Honors 1360 Planet Earth Last time: Hyp : Earthquakes release accumulated stress & strain

1. 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 • May someday be possible to predict EQs; need improved understanding of physics & MUCH better measurements • Today: • Volcanism Read for Mon: 181-211

2. Global Volcanism (last 10,000 years) Note: There are more under the oceans!

3. Recall The Typical Geotherm: 0 Temperature ~1300 ºC 0 Conductive ~150 km Depth Convective

4. Normally does not intersect melt temperature for dry mantle rock! 0 Temperature 0 Conductive Melt Depth Convective

5. Option 1: Raise the Geotherm! 0 Temperature 0 Conductive Melt Depth Convective (There are two ways this can happen:)

6. 1.i: Extension at mid ocean ridges, continental rifts: Deep rock moves upward to fill the space created carrying heat with it!

7. 1.ii: Bring hot rock to the base of the lithosphere by convection (mantle plume or “hotspot”) E.g., Yellowstone, Hawaii!

8. Heat Flow: Q = k DT/Dz Recent Volcanism

9. Option 2: Add Water! 0 Temperature 0 Conductive Melt Depth Water Reduces Melting Temperature of Rock By 200-300 Degrees Convective

10. 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

11. Basalt Andesite Rhyolite (darker)  (lighter) SiO2 (Quartz) ~50% ~60% ~70% Melt Temperature: ~1200 ºC ~700 ºC Viscosity: (low)  (high) Basalt Andesite Rhyolite

12. Why does viscosity matter? Weight-% solubility of water 0% 5% 10% 0 Basaltic 3 Depth (km) 6 Andesitic 9 Rhyolitic 12 Steam, CO2, other gases are 90% of volume at surface!

13. 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!

14. 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)

15. Major Volcano Landforms: Basaltic: Shield volcanoes, cinder cones Andesitic: stratovolcanoes (Mt Rainier, Mt St Helens) Rhyolitic: Large Calderas (Taal, Yellowstone)

16. 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