Tectonism

Igneous Processes

Volcanoes


- 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