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Today – 4/04
• Spring Winds
• Mineralogy of the Earth
Jetstream Monday 5 am
NWS 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
RodeoChediski
Fire,
2002
Aspen Fire, 2003
Last Time
Atomic scale structure of the core and lower
mantle
Figuring out mantle composition
Lower 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, SiO2
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
Olivine
Olivine
Olivine
Pyroxene (Diopside)
Garnet (Pyrope)
Crust
Oceanic – basalt
 Basalt: 50% feldspar, 50% pyroxene
 Continental – granite
 Granite: 30% quartz, 70% feldspar

Feldspar (Amazonite)
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
Geotherm
Decompression Melting

Hydration Melting
Montserrat 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?