GENERAL GEOLOGY (GEOSPHERE) Flashcards
OUtermost layer of the geosphere of the Earth
Crust
Describe the crust
Very thin and brittle like very thin skin of apple
What separates crust from the underlying mantle?
Mohorovicic (Moho) discontinuity
What is the primary composition of the oceanic curst?
Mafic rocks enriched with oxides of iron, magnesium and calcium
What does elevated Fe content do to the oceanic crust?
It gives the O.C. darker color and higher density
What causes OC to be less buoyant aside from its high density?
its thinness
What are the principal rocks that comprises the OC?
Basalt and Gabbro which are largely composed of pyx and Ca Plag
Describe layers of the OC
Same as ophiolite sequence
Layer 1 - Sediments ,
Layer 2 - Pillow Basalt,
Layer 3 Sheeted Dikes
Layer 4 Gabbro
Layer 5 Ultramafics and
Layer 6 Mantle peridotites
What is the density of OC?
3.0 g/cc
Location and age of the oldest oceanic crust
W and E borders of North Atlantic Ocean and W Pacific Ocean, 180 Ma
Why is the oceanic relatively young?
Because old oceanic crusts are destroyed in subduction zones
How can fragments of old oceanic crusts be preserved
Through obduction of ophiolites
What are ophiolites?
Slices of oceanic crust thrust/obducted onto continental margins or volcanic arcs
Generalized composition of the continental crust?
Granitic/Granodioritic
Oxides which enrich the continental crust?
POtassium, Sodium and Silica (K, Na, Si)
Differentiate composition of upper and lower portion of the cont. crust
Upper is more granitic while lower is more dioritic and/or gabbroic
What makes Cont Cr more buoyant than OC?
Its lower density and greater thickness
These are the thinnest portions of continental crust which normally occur below sea level
Thinned continerntal margins and rifts
Age and Location Oldest documented Continental Crust?
4.03Ga, NW Territories of Canada
Age of greenstone belts in Greenland?
4.28 Ga
Age of Metamorphosed Sed Rocks in Jack Hill Craton in Australia
4.4 Ga
Relative composition of the mantle in terms of oxide content?
30-40% MgO, less FeO, 40-45 SiO2
Relative composition of mantle
Ultrabasic
Dominant rocks in the upper mantle
Ultramafic such as Peridotite
Depth of the Upper Mantle
400km
Dominant Minerals in the Mantle
Pyx and Olivine
Minor minerals in the Mantle
plag. garnet, spinel
What comprises the Lithosphere?
Crust and Uppermost mantle
Rheology of Lithosphere
Brittle
Why is the lithosphere the site for most earthquakes
Because it is strong enough to rupture in response to stress and is divided into fragments called plates
What is the Low Velocity Zone?
A layer in which P-wave velocity is anomalously low coming from high velocity in lithosphere. The change in velocity is attributed to small amounts of partial melting due to temperature and devolatilization of hydrous minerals
Depths of the LVZ?
100-250km below the surface
What marks the top of the LVZ?
Contact between the strong lithosphere and the weakly plastic astenosphere
deepest part of the Asthenosphere?
660km
Rehology of asthenosphere?
Plastic and flows slowly rather than rupturing
What happened to seismic veolocity as it passes through the TZ? and why?
Increases - This sudden jump is caused by increase in rigidity and incompressibility (Mas matigas) which is a result of the transformation of olivine which transforms to beta spinel of Wadsleyite (Mg2SiO4). At much lower depth within the TZ, B-Spine/Wadsleyite transforms to higher pressure varation Ringwooite (Mg2SiO4).
At which depth and pressure conditions does ringwoodite and garnet transform into perovskite (Mg,Fe,Al)siO3?
appro 660 km (base of TZ) and approx 24 GPa
Oxide phase which coexist with Perovskite?
Periclase MgO
Sequence of Pyroxene phase change within the TZ (Transition Zone)?
Inversion to Garnet, Garnet to Ilmenite and Perovskite
What does the base of the TZ at 660 marks?
The boundary between the astenosphere and the mesosphere or lower mantle
Silica Polymorph in the Lower Mantle?
Stishovite
Depth range of the mesosphere?
660-2900km
How do we study the lower mantle?
Through Seismic Anomaly
A complex zone near the Core-Mantle Boundary/Gutenberg where, they say that deep mantle plumes originate and where subducting slabs terminate.
D’’ or D double prime Layer
Location of D’’
130-340km above the CoreMantle Boundary 2560 - 2770 km
Relative Composition of the Core
85% Fe,
5% Ni,
8-10% light elements, O,S, H
How was the liquid nature of the outer core discovered?
There was a dramatic decrease in P-wave velocity and termination of S-wave which signals that the material within 2900km is non-rigid such as fluids
How does Geophysical studies describe the outer core?
highly compresed liquid with density of 10-12g/cc
The discontinuity between the outer and inner core and its depth
Lehmann Discontinuity at 5150km
What happens at lehmann in terms of seismic waves?
There is a rapid increase in P-wave velocity and reappearance of low velocity Swave
density of the inner core
13g/cc
Why is the inner core seismically anisotropic?
Because seismic velocity varies with direction which is caused by:
1) the parallel stacking of iron rich crystals
2) single composition with fast velocity direction
Upper-Lower crust discontinuity
Conrad
Crust-Mantle Discontinuity
Mohorovicic
Upper-Lower Mantle Discontinuity
Repetti Discontinuity
Mantle-Core Discontinuity
Guttenberg Discontinuity
Outer Core Inner Core Boundary
Lehmann Discontinuity
