Quiz:
1.
What would be a reasonable temperature for a
basaltic magma?
2.
Under what conditions is Ca a network-forming ion?
Geo 406 - Volcanic Structures
Announcements
Quizzes – every 2-4 lectures, not always on Monday
Questions from last time?
Goals for today
Understand Volcanic Structures
Other controls on viscosity
Phenocrysts
increase viscosity (WHY?)
Not trivial: when they rub together or constrict the flow of melt between them do they have
a big effect, but there is an effect even at low amts of crystallization. [ show figure]
Volatiles
Main gases: H2O, CO2, SO2, H2, CO, H2S, S2, HCl
Solubility = f(T, P, Xgas, Xmagma)
Solubility & Temperature
H2O is less soluble with increasing Temp
Note that this is counter-intuitive!
What would happen to a rhyolitic or dacitic melt saturated wrt H2O if a pulse of
basalt was injected into the magma chamber?
Explosive eruption!
Solubility & Pressure
H2O more soluble with increasing pressure
What happens when pressure is abruptly released on a magma chamber?
Mt. St. Helens!
Volume change on volatile exsolution is huge
can cause an explosion
“vesiculation”
volatiles concentrate near tops of magma chambers
earlier parts of eruptions often are more violent
Volc. Structures
Two types: Lava vs. Pyroclastics
Lava-related
Shield volcanoes
Lava rivers, high T, basalt, runny - pahoehoe
Lower T, more viscous – aa
[Show movies]
Domes
[Show MSH dome/spine movies]
Flood basalts – fissure eruptions
Requires extensional tectonics
Columnar basalt
Cooling and contraction forms cracks normal to cool surface
Like mudcracks
Pillow basalt
Pyroclastic
Tuff – rock formed of pyroclastic material
Tephra - all materials erupted explosively and deposited from the air
not necessarily lithified
Pyroclastic flow (nuees ardentes) – Down-flowing mixture of tephra and hot gases;
flows down valleys to plains
Surge – energetic, cross-bedded deposit
Ignimbrite – The deposit formed by a pyroclastic flow
Fills valleys
Welded in middle
Airfall deposit – tuff formed from ash falling through the air.
Mantles hillsides
well-sorted
layered
poorly welded