(J. Louie) Composition, and mechanical properties, vary widely within the earth. The differentiation of the earth reflects the processes of differentiation that occurred with the formation of the solar system 5.5 billion years ago. The Sun holds 95% of the mass of the solar system, and the early solar nebula was thus mostly hydrogen and helium. As the nebula cooled, minor constituents condensed out of the nebula, starting with the heavier and more refractory (Ni, Fe), and ending with the lighter and more volatile (water, methane).
Question: which is more refractory, pyroxene or quartz? Why is there so much oxygen in any of the planets?
Earth | Moon 1 2 3 | 4 ----------------------+------ Fe 34.6 29.3 29.9 | 9.3 O 29.5 30.7 30.9 | 42.0 Si 15.2 14.7 17.4 | 19.6 Mg 12.7 15.8 15.9 | 18.7 Ca 1.1 1.5 1.9 | 4.3 Al 1.1 1.3 1.4 | 4.2 Ni 2.4 1.7 1.7 | 0.6 Na 0.6 0.3 0.9 | 0.07 S 1.9 4.7 - | 0.3
1 32.4% iron meteorite (with 5.3% FeS) and 67.6% oxide portion of bronzite chondrites.Certainly everything aside from Fe, O, Si, and Mg can be considered a minor constituent of the earth.
2 40% Type I carbonaceous chondrite, 50% ordinary chondrite, and 10% iron meteorite (containing 15% sulphur).
3 Nonvolatile portion of Type I carbonaceous chondrites with FeO/FeO+ MgO of 0.12 and sufficient SiO2 reduced to Si to yield a metal/silicate ratio of 32/68.
4 Based on Ca, Al, Ti = 5 x Type I carbonaceous chondrites; FeO = 12% to accommodate lunar density; and Si/Mg = chondritic ratio.
Question: outline two theories explaining why the Moon is so light.The earth continues to differentiate since its accretion, with the result that most Fe has sunk to the core, while Fe, Mg silicates compose the mantle. The compositions of the core and inner core can be surmised from their seismic wave velocities because experimental data has been taken on core materials at the enormous pressures and temperatures present in the core. Work to improve military armor led to the development of gas-driven guns capable of firing a slug at velocities up to 7 km/s. On impact to a sample, the slug produces shock waves and radiance from the core-like pressures and temperatures that can be measured for a few nanoseconds before the apparatus flies apart.
Question: just what is being enriched in basalt and depleted from dunite?(from Serdar Ozalaybey, UNR) Olivine has the important property of anisotropy, where seismic waves travel about 10% faster along its stubby crystals than across them. Shear strains in the mantle can align the crystals in a preferred direction, where seismic analysis can map the strain orientations.
(from Kearey & Vine, copyright Blackwell Sci. Publ.) Although though the mantle is nearly homogeneous, it exhibits changes in mineral phase with increasing depth as revealed by seismic velocity gradients and shock-wave experiments. These changes in crystal structure are the simplest explanations for the existence of the two seismic velocity discontinuities in the upper mantle, at 400 and 670 km depth.
Question: what is likely to be a geological deposit close to Reno where you might find chunks of mantle?
The oceanic crust, excluding the overlying sediments, is uniformly composed
of rocks having a basaltic composition.
It is however, found by worldwide seismic refraction experiments to
consist of three layers of increasing velocity downwards.
Layer 1 has P-wave velocities between 1.6 and 2.5 km/s; Layer 2 between
3.4 and 6.2 km/s; and Layer 3 between 6.4 and 7.0 km/s.
These differences reflect the differing origins of each layer, and their
The underlying upper mantle has velocities between 7.4 and 8.6 km/s.
(from Kearey & Vine, copyright Blackwell Sci. Publ.) Early and late views of the layering of oceanic crust, with velocities in km/s.
All oceanic crust is produced at spreading centers by a very specialized process. Hot mantle pyrolite in a thermal convection boundary layer partially melts near the surface due to the decrease in pressure. The exsolved basalt magma cools into gabbroic intrusions below 2 km depths, while large quantities erupt at the sea bed to flow into pillow lavas. The top of the magma chamber is being continually intruded by dikes taking magma to the surface. Because this is occurring at a spreading center, each dike is always split by a new one, forming a vast stack of vertically-oriented dikes called a sheeted dike complex.
Question: How could you tell in outcrop or hand sample that every dike is always split by a new one? Can you tell the past direction of spreading?
(from Kearey & Vine, copyright Blackwell Sci. Publ.) Remnants of complete sections of oceanic crust have been found in outcrop around the world, thrust up by past continental or island-arc collision zones. The generalized ophiolite sections above are (left to right) from Oman, Newfoundland, and Cyprus. They may be more representative of young, hot oceanic crust from back-arc basins and young rifts that resists subduction.
Schweickert discovered a highly-weathered ophiolite locally in the
Stillwater Range east of Fallon.
Complete ophiolite sequences exist on the western slope of the Sierra
Navada, and in the Klamath Mountains.
(from USGS; click for a larger image, or
for a 0.7 Mbyte printable PostScript file)
The continents have grown over geologic time, with their ancient cratonic
cores wrapped in successively-younger layers of obducted mountain belts.
Continental seismic reflection surveys have shown the continental crust
to be complexly-assembled and faulted at all levels, with some areas
perhaps active tectonically down to and even below the Moho.
The cratons may also have roots of old, cold subducted mantle lithosphere
that extend down to the 400 km discontinuity.
Although the crust has an average composition between diorite and granodiorite, many regions have relatively high concentrations of quartz, shown at left. Quartz content is mechanically crucial because, particularly in the presence of minor amounts of water, quartz-rich rock becomes relatively ductile at mid-crustal temperatures and pressures.
Question: discuss the role of metamorphic core complexes in crustal deformation.
Question: contrast the style of volcanism from an enriched, evolved, volatile-rich magma against that from a depleted, primitive, dry magma.