Lecture 3

Physical Descriptions of Magmas - Chapter 5

I. What are magmas? Mixtures of

• Lquid: silicate magma
• Solid: crystals
• Gas: dissolved or undissolved (bubbles)

A. Liquid

• Liquid is a network of silica tetrahedra and wide range of other elements.
• Not like water (H2O). Single constituent melts and freezes at the same temperature. Molten magma is more chemically complex. No clear-cut freezing point.
• Linked silicate tetrahedra - strongly bonded Si-O polymers.

B. Solid - Phenocrysts

• Magmas crystallizing before they are erupted (i.e., beginning the solidification process). Mm to cm across.
• Can track evolution of physical conditions within magma chamber (i.e., zoning, etc.).
• Abundance of obsidian (rhyolitic glass), but not much tachylite (basalt glass) due to slow nucleation of crystals in silicic magmas

C. Volatiles (gases)

• H2O as steam, most abundant
• CO2
• SO2
• Lesser H, HCl, HF, HS, CO, Cl, F

Typical is 97% H2O, 2.7% CO2, 4.8

Measurement of volatiles

• Trap gases in flasks (Fig. 5.2)
• Gas bubbles within crystals or glass

How does pressure affect solubility of gases?

How does temperature affect solubility of gases?

• There is typically a huge change in volume when gas comes out of solution. So if it happens rapidly, this causes an explosion. Vesiculation
• Shards in ash are often just remains of bubble walls. Shattered magma.
• However, gas may just slowly exsolve to form bubbles in magma.

II. Chemical composition

More mafic lavas have less linked silicate tetrahedra, more felsic lavas have fewer

Linking of silicate tetrahedra affects viscosity of magmas.

III. Temperature -

• increases from rhyolite to basalt.
• Can exist over a wide range (no clear freezing/melting point)

Fig. 5.1 of text

• Note that melting T increases with pressure.

IV. Viscosity (n) = Resistance to flow.

Internal resistance to flow by a substance when a shear stress is applied: Shear stress/shear strain

• Water - low viscosity, Honey - high Viscosity. Both are Newtonian fluids
• Toothpaste is Bingham fluid

A. Compositional effect - Why such a difference in viscosity from basalt to rhyolite?

• More Si, better network, stronger, more viscous
• Network modifiers Mg, Fe, Ca, Ti, Mn higher in basalt than rhyolite
• Network formers Al, Na, K higher in rhyolite than basalt

B. Temperature & dissolved H2O content

Both temperature and water content of magma affect viscosity

Water breaks silicon-oxygen bonds by forming OH- ions, thus decreases viscosity.

Increasing temperature causes bonds to break (atoms vibrating faster), so also decreases viscosity

Example: Mono Craters

Viscosity in poise:

	Dry = 0% H2O	1% H2O	3%H2O	5% H2O
815°	8 x 1010	3 x 109	2 x 107	8 x 105
900°	5 x 109	2 x 108	7 x 106	1 x 105

From above, you can see that water is much more important than temperature in affecting viscosity

• 0 - 5% H2O, 5 orders of magnitude change in viscosity
• 100°C change, 1 order of magnitude change in viscosity

C. Phenocrysts - high concentrations strongly effect viscosity

V. Rheology

• Laminar flow - very orderly, low velocity
• Turbulent flow - disorderly, high velocity

Defined by Reynold's number

Re = flow velocity (U) * depth (h) / viscosity (n)

Transition from laminar to turbulent flow happens at Reynolds numbers greater than about 1000-2000.