Magma and Its Properties 


Nature of magma:

- Igneous rocks form from molten material . Most are silicate melts

- As silicate magmas cool, the common "rock-forming minerals。ヲ crystallize

- When enough magma has formed, buoyancy causes it to rise.

- Magma may: erupt, crystallize slowly underground, texture develops as rock cools

Magmatic temperatures

- Direct measurement possible for some volcanic eruptions.

- Lab experiments: measure melting point of samples

- Since a magma = mixture of minerals, it melts over a range, not a single temp: basaltic magma 1100 - 1200 oC, rhyolitic lava 800 - 1000 oC

- Increased pressure increases melting temperatures

- Addition of water lowers melting temperatures, by up to several 100 deg

II. Magma Densities

A. Densities control many processes (buoyant rise, differentiation, magma mixing)

B. Measuring density of rocks

- Density may be measured directly

- Or calculated as pハハat T = p1 [1 - a (T - T1)]; pハ = density , T = temperature , aハ = coefficient of thermal expansion = relative increase in volume per unit
increase in T at constant P

C. Density by partial molar volume

- Def: Vi = change in volume resulting from change in # of moles of component i at constant T, P, moles of all other components :V of magma with many components = Va na + Vb nb + ... + Vi ni = SUM Vi ni ; Xi = mole fraction of i = ni / (na + nb + ... + ni); dividing by (na + nb + ... + ni), V = Va Xa + VbXb + ... + ViXi = S ViXi = molar volume
of magma; similarly, molecular weight of magma = SUM MiXi

- Finally, pハ = SUM MiXi / SUM ViXi

D. Density by simple rule of thumb

Volume expansion on melting = 10% for most rocks

- Thus, magma p = 90% rock p

III. Magma Viscosity

A. Basics ''

-Def: resistance to flow = viscosity = n (eta) = Pa s

-Important for: rates of emplacement; shapes of igneous bodies; separation of crystals from magma

-Fluids flow when subjected to shear stress: viscosity = shear stress t / rate of shear strain e

- May be measured or calculated

B. Variation with temperature

-Obeys an Arrhenius relation: n = n0 exp (E / RT) , where E = activation energy, R = gas constant = 8.3144 J/mol K, T = absolute temperature (Kelvin)

-In log form this becomes: ln n = ln no + E / RT

-In lab, measure n for various T, plot ln n versus 1/T to get E and no

C. Variation with pressure

- Much smaller effect than T

- Indirect effect: increasing P permits more water to dissolve in magma, which greatly lowers n

D. Effect on lava flow rate

-For a lava that is a Newtonian fluid in laminar flow (v), neglecting cooling:

-Calculations for various n show how v changes

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