9 11.3 0.8

Fe2O3 1.2 2.1 2.2 2.5 2.3 3.5 2.9 2.4 3.2 2.5 2.8 4.9 1.9

FeO 0.8 2.2 3.0 2.0 1.8 5.5 9.1 7.4 8.7 9.8 4.3 7.7 11.9

MgO 0.3 1.6 2.1 2.1 0.6 4.4 6.3 21.7 9.4 34.0 36.0 13.3 43.2MnO 0.3 0.1 0.1 0.1 0.2 0.1 0.2 0.1 0.2 0.2 - 0.2 0.2

CaO 1.1 3.7 5.5 4.2 1.9 7.9 10.4 12.0 10.7 3.5 0.7 13.0 0.8

Na2O 3.0 4.1 4.0 3.8 8.7 3.7 2.2 0.4 2.6 0.6 3.1 0.3

K2O 5.4 3.0 1.4 7.4 5.4 1.1 0.8 0.2 1.0 0.2 0.2 1.4 0.1

H2O+ 0.8 0.7 0.6 0.5 1.0 0.9 0.9 0.5 0.8 0.8 10.5 1.1 0.4

P2O5 0.1 0.2 0.2 0.2 0.2 0.3 0.2 0.1 0.4 0.1 tr 0.8 0.1

after Nockolds SR (1954): Average chemical compositions of some igneous rocks

Geol Soc Am Bull, 65, 1007-1032.

Name of volcanic rock with

Characteristic (or case type) composition

Place into 4 classes: Felsic, Intermediate, Mafic & Ultra-Basic

Define the oxide amounts that define each class: are there anomalies?


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Rhyolite Rhyodacite Dacite Trachyte

SiO2 73.6 66.3 63.6 58.3

TiO2 0.2 0.7 0.6 0.7Al2O3 13.4 15.4 16.7 18.0

Fe2O3 1.2 2.1 2.2 2.5

FeO 0.8 2.2 3.0 2.0

MgO 0.3 1.6 2.1 2.1MnO 0.3 0.1 0.1 0.1

CaO 1.1 3.7 5.5 4.2

Na2O 3.0 4.1 4.0 3.8

K2O 5.4 3.0 1.4 7.4


Acid-to-Intermediate (Felsic) End


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Intermediate-to-Basic (Mafic) End

Andesite Tholeiite Pyroxenite Alkali

Basalt Basalt

SiO2 54.2 50.8 50.5 45.8

TiO2 1.3 2.0 0.5 2.6Al2O3 17.2 14.1 4.1 14.6

Fe2O3 3.5 2.9 2.4 3.2

FeO 5.5 9.1 7.4 8.7

MgO 4.4 6.3 21.7 9.4MnO 0.1 0.2 0.1 0.2

CaO 7.9 10.4 12.0 10.7

Na2O 3.7 2.2 0.4 2.6

K2O 1.1 0.8 1.4 1.0


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Ultra-Basic (Ultra-Mafic) End

Peridotite Serpentine Nephelinite Dunite

SiO2 43.5 40.4 40.3 40.2

TiO2 0.8 tr. 2.9 0.2Al2O3 4.0 1.9 11.3 0.8

Fe2O3 2.5 2.8 4.9 1.9

FeO 9.8 4.3 7.7 11.9

MgO 34.0 36.0 13.3 43.2MnO 0.2 - 0.2 0.2

CaO 3.5 0.7 13.0 0.8

Na2O 0.6 3.1 0.3

K2O 0.2 1.4 0.10.2


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Al2O3Saturation Classes

Peraluminous:

Molecular Al2O3> (CaO + Na2O + K2O)

Metaluminous:

Molecular Al2O3< (CaO + Na2O + K2O)

but

Molecular Al2O3> (Na2O + K2O)

Subaluminous:

Molecular Al2O3~ (Na2O + K2O)

Peralkaline:

Molecular Al2O3< (Na2O + K2O)


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Simple Chemical Classification for

the Common Volcanic Rock Types

Rock Type SiO2 Al2O3Saturation Class Class(wt%)

Rhyolite >68 Peraluminous+Metaluminous Acid (silicic, salic)

(FELSIC)

Dacite 63-68 Peraluminous+Metaluminous

Andesite 57-63 Metaluminous Intermediate

Basalt

Tholeiitic 52-57 Metaluminous-to-Subaluminous

Hawaiite

Alkali Basalt 45-52 Metaluminous-to-Subaluminous Basic

(MAFIC)

Leucite


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Some physical properties of magma (or lava) related to differences

in chemical composition. Basaltic lavas have lower SiO2contents,

higher temperatures, and lower viscosity when compared to more

evolved lavas such as dacites and rhyolites.

Credit: J. Johnson, U.S. Geological Survey Photoglossary of Volcanic Terms.


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Density

The density of a magma is determined by its temperature, pressure, and

composition. Increasing temperature causes magma to expand, which decreases its

density, while increasing pressure causes magma to compress, which

increases its density.

The most important factor controlling magma density is composition.

In particular iron, which is relatively abundant in basalts, has a highermass-to-radius ratio than elements like potassium and silicon, which

are enriched in evolved magmas.

The density of most magmas ranges from 2.2 to 3.1 g/cm3, with basalts

being densest and rhyolites being least dense.

Viscosity


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Viscosity

One of the most important physical properties of magma in terms of determining its

behavior is its viscosity.

Viscosity is a measure of resistance to flow, i.e. the thickness or stickiness of a liquid.

A low-viscosity liquid is runny(water). A high-viscosity liquid is thicker and lessrunny(honey or caramel). The viscosity of a silicate liquid is determined by its

composition.

Viscosity is typically expressed in units of Pas (Pascal-seconds) or P (poises), where

1 Pas = 10 P

Silica (SiO2) in a melt tends to form an interconnectednetwork of ions, like a polymer. The more silica there

is in a melt, the more developed the network becomes,

giving the liquid a higher viscosity. Conversely,

increasing the temperature of a melt has the effect of

decreasing its viscosity.

Because basaltic liquids are characterized by both

higher temperatures and lower SiO2contents than

rhyolitic liquids, basaltic liquids will also have lower

viscosities.

.


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Viscosity, or the resistance of magma to flow when

subjected to a force.

Viscosity is measured in the units of Poise and there

is a tremendous rangedisplayed by magmas erupted

on Earth.

In general, the viscosity of magma increases as the

silica content increases from basalt to rhyolite.

Magmatic viscosities are much larger than fluids that

we are familiar with, such as water or olive oil.


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A natural magma or lava is more complex than a pure silicate melt. A

magma (or lava) is typically a mixture of melt and crystals. Thus, the

viscosity of a magma also depends on the degree of crystallization.

The greater the percentage of crystals, the higher the viscosity. We

refer to this as the effective viscosity of the magma. The effectiveviscosity is related to the liquid viscosity by the relationship

eff=0(11.35)2.5

where heffis the effective viscosity, h0is the crystal-free liquid

viscosity, andfis the fractional volume of crystals.

Viscosity of silicate liquids plotted as a

function of SiO2content and

temperature.Note that magmas at lower temperature

are more viscous than those at higher

temperature for any given SiO2content. For a

given temperature, the viscosity increases

with SiO2content.


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SiO2: Control on Viscosity

Viscosity is a measure of the resistance of a fluid to flow

Viscosity (h) increases logarithmically with SiO2

Log(h)

SiO2(wt%)

1 Pa s

1015Pa s

50

wt%

70

wt%

Lava lakes (10-103Pa s)Lava flows (100105Pa s)

Lava domes (>109pa s)

Silicic lava flows (108-1010Pa s)


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Density increases with decreased SiO2Why?

McBirney (2007), Fig. 2.11

Because:(i) silica is one of the lightest

oxides, and

(ii) silica is the most dominant

oxide:

density decreaseswith

increasedSiO2

Basaltis the most densemelt(least SiO2).

Rhyoliteis the least densemelt

(most SiO2).


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Melt Properties II, Temperature

Melt temperature varies with composition

Generally, the lower the SiO2content, the higher the melt T

(Plus le taux de silice est bas plus la temprature est leve)

Rock Type Temperature (C)

Rhyolite 700900

Dacite 8001100

Andesite 9501200

Basalt 10001200

From Cas & Wright (1987)


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Location Date Temp, C Technique (source)

Santiaguito 1940 725 Pyrometer (1)Santiaguito 1990 850 Chemistry (2)

Etna 1043-1111 Chemistry (3)

Etna 2001 1074 Radiometer

Etna 1975 1070-90 Thermocouple (4)Etna 1991-93 1080 Thermocouple (5)

Mauna Loa 1984 1143 Thermocouple (6)

Stromboli 1901 1150 Thermocouple (7)

Kilauea 1999 1150 Thermocouple (h)

Erta Ale 1970 1175 Thermocouple (8)

Vesuvius 1913 1200 Thermocouple (7)

Mauna Loa 1859 1205 Chemistry (9)

(1) Zies (1941); (2) Scaillet et al. (1998); (3) Pompilio et al. (1998); (4) Pinkerton & Sparks (1976); (5) Calvari et

al. (1994); (6) Lipman and Banks (1987); (7) from Table 4.2 of MacDonald (1972); (8) Le Guern et al. (1979); (9)Riker et al. (2009); (h) field-measurement.

Some Lava Interior (Eruption) Temperatures


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Blue glassy (p-type) pahoehoe lobe, Kilauea

~50 cm

Lava Surface Temperature


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The surface temperature of the lava is always less than that of

the interior

Tinterior= Terupt= 1150 C

Active pahoehoe lobe, Kilauea, 1999

900-950 C

750-850 C

Lava Interior versus Surface Temperature


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Temperature: Control on Viscosity

Viscosity is a measure of the resistance of a fluid to flow

Viscosity (h) increases logarithmically as Temperature decreases

Log(h)

Temperature (C)


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The large range of magmatic viscosities is related to differences in the structure of

magmas at the molecular level, which varies as a function of silica content.

Magmas are fluids but they possess structure in the form of silicon-oxygen building

blocks, that are linked together to form tetrahedra.

Magmas that are rich in silicon form many linked tetrahedra that result in the

polymerization of the liquids. The bonds that form between shared oxygen atoms are

strong, thus giving the liquid strength and high viscosity.

Lower silica magmas have higher contents of anions such as Ca, Mg and Fe that form

weaker bonds and inhibit the development of strongly linked silicon tetrahedron. Thereforelow-silica magmas have a lower viscosity.

Structure of silicon tetrahedron (A and B)

and the linkage of two tetrahedron by

sharing an oxygen (C)


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Viscosity

increases with

SiO2

From Oppenheimer (2003)

Viscosity Relations and Data

Viscosity

decreases with

increased temperature


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Vesicle Shapes: Rounded

No shearing; bubbles can grow in all directions

~2 cm


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Vesicle Shapes: Sheared

Shearing, so bubbles are deformed, stretched and elongated

X500 50mm 10 mm

in the conduit in a lava flow


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Highly Vesicular Lava: Surface


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Vesicle Volume Fractions

For Lava

Location Range (%) Mean (%) Notes (Study)

Etna 442 20 'a'a-dominated (1)

Etna 771 32 'a'a-dominated (2)

Kilauea 1070 4060 pahoehoe-dominated (3,4)

Kilauea 4294 70 at-vent (1997 overflows)

(1) Herd & Pinkerton (1997); (2) Gaonach et al. (1996);

(3) Wilmoth & Walker (1993); Cashman et al. (1994).


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For round bubbles: Viscosity (h) increases with number of bubbles

For sheared bubbles: Viscosity (h) decreases with number of bubbles

h

Bubble Content

Power Law

(Einstein-Roscoe Relation)

Round bubbles

Sheared bubbles

Bubbles: Control on Viscosity


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Magma Properties III: CrystalsLavas and pyroclasts usually contain a mixture of

glass, phenocrysts, microphenocrysts and bubbles

Phenocrysts: >100-200 m

microphenocrysts: 30-200 m

microlites:


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Crystals and Temperature

As temperature decreases

the relative proportions ofmelt and crystals changes

Crystal content also

depends on cooling rate:

(1) Slow cooling:

Phenocrysts

(2) Fast Cooling:

Micro-phenocrysts

(3) Very Fast Cooling

Microlites

(4) Quenching:

Glass

McBirney (2007), Fig. 3.2, page 76Proportions of crystals and liquid for a basalt


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Kilauea lava cooled to 1170 C, then quenched:

Olivine phenocrysts + pyroxene microphenocrysts + melt (glass)

Crystal-Glass Mix in a Cooling Lava

(i) 1170 C


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Kilauea lava cooled to 1130 C, then quenched:

Olivine phenocrysts + larger pyroxene microphenocrysts


(ii) 1130 C


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Kilauea lava cooled to 1020 C, 80 % crystals:

Small pools of Fe-poor, SiO2-rich liquid remain in a crystal mush


(iii) 1020 C


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Kilauea lava below 990 C:

Rock is almost totally crystalline


(iv) 990 C

C l C l Vi i


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Crystals: Control on Viscosity

Mixture viscosity increases with crystal content

Power Law (Einstein-Roscoe Relation)

h

Crystal Content (f)

h0

h= h0 (1 - 1.67 f)-2.5

Offset

viscosity of the liquid

viscosity of the mixture

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AH1 - Magma Properties