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?

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.