Today – 4/04 Mineralogy of the Earth

Last Wednesday’s Storm Report

Final Exam May 11, points matching, 50 points diagrams, 20 points multiple choice Review sheet posted Wednesday, April 27 Last 2 classes as much time as needed for review questions Multiple choice from new material only; diagrams and matching will include material from the last two tests

Tsunami characteristics High speed Long wavelength

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

Inside Lunar Basalt

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?

Tucson Meteorite

Iron at Earth’s Surface

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 formed Crust, which we can directly observe, derived from primitive mantle Mantle 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 minerals Lower mantle – 70% Mg-perovskite, 20% magnesiowüstite, 10% Ca-perovskite In minerals, negatively charged oxygen atoms are thought of as big, positively charged atoms as small

Perovskite On Earth’s surface, Si atoms are almost always bonded to four O atoms. O atoms in minerals are negatively charged, and so repel each other. In the lower mantle, pressure forces the oxygens closer together, so six fit around a silicon On Earth’s surface, Si atoms are almost always bonded to four O atoms. O atoms in minerals are negatively charged, and so repel each other. In the lower mantle, pressure forces the oxygens closer together, so six fit around a silicon

Magnesiowüstite Same structure as table salt Same structure as table salt Thought of as closest-packed oxygens with little irons placed in the spaces between the oxygens Thought of as closest-packed oxygens with little irons placed in the spaces between 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

Olivine

Olivine

Olivine

Pyroxene (Diopside)

Garnet (Pyrope)

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

Feldspar (Amazonite)

Quartz (Variety Amethyst)

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

Why Do Rocks Melt? Decompression – MOR’s Hydration – subduction zones