G141 REVIEW: MAGMA
GENERATION AND MODIFICATION
Magma Melting
- Source of heat = decay of radioactive elements
- Geothermal gradient = temperature increase with depth
- Melting curve: increase in Tmelt with depth (due to
pressure increase)
Mechanisms of magma generation
- Temperature increase (hot-spots)
- Pressure decrease = ‘decompression melting’ (Divergent
boundaries)
- Addition of water ŕ
decrease in Tmelt (Subduction zones)
Buoyant ascent of magma
- Melting ŕ
expansion of volume
- Decrease in density
- Upward flow due to gravity
Fractional Crystallization
- Systematic change in magma composition due to
crystallization
- SiO2 poor ŕ
SiO2 rich,
- Fe, Mg rich ŕ
Fe, Mg poor
- Removal of crystals changes composition of remaining melt
- Increase viscosity, increase gas content ŕ increase explosivity
Other processes of magma modification
- Assimilation = dissolution of chemical components of
surrounding rock
- Magma mixing = mixing with other magma sources
- Partial melting of crust