Download 26 Sep: Volcano Processes

Honors 1360 Planet Earth
26 September 2008
Read for Mon: 181-211
Last time:
Hyp: Earthquakes release accumulated stress & strain
Obs: Earthquake “sequences” (Sumatra, Turkey) where
large stress changes following one event favor another
Obs: “Slow fault slip” events, harmonic (seismic) tremor
 May someday be possible to predict EQs; need improved
understanding of physics & MUCH better measurements
Today:
• Volcanism
Global Volcanism (last 10,000 years)
Note: There are more under the oceans!
Recall The Typical Geotherm:
0
0
Depth
~150
km
Temperature
~1300 ºC
Normally does not intersect melt temperature for dry
mantle rock!
Depth
0
0
Temperature
Depth
0
0
Option 1: Raise the Geotherm!
Temperature
(There are two ways this can happen:)
1.i: Extension at mid ocean ridges, continental rifts:
Deep rock moves upward to fill the space created
carrying heat with it!
1.ii: Bring hot rock to the base of the lithosphere by
convection (mantle plume or “hotspot”)
E.g., Yellowstone, Hawaii!
Heat Flow:
Q = k DT/Dz
Recent Volcanism
Option 2: Add Water!
Depth
0
0
Temperature
Water Reduces Melting
Temperature of Rock
By 200-300 Degrees
If it forms in the mantle, why does it come to the surface?
Basalts:
--found at mid-ocean ridges, hotspots, continental rifts
Oceanic crust is entirely made of basalt/gabbro
Andesites:
--found mostly over subduction zones
Silicic Volcanoes:
--usually in continental rifting settings, continental
hotspots
Basalt
(darker)
~50%
Andesite
Rhyolite

(lighter)
SiO2 (Quartz)
~60%
~70%
Melt Temperature:
~1200 ºC

~700 ºC
(low)
Basalt
Viscosity:

Andesite
(high)
Rhyolite
Why does viscosity matter?
Weight-% solubility of water
0%
5%
10%
0
Basaltic
Depth (km)
3
6
Andesitic
9
Rhyolitic
12
Steam, CO2, other gases are 90% of volume at surface!
Why different compositions?
Intrusion!
Magma rises to level of
neutral buoyancy in the
crust!
Large enough melt body
can fractionate (lighter
melt fraction floats to top)
Need some combination
of hotter magma, more
silicic magma, and/or
more dense crust to get
all the way to surface!
Volcanism & Intrusion important because:
• Transports lighter components of mantle upward
to form crust
• Cycles Volatile components (water, CO2, SO2 etc.)
back into the atmosphere/hydrosphere
(would completely recycle every ~1.5 billion years!)
• Concentrates resources (geothermal and mineral)
Major Volcano Landforms:
Basaltic: Shield
volcanoes, cinder cones
Andesitic:
stratovolcanoes
(Mt Rainier,
Mt St Helens)
Rhyolitic: Large
Calderas (Taal,
Yellowstone)
Interesting Aside:
• Calderas characterized by “unrest” (changing deformation…)
• Since we’ve never seen a caldera eruption, would we know
what to look for?
Taal
Volcano
Philippines
GPS
time
series
Yellowstone
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.