Honors 1360 Planet Earth Last time: Hyp : Earthquakes release accumulated stress & strain
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s 1360 Planet Earth rthquakes release accumulated stress & strain rthquake “sequences” (Sumatra, Turkey) where large stress changes following one event favor low fault slip” events, harmonic (seismic) tremor omeday be possible to predict EQs; need improved derstanding of physics & MUCH better measurements sm 26 September 2008 Read for Mon: 181-211
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26 September 2008. Honors 1360 Planet Earth 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 - PowerPoint PPT Presentation
Transcript of Honors 1360 Planet Earth Last time: Hyp : Earthquakes release accumulated stress & strain
Honors 1360 Planet Earth
Last time:
Hyp: Earthquakes release accumulated stress & strain Obs: Earthquake “sequences” (Sumatra, Turkey) where large stress changes following one event favor anotherObs: “Slow fault slip” events, harmonic (seismic) tremor
May someday be possible to predict EQs; need improved understanding of physics & MUCH better measurements
Today:• Volcanism
26 September 2008
Read for Mon: 181-211
Global Volcanism (last 10,000 years)
Note: There are more under the oceans!
Recall The Typical Geotherm:
Dep
thTemperature
00 ~1300 ºC
~150 km
Conductive
Convective
Normally does not intersect melt temperature for dry mantle rock!
Dep
thTemperature
00
Conductive
Convective
Mel
t
Option 1: Raise the Geotherm!
Dep
th
Temperature00
Conductive
Convective
Mel
t
(There are two ways this can happen:)
1.i: Extension at mid ocean ridges, continental rifts:
Deep rock moves upward to fill the space createdcarrying 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 T/z
Recent Volcanism
Option 2: Add Water!
Dep
thTemperature
00
Conductive
Convective
Mel
t
Water Reduces MeltingTemperature 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 Andesite Rhyolite
(darker) (lighter)
SiO2 (Quartz)~50% ~60% ~70%
Melt Temperature:~1200 ºC ~700 ºC
Viscosity:(low) (high)
Basalt
Andesite
Rhyolite
Why does viscosity matter?
0
6
9
12
3
Dep
th (
km)
0% 5% 10%Weight-% solubility of water
Steam, CO2, other gases are 90% of volume at surface!
Basaltic
Andesitic
Rhyolitic
Why different compositions?
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!
Intrusion!
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)
TaalVolcano
PhilippinesYellowstone
GPStime
series
Interesting Aside:• Calderas characterized by “unrest” (changing deformation…)• Since we’ve never seen a caldera eruption, would we know what to look for?
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