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 26 September 2008 Read for Mon:

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

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

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

Option 1: Raise the Geotherm! Depth Temperature 0 0 Conductive Convective Melt (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  T/  z Recent Volcanism

Option 2: Add Water! Depth Temperature 0 0 Conductive Convective Melt Water Reduces Melting Temperature of Rock By 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) SiO 2 (Quartz) ~50% ~60% ~70% Melt Temperature: ~1200 ºC  ~700 ºC Viscosity: (low)  (high) Basalt Andesite Rhyolite

Why does viscosity matter? Depth (km) 0%5%10% Weight-% solubility of water Steam, CO 2, 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, CO 2, SO 2 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)

Taal Volcano Philippines Yellowstone GPS time series Interesting Aside: Calderas characterized by “unrest” (changing deformation…) Since we’ve never seen a caldera eruption, would we know what to look for?