Physical Properties of Magmas
What do magmas consist of?
- liquid (melt) - usually silicate, may be carbonate, sulfide, iron
- solid - crystals
- gas - may be dissolved in liquid (not a separate phase) or may be exsolved as a free vapor
What is a silicate liquid? - 3 dimensional network of silicate tetrahedra. See handout
Click here for table of
silicate melt properties
- Some replacement of Si4+ with Al3+ which causes charge imbalance.
Insert other +1 cations to balance charge (Na+, K+ which fit
- Network formers - Si4+, Al3+, Na+, K+
- Network modifiers (break Si-O bonds and network) - Fe2+, Fe3+, Mg2+, Ca
2+, Ti4+, H2O
- Keep in mind, if Na+ + K+ < Al3+, then they are network formers.
HOWEVER, Na+ + K+ > Al3+, then they become network modifiers
If Na+ + K+ << Al3+, then Ca2+ becomes network former.
- So what does this mean physically?
strong network = viscous magma = rhyolite
weak network = fluid magma = basalt
Temperature (T) of magmas
Direct measurements - sheathed thermocouple, optical pyrometer
Indirect measurements - lab experiments to melt rock, geothermometry on
Viscosity (n) of magmas
- Describes resistance to flow. Can
think of ratio of shear stress to strain rate (rate at which rock strains
relative to the stress applied).
- See class notes for plots of stress vs. strain, Newtonian
versus non-Newtonian fluids
- n = f(T,P,X,presence of dissolved vapor, %crystals)
- As T increases, n decreases
- Pressure effect can be ignored
- Composition - more network formers n increases, more network modifiers
- Dissolved vapor - as H2O increases, n decreases
BUT, CO2 is network former, so as CO2 increases, n increases
- %crystals - increase % crystals, increase n
Density - p in g/cm3
- Silicic magmas (~2.2 g/cm3) to mafic magmas (~2.7 g/cm3, or 2.8
- Very important when thinking about upward movement of magmas - buoyancy
- Important when thinking about crystal settling
mafic minerals - 3.0 g/cm3, will always sink
See class notes for density versus depth plot. Explanation of why we
don't see magmas that derived from great than 400 km depth.
plagioclase - 2.7 g/cm3, will sink in Fe-poor liquids, will float in
leucite - 2.47 g/cm3, will generally float
- Major volcanic gases are H2O, CO2, SO2, H2, CO, H2S, S2, HCl
- Solubility - f(T,P,Xgas,Xmagma)
H2O as an example
1. Temperature - H2O less soluble as temperature increases
2. Pressure - H2O more soluble as pressure increases
- Important in generating explosive eruptions when hot basalt injects base of more felsic magma chamber
3. Can ignore Xmagma for now.
- Important in generating explosive eruptions with sudden lowering of pressure