Transcript Physicochemical controls on volcanic eruptions.

Lecture topics
• Elements, stars and the solar
system
• Earthquakes
• Volcanoes
• Tsunamis
• Plate tectonics
• Nov. 2 - Midterm
Today’s Lecture
• Review of volcano classification
• Volcano explosivity index (VEI)
• Controls on eruption style
• Monitoring volcanoes
• Volcanoes and plate boundaries
Last lecture ...
Shield volcano
Stratovolcano
Scoria cone
Caldera
http://www.geology.sdsu.edu/how_volcanoes_work/
Types of eruptions
Hawaiian
Non-explosive
eruption
with effusive
lava flows
Vulcanian
Plinian
Explosive
eruption with
voluminous
plume of
Strombolian
http://www.geology.sdsu.edu/how_volcanoes_work/
tephra
MEASURING EXPLOSIVENESS:
The Volcano Explosivity Index (VEI)
VEI
TEPHRA
PLUME
HEIGHT
TEPHRA
VOLUME
CLASSIFICATION
EXAMPLE
0
<100 m
103 m3
Hawaiian
Kilauea
1
100-1000 m
104 m3
Hawaiian/Strombolian
Stromboli
2
1-5 km
106 m3
Strombolian/Vulcanian
Galeras (1992)
3
3-15 km
107 m3
Vulcanian
Ruiz (1985)
4
10-25 km
108 m3
Vulcanian/Plinian
Galunggung (1982)
5
>25 km
1 km3
Plinian
St. Helens (1980)
6
>25 km
10 km3
Plinian/Ultra-Plinian
Krakatau (1883)
7
>25 km
100 km3
Ultra-Plinian
Tambora (1815)
8
>25 km
1000 km3
Ultra-Plinian
Toba (74 ka)
http://www.geology.sdsu.edu/how_volcanoes_work/
PHYSICOCHEMICAL CONTROLS
ON ERUPTION STYLE
• Magma viscosity
• Bulk composition and temperature
• Dissolved gas
http://www.geology.sdsu.edu/how_volcanoes_work/
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/
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/
Bulk Composition and Temperature
SiO2
MAGMA
TYPE
TEMP.
(Celsius)
VISCOSITY
GAS
CONTENT
ERUPTION
STYLE
~50%
mafic
~1100
low
low
nonexplosive
~60%
intermediate
~1000
~70%
felsic
~800
intermediate intermediate intermediate
high
high
explosive
More SiO2 = lower melting temperature = higher
viscosity
http://www.geology.sdsu.edu/how_volcanoes_work/
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/
1. Bubbles form in rising magma
2. Exsolution of gas causes
explosive fragmentation and
eruption.
http://www.geology.sdsu.edu/how_volcanoes_work/
Monitoring Volcanoes
• Deformation (tilt)
• Seismic activity (focal depth)
• Gas chemistry
http://volcano.und.nodak.edu/vwdocs/vwlessons/monitors.html
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/monitors.html
Instrumentation for measuring tilt
Tiltmeter - a
sophisticated form of a
carpenter’s level
GPS station - measures
changes in location using
satellites
http://volcano.und.nodak.edu/vwdocs/vwlessons/monitors.html
Monitoring Volcanoes using
Seismic activity
• The frequency and distribution of earthquakes
provides information about magma movement and
volcano structure
Monitoring Volcanoes using
Seismic activity
1. Rising magma
produces a few small,
deep earthquakes
2. Earthquakes more
frequent and
shallower
Monitoring Volcanoes using
Seismic activity
3. Earthquakes move
to shallow crust
4. Eruption may be
triggered by
earthquake
Worldwide Distribution of Volcanoes
Most of the ~550 active volcanoes are at plate boundaries
Divergent
Intraplate
(hotspots)
Convergent
Volcanism at divergent plate boundaries
Fissure eruptions at
mid-oceanic ridges
Cooling at ocean floor
produces pillow basalt
Venting of hot
fluids - black
smokers
Volcanism at convergent plate boundaries
• Volcanoes form above subduction zones due
to release of water
• Typically stratavolcanoes
Intraplate (hotspot) volcanism
Emperor seamounts
Hawaiin
seamounts
Hawaiian volcanoes are
believed to be located
above a mantle hotspot
Old volcanoes
(seamounts) reveal
movement of Pacific plate
for last 70 million years