- Where do volcanoes occur?
- What is the driving force behind a volcanic eruption?
- gasses bubbling out of solution
Why are some volcanoes explosive? - magma viscosity
- Silica in Magma
=> more SiO2 -> increased complexity = high viscosity
=> less SiO2 -> less complexity = low viscosity
- fluid versus explosive…
Controls on Magma Behavior
-> One of the most important characteristics of magma = viscosity (resistance to flow)
-> Controls on viscosity=1) composition, 2) volatiles,3) pressure
MAGMA
felsic 70% Si 10-15% H2O
intermediate 60% Si 5-7% H2O
mafic 50% Si 1-2% H2O
Extrusive Igneous Activity - Volcanism
=> magma erupted at the surface
ERUPTED MATERIALS
1. Lava Flows
- pillow lava
- pahoehoe
- aa
2. Volatiles - gasses
- H2O
- CO2
- SO2 and H2S
- the driving mechanism for most eruptions
- a method for predicting eruptions
3. Pyroclastics
- ash and cinders
- Flow and fall
- blocks and bombs
Þ tuff
4. Mudflows (lahars)
• The hazard to humans 10’s of km from the volcano
Volcanic Architecture
• Magma chamber
• Chimney & vent
• Caldera
• Resurgent dome
Eruptions and Volcanoes
1. Fissure Eruptions *MOR's and “Hot Spots”
=> very fluid lava = basalt = mild eruptions
2. Shield Volcanoes *MOR's and “Hot Spots”
=> fluid lava
=> basaltic flows, few pyroclastics = mild eruptions
Volcano Size
3. Cinder Cones *”Hot Spots” & Subduction
=> all pyroclastics = violent eruptions
=> small (< 300 m) symmetrical cone w/steep slopes (~30°)
=> short lived eruptions, and volcano easily eroded
EX.: Paracutin (Mexico)
4. Composite Cones (stratovolcanos) * Subduction
=> andesitic lava flows and pyroclastic eruptions
= very violent eruptions
Þ alternating layers of lava flows and pyroclastics
Þ Eruptive cycles
5. Calderas *continental hot spots
=> regional volcanic collapse features
=> pyroclastic eruptions = extremely violent
Western Calderas - plate movement over hot spot
Volcanoes & Human History
• The loss of entire cities
• Climate change
Predicting Eruptions =>
- Precursors
- Shape
- Seismic Activity
- Gasses