
Magma and Its Properties  
C http://ijolite.geology.uiuc.edu/08SprgClass/geo436/436%20lectures/L3Magma.html
http://ijolite.geology.uiuc.edu/08SprgClass/geo436/lectures.html
Nature of magma:
 Igneous rocks form from molten material . Most are silicate melts
 As silicate magmas cool, the common "rockforming 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
ハ
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
