Geol 456/656
Composition of the Earth
In studying plate tectonics we will be concerned with the mechanical
properties of parts of the earth, such as their strength, elasticity, and
viscosity.
Although such properties do depend on pressure, temperature, and the
state of stress, they are most sensitive to the composition of
the material in question.
(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?
Relics of this condensation process still exist, and occasionally fall to
earth as meteorites.
Above at left is a refractory Fe-Ni meteorite (cut to show crystal fabric),
and at right is the ablated surface of a rocky chondritic meteorite.
One can ``mix up'' a composition for the whole earth by combining proportions
of iron and chondritic meteorites:
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.
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.
Certainly everything aside from Fe, O, Si, and Mg can be considered a minor
constituent of the earth.
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.
Composition of the Mantle
Samples of the upper mantle occasionally appear where faulting has
exposed it in oceanic fracture zones, thrust it up in collision zones, or
where brought up in diatremes and basalt eruptions.
The rock revealed is usually perdotite, which Ringwood explains as
three-quarters dunite (or pure olivine) and one-quarter basalt.
Basalt can be formed by partial melting of this pyrolite,
which drives off the
enriched basalt magma, leaving behind the depleted dunite.
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?
Composition of the Oceanic Crust
Though almost everywhere less than 10 km thick, the oceanic crust plays
a crucial role in plate tectonics.
Oceanic crust is produced wherever hot mantle material comes into
contact with the surface of the solid earth, so in a way it can be thought
of as an atmospheric weathering product of the mantle.
The albite feldspar at left (Na plagioclase) , and many other common, lighter minerals
form as a result of this atmospheric interaction.
In this way the oceanic crust reflects much about the composition and
motion of the underlying 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
mechanical states.
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.)
Sea water circulates into the cooling oceanic crust to depths of
3 km, driven by the heat of the magma intrusion.
This circulation provides the primary chemical interaction between the
lithosphere and the atmosphere, releasing metal sulphides, water, methane, and carbon
dioxide into the ocean, and entraining water and metal oxides into the crust.
The circulation is shown by the metamorphism and serpentinization of
the basalts and gabbros.
Prominent metallic ore deposits originate at such spreading ridges.
(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.
Composition of the Continental Crust
By virtue of its very low-density composition (primarily Na and K feldspars
like the sample at left) the earth's continental crust floats as a frothy
scum atop the convecting mantle and attached oceanic crust.
The highly-enriched minerals beached on the continents have collected
over most of the earth's history, and the oldest rocks preserved on the
planet are found on the continents (except meteorites, which are often
older).
(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.
Differences between the oceanic and continental crust:
- 0.2 Ga versus 3.5 Ga maximum age;
- 5 km versus 35 km average thickness;
- global layering versus great heterogeneity;
- broad seismic quiescence versus tectonic activity; and
- large-scale versus spotty volcanism.
Contamination from the Biosphere
Interaction between the biosphere and the oceanic and continental crust
has significantly modified the function of plate tectonics over geologic
time.
Biotic activity entrains large amounts of atmospheric volatiles in rocks
such as the limestone at left and the coal at right.
Such rocks can be dragged to great depths in subduction collision zones,
where the water and carbon dioxide will exsolve out and promote the
formation of light magmas that carry tremendous amounts of heat to the
surface at island arcs and continents.
Such highly evolved magmas appear throughout continental regions.
Question: contrast the style of volcanism from an enriched, evolved,
volatile-rich magma against that from a depleted, primitive, dry magma.
