Today – 4/04 Mineralogy of the Earth. Last Wednesday’s Storm Report.
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Transcript of Today – 4/04 Mineralogy of the Earth. Last Wednesday’s Storm Report.
Final Exam
May 11, 11-130 points matching, 50 points diagrams, 20 points multiple choiceReview sheet posted Wednesday, April 27Last 2 classes as much time as needed for review questionsMultiple choice from new material only; diagrams and matching will include material from the last two tests
Possible Test Question
Atmospheric instability results from:
a) Temperature gradients in the atmosphere such that a parcel of air rising from the surface will always be warmer than the surrounding air
b) Convection
c) High pressure systems
d) All of the above
Possible Test Question
The defining characteristic of a supercell is:
a) Tornadoes
b) A persistently rotating updraft
c) Vertical wind shear
d) All of the above
Last Time
Supercells – thunderstorms characterized by persistently rotating updrafts
Produce tornadoes, hail, damaging straight-line winds, and lightning
About 1% of tornadoes are F4 or F5 – these are the bad ones
How is Earth Put Together?
Earth is made of various kinds of atoms
The atoms are arranged into minerals
Collections of mineral grains form rocks
Minerals
Characterized by translational symmetry – arrangement of atoms repeats over and over again.
Atoms are held together by chemical bonds – electrical interactions
Minerals
A mineral structure is the lowest energy arrangement for the conditions of formation. If conditions change, for instance in a subducting slab (pressure and temperature rising), a mineral may change its structure (or melt). This is called a phase transition
What phase transitions are useful to you?
The Core
85% Fe, 5% Ni, 10% other
Iron is a closest-packed crystal at inner core conditions – most efficient way to pack equal-sized spheres in space
Iron is a liquid at outer core conditions
Mysteries of the Core
Initiation of the geodynamo
Age of the inner core
Superrotation of the inner core!
Finding the Composition of the Mantle
“Primitive mantle” composition deduced from meteorites – starting composition before crust formedCrust, which we can directly observe, derived from primitive mantleMantle today = primitive mantle – crust Also some samples (xenoliths)Mantle has more Fe, Mg; less Si, O than the crust – mantle is denser than crust
Composition of the Mantle
Mantle composition is thought to be chemically homogeneous – same kinds and number of atoms in upper and lower mantle, but because of the phase transition thing, different mineralsLower mantle – 70% Mg-perovskite, 20% magnesiowüstite, 10% Ca-perovskiteIn minerals, negatively charged oxygen atoms are thought of as big, positively charged atoms as small
PerovskitePerovskite On Earth’s surface, Si atoms are almost On Earth’s surface, Si atoms are almost
always bonded to four O atoms. O atoms always bonded to four O atoms. O atoms in minerals are negatively charged, and in minerals are negatively charged, and so repel each other. In the lower mantle, so repel each other. In the lower mantle, pressure forces the oxygens closer pressure forces the oxygens closer together, so six fit around a silicontogether, so six fit around a silicon
MagnesiowMagnesiowüüstitestite Same structure as table saltSame structure as table salt Thought of as closest-packed oxygens Thought of as closest-packed oxygens
with little irons placed in the spaces with little irons placed in the spaces between the oxygensbetween the oxygens
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
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
Why is the Crust Different from the Mantle?
When partial melting begins in the asthenosphere, the elements Si, Al, Ca, Na, and K migrate into the melt, while Mg stays 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
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 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