Extrusive silicic rocks
In order to understand silicic magmatism, must look at volcanic counterparts of plutons
Mineralogy: same as granites
Volcanic rocks: >90% ignimbrites + pumiceous airfall , <10% glassy lavas - dacites & rhyolites
Size: ash, lapilli, blocks & bombs
Ignimbrites - a.k.a. ash flow tuffs
Plinian columns - Vertical eruptive column that contains gas, ash sized magma, crystals, lapilli, pumice, lithics, etc. Get air fall first (coarsest material right around vent, finest material further away)
Column collapses, get turbulent flow of ash, gas, and dust
Welding: glass shards and lumps of frozen magma can soften and weld together inside ignimbrite flow after deposition - 500º - 600ºC
Examples of ignimbrites:
- Voluminous ones: mid-Tertiary western US
- Recent ones: 1912 Mt. Katmai, AK Valley of 10,000 smokes
- Often, basal part is more differentiated than top
- Why? Stratified magma chamber is tapped from the top down. Lowest ash flow sheet is from top of chamber, higher up sheets from deeper.
- But generally, ignimbrites only tap upper levels of chamber. Sudden interruption of solidification.
- Does zonation indicate lack of convection?
Long Valley Caldera
Zoning of magma chamber deduced from compositions of erupted products
Progressive inward crystallization at shallow depth with recurrent injection of new mafic magma.
Fractionation of light liquids upwards during sidewall crystallization.
Multiple convection cells
Can't have one big convection cell because agma too viscous, but smaller scale convection cells ok
How does compositional zonation originate to begin with?
Crystallization at base of walls and floor, creates differentiated liquid L1
Continuous rise of felsic liquids
Size of eruptions
Crude relationship between caldera size and size of associated ignimbrite