GENERAL GEOLOGY (GEOSPHERE)

Question 1

OUtermost layer of the geosphere of the Earth

Answer 1

Crust

Question 2

Describe the crust

Answer 2

Very thin and brittle like very thin skin of apple

Question 3

What separates crust from the underlying mantle?

Answer 3

Mohorovicic (Moho) discontinuity

Question 4

What is the primary composition of the oceanic curst?

Answer 4

Mafic rocks enriched with oxides of iron, magnesium and calcium

Question 5

What does elevated Fe content do to the oceanic crust?

Answer 5

It gives the O.C. darker color and higher density

Question 6

What causes OC to be less buoyant aside from its high density?

Answer 6

its thinness

Question 7

What are the principal rocks that comprises the OC?

Answer 7

Basalt and Gabbro which are largely composed of pyx and Ca Plag

Question 8

Describe layers of the OC

Answer 8

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

Question 9

What is the density of OC?

Answer 9

3.0 g/cc

Question 10

Location and age of the oldest oceanic crust

Answer 10

W and E borders of North Atlantic Ocean and W Pacific Ocean, 180 Ma

Question 11

Why is the oceanic relatively young?

Answer 11

Because old oceanic crusts are destroyed in subduction zones

Question 12

How can fragments of old oceanic crusts be preserved

Answer 12

Through obduction of ophiolites

Question 13

What are ophiolites?

Answer 13

Slices of oceanic crust thrust/obducted onto continental margins or volcanic arcs

Question 14

Generalized composition of the continental crust?

Answer 14

Granitic/Granodioritic

Question 15

Oxides which enrich the continental crust?

Answer 15

POtassium, Sodium and Silica (K, Na, Si)

Question 16

Differentiate composition of upper and lower portion of the cont. crust

Answer 16

Upper is more granitic while lower is more dioritic and/or gabbroic

Question 17

What makes Cont Cr more buoyant than OC?

Answer 17

Its lower density and greater thickness

Question 18

These are the thinnest portions of continental crust which normally occur below sea level

Answer 18

Thinned continerntal margins and rifts

Question 19

Age and Location Oldest documented Continental Crust?

Answer 19

4.03Ga, NW Territories of Canada

Question 20

Age of greenstone belts in Greenland?

Answer 20

4.28 Ga

Question 21

Age of Metamorphosed Sed Rocks in Jack Hill Craton in Australia

Answer 21

4.4 Ga

Question 22

Relative composition of the mantle in terms of oxide content?

Answer 22

30-40% MgO, less FeO, 40-45 SiO2

Question 23

Relative composition of mantle

Answer 23

Ultrabasic

Question 24

Dominant rocks in the upper mantle

Answer 24

Ultramafic such as Peridotite

Question 25

Depth of the Upper Mantle

Answer 25

400km

Question 26

Dominant Minerals in the Mantle

Answer 26

Pyx and Olivine

Question 27

Minor minerals in the Mantle

Answer 27

plag. garnet, spinel

Question 28

What comprises the Lithosphere?

Answer 28

Crust and Uppermost mantle

Question 29

Rheology of Lithosphere

Answer 29

Brittle

Question 30

Why is the lithosphere the site for most earthquakes

Answer 30

Because it is strong enough to rupture in response to stress and is divided into fragments called plates

Question 31

What is the Low Velocity Zone?

Answer 31

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

Question 32

Depths of the LVZ?

Answer 32

100-250km below the surface

Question 33

What marks the top of the LVZ?

Answer 33

Contact between the strong lithosphere and the weakly plastic astenosphere

Question 34

deepest part of the Asthenosphere?

Answer 34

660km

Question 35

Rehology of asthenosphere?

Answer 35

Plastic and flows slowly rather than rupturing

Question 36

What happened to seismic veolocity as it passes through the TZ? and why?

Answer 36

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).

Question 37

At which depth and pressure conditions does ringwoodite and garnet transform into perovskite (Mg,Fe,Al)siO3?

Answer 37

appro 660 km (base of TZ) and approx 24 GPa

Question 38

Oxide phase which coexist with Perovskite?

Answer 38

Periclase MgO

Question 39

Sequence of Pyroxene phase change within the TZ (Transition Zone)?

Answer 39

Inversion to Garnet, Garnet to Ilmenite and Perovskite

Question 40

What does the base of the TZ at 660 marks?

Answer 40

The boundary between the astenosphere and the mesosphere or lower mantle

Question 41

Silica Polymorph in the Lower Mantle?

Answer 41

Stishovite

Question 42

Depth range of the mesosphere?

Answer 42

660-2900km

Question 43

How do we study the lower mantle?

Answer 43

Through Seismic Anomaly

Question 44

A complex zone near the Core-Mantle Boundary/Gutenberg where, they say that deep mantle plumes originate and where subducting slabs terminate.

Answer 44

D'' or D double prime Layer

Question 45

Location of D''

Answer 45

130-340km above the CoreMantle Boundary 2560 - 2770 km

Question 46

Relative Composition of the Core

Answer 46

85% Fe, 5% Ni, 8-10% light elements, O,S, H

Question 47

How was the liquid nature of the outer core discovered?

Answer 47

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

Question 48

How does Geophysical studies describe the outer core?

Answer 48

highly compresed liquid with density of 10-12g/cc

Question 49

The discontinuity between the outer and inner core and its depth

Answer 49

Lehmann Discontinuity at 5150km

Question 50

What happens at lehmann in terms of seismic waves?

Answer 50

There is a rapid increase in P-wave velocity and reappearance of low velocity Swave

Question 51

density of the inner core

Answer 51

13g/cc

Question 52

Why is the inner core seismically anisotropic?

Answer 52

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

Question 53

Upper-Lower crust discontinuity

Answer 53

Conrad

Question 54

Crust-Mantle Discontinuity

Answer 54

Mohorovicic

Question 55

Upper-Lower Mantle Discontinuity

Answer 55

Repetti Discontinuity

Question 56

Mantle-Core Discontinuity

Answer 56

Guttenberg Discontinuity

Question 57

Outer Core Inner Core Boundary

Answer 57

Lehmann Discontinuity