Today – 4/04

• Spring Winds

• Mineralogy of the Earth

Jetstream Monday 5 amJetstream Monday 5 am

NWS Warnings MondayNWS Warnings Monday

Red Flag Warning

Dangerous fire weather!

Locally defined criteria

Here: relative humidity below 15%, sustained winds of 20 mph and/or gusts of 35 mph, dangerous fuel conditions

Rodeo-Rodeo-ChedisChediski Fire, ki Fire, 20022002

Aspen Fire, 2003Aspen Fire, 2003

Last Time

Atomic scale structure of the core and lower mantleFiguring out mantle compositionLower mantle: negatively charged oxygens are big, smaller positively charge atoms (Mg, Si) are small, fit in between oxygens. Pressure forces oxygens close together, so each Si is surrounded by (bonded to) six oxygens

Stishovite, SiOStishovite, SiO22

Little blue sphere = silicon atom

Big red spheres = oxygen atoms

Possible Test Question

Would you expect to find stishovite in the:

a) Crust

b) Upper mantle

c) Lower mantle

d) Core

Upper Mantle

Same kinds and numbers of atoms as the lower mantle, but combined differently to form different minerals

70% olivine, 25% pyroxene, 5% garnet

In olivine and pyroxene, silicon atoms are surrounded by four oxygens. In mantle garnets, half of the silicons are surrounded by four oxygens, half by six

OlivineOlivine

OlivineOlivine

Olivine

Pyroxene (Diopside)Pyroxene (Diopside)

Garnet (Pyrope)Garnet (Pyrope)

CrustCrust

Oceanic – basalt Oceanic – basalt Basalt: 50% feldspar, 50% pyroxeneBasalt: 50% feldspar, 50% pyroxene Continental – graniteContinental – granite Granite: 30% quartz, 70% feldspar Granite: 30% quartz, 70% feldspar

Feldspar (Amazonite)Feldspar (Amazonite)

Quartz (Variety Amethyst)Quartz (Variety Amethyst)

Important Points

Extreme conditions of the mantle limit the number of structure types to a handful, but huge variety in the low P, T crust

Si bonded to 6 oxygens in lower mantle, 4 in upper mantle and crust

Negatively charged oxygens thought of as big, with small positively charged atoms in the spaces between oxygens

Why is the Crust Different from the Mantle?

When partial melting begins in the asthenosphere, the elements Si, Al, Na, and K migrate into the melt, while Mg and Fe stay put. The magma rises, preferentially separating out these elements. Additionally, minerals with lots of Mg and little Si crystallize out first, the second step in the mantle’s Mg loss prevention program

Why is the Crust Different from the Mantle?

When rocks that are already enriched in Si and depleted in Mg in the lithosphere begin to melt again for some reason, the process repeats, widening the compositional gap between these “evolved” rocks and the mantle they originally came from

Why Do Rocks Melt?

Decompression – MOR’s. As convecting hot rock rises, confining pressure is reduced, and the rock begins to melt

Hydration – subduction zones. As a subducting slab goes down, water trapped in cracks and hydrous minerals is released into the overriding mantle. This lowers the melting point of the overriding rocks, and magma is generated

GeothermGeotherm

Decompression MeltingDecompression Melting

Hydration MeltingHydration Melting

Montserrat Tectonic SettingMontserrat Tectonic Setting

Movie Notes and Questions

How did the oceans and atmosphere form?

Pyroclastic flow – cloud of hot gas, ash, and rock that flows down the side of a volcano at high speed

Eruption mechanism

How do the scientists predict eruptions?

How high does the eruption column reach?