Lecture topics

1
Lecture topics

• Elements, stars and the solar system
• Earthquakes
• Volcanoes
• Tsunamis
• Plate tectonics
• Nov. 2 - Midterm

2
Todays Lecture

• Review of volcano classification
• Volcano explosivity index (VEI)
• Controls on eruption style
• Monitoring volcanoes
• Volcanoes and plate boundaries

3
Last lecture ...
Shield volcano
Stratovolcano
Scoria cone
Caldera
http//www.geology.sdsu.edu/how_volcanoes_work/
4
Types of eruptions
Hawaiian
Non-explosive eruptionwith effusive lava flows
Vulcanian
Plinian
Explosive eruption with voluminous plume of
tephra
Strombolian
http//www.geology.sdsu.edu/how_volcanoes_work/
5
MEASURING EXPLOSIVENESS The Volcano Explosivity
Index (VEI)
http//www.geology.sdsu.edu/how_volcanoes_work/
6
PHYSICOCHEMICAL CONTROLS ON ERUPTION STYLE

• Magma viscosity
• Bulk composition and temperature
• Dissolved gas

http//www.geology.sdsu.edu/how_volcanoes_work/
7
Magma Viscosity
Viscosity Ability of a substance to resist flow.

• The most abundant elements in magma are Si and O
• While magma is cooling, Si and O bond to form
  tetrahedra (A and B)

http//www.geology.sdsu.edu/how_volcanoes_work/
8
Magma Viscosity

• With further cooling, Si-O tetrahedra combine to
  form double tetrahedra (C), chains, double chains
  and networks
• This process (polymerization and
  crystallization) increases viscosity
• Higher magma viscosity more explosive!

http//www.geology.sdsu.edu/how_volcanoes_work/
9
Bulk Composition and Temperature
More SiO2 lower melting temperature higher
viscosity
http//www.geology.sdsu.edu/how_volcanoes_work/
10
Dissolved gas

• Gas bubbles in magma form vesicles in volcanic
  rock
• Rising of magma causes decompression
• Sudden formation of bubbles (gas exsolution) can
  trigger an eruption - like opening a can of pop

http//www.geology.sdsu.edu/how_volcanoes_work/
11
http//www.geology.sdsu.edu/how_volcanoes_work/
12
Monitoring Volcanoes

• Deformation (tilt)
• Seismic activity (focal depth)
• Gas chemistry

http//volcano.und.nodak.edu/vwdocs/vwlessons/moni
tors.html
13
Monitoring Volcanoes using Tilt

• As magma accumulates beneath a volcano, it
  exerts pressure on the overlying and surrounding
  rocks.
• The pressure causes the summit of the volcano to
  move upward and outward

http//volcano.und.nodak.edu/vwdocs/vwlessons/moni
tors.html
14
Instrumentation for measuring tilt
Tiltmeter - a sophisticated form of a carpenters
level
GPS station - measures changes in location using
satellites
http//volcano.und.nodak.edu/vwdocs/vwlessons/moni
tors.html
15
Monitoring Volcanoes using Seismic activity

• The frequency and distribution of earthquakes
  provides information about magma movement and
  volcano structure

16
Monitoring Volcanoes using Seismic activity
1. Rising magma produces a few small, deep
earthquakes
17
Monitoring Volcanoes using Seismic activity
3. Earthquakes move to shallow crust
18
Worldwide Distribution of Volcanoes
Most of the 550 active volcanoes are at plate
boundaries
19
Volcanism at divergent plate boundaries
Fissure eruptions at mid-oceanic ridges
20
Volcanism at convergent plate boundaries

• Volcanoes form above subduction zones due to
  release of water
• Typically stratavolcanoes

21
Intraplate (hotspot) volcanism
Hawaiian volcanoes are believed to be located
above a mantle hotspot