Physical properties of magmas!

Naming volcanic rocks!

Physical properties of magmas 1. density (ρ), 2.viscosity (η), 3. thermal conductivity (κ)(control flow rates, cooling rates, eruption rates etc.)!

1. Density ρ = M / V (g cm-3)

ρ (g

cm

-3)

900 1000 1100 1200 TºC

2.8

3.0

10 20 30 P (kbars)

Density of Kilauea basaltic magma as function of Pressure

Plag An80

ρ (g

cm

-3)

2.0

2.2

2.4

2.6

2.8 Basalt 1

Basalt 2

Andesite

Rhyolite

Liquidus T

Some lab measurements

Viscosity!

ü The physical property that most influences the observed eruptive behavior of magmas!

ü Viscosity depends on !ü  Chemical composition (most strongly correlated with

SiO2 content and volatile content), !ü  Temperature!ü  Pressure!ü  Crystallinity!

Definition of viscosity!

= σ/ε .

a. Calculated viscosities of anhydrous silicate liquids at one atmosphere pressure, calculated by the method of Bottinga and Weill (1972) b. Variation in the viscosity of basalt as it crystallizes (after Murase and McBirney, 1973), Geol. Soc. Amer. Bull., 84, 3563-3592. c. Variation in the viscosity of rhyolite at 1000ºC with increasing H2O content (after Shaw, 1965, Amer. J. Sci., 263, 120-153).

ViscosityComparison with familiar liquids!

Baker et al., 2004

Measuring viscosity!

ü Viscometer (e.g., falling sphere)!

ü Pitch Drop experiment!ü Started in 1927 ü Ig-Nobel prize in

2005

Why should melt viscosity depend on composition?!ü Need to think about molecular

level melt structure to understand the connection!

Crystalline and molten SiO2 structure!

Crystalline SiO2 Liquid SiO2

Addition H2O, Na2O, MgO,… leads to depolymerization (breaking of large

“molecule”)

Contrasting structures of melts rich in network-forming and network-modifying components!

fully polymerized

depolymerized

Thermal conductivity!

ü Rate at which heat is conducted through rocks and magmas. [Units: W/m-1K-1].

ü Values fairly constant for silicate (~2-4 W/m-1K-1).

ü In cooling rate calculations, we use the thermal diffusivity: k = K / ρCp (Cp is the specific heat). [Units of k: m2 s-1].

Magmas: multiphase systems!

Why do bubbles nucleate in magmas?!

Supersaturated in volatile elements!

ü H2O!

From Moore et al., 1998

CO2

Explosive eruptions!

Why do crystal grow in magmas?!

ü Cooling!ü Decompression

in gas-saturated conditions!

10

20

Pre

ssur

e (K

b)

Silicic magma

Liquidus (dry)

Solidus (dry) Solidus + xs. H2O

Liquidus + xs. H2O