Today – 4/04 Spring Winds Mineralogy of the Earth.
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Transcript of Today – 4/04 Spring Winds Mineralogy of the Earth.
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Today – 4/04
• Spring Winds
• Mineralogy of the Earth
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Jetstream Monday 5 amJetstream Monday 5 am
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NWS Warnings MondayNWS Warnings Monday
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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
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Rodeo-Rodeo-ChedisChediski Fire, ki Fire, 20022002
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Aspen Fire, 2003Aspen Fire, 2003
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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
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Stishovite, SiOStishovite, SiO22
Little blue sphere = silicon atom
Big red spheres = oxygen atoms
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Possible Test Question
Would you expect to find stishovite in the:
a) Crust
b) Upper mantle
c) Lower mantle
d) Core
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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
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OlivineOlivine
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OlivineOlivine
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Olivine
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Pyroxene (Diopside)Pyroxene (Diopside)
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Garnet (Pyrope)Garnet (Pyrope)
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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
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Feldspar (Amazonite)Feldspar (Amazonite)
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Quartz (Variety Amethyst)Quartz (Variety Amethyst)
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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
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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
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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
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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
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GeothermGeotherm
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Decompression MeltingDecompression Melting
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Hydration MeltingHydration Melting
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Montserrat Tectonic SettingMontserrat Tectonic Setting
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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?