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 - 500 - 600C

 

 

Examples of ignimbrites:

- Voluminous ones: mid-Tertiary western US

- Recent ones: 1912 Mt. Katmai, AK Valley of 10,000 smokes

  1. Often, basal part is more differentiated than top
  2. Why? Stratified magma chamber is tapped from the top down. Lowest ash flow sheet is from top of chamber, higher up sheets from deeper.
  3. But generally, ignimbrites only tap upper levels of chamber. Sudden interruption of solidification.
  4. 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.

Model:

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