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GLE 2024 (Day 1) > GENERAL GEOLOGY (GEOSPHERE) > Flashcards

GENERAL GEOLOGY (GEOSPHERE) Flashcards

1
Q

OUtermost layer of the geosphere of the Earth

A

Crust

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2
Q

Describe the crust

A

Very thin and brittle like very thin skin of apple

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3
Q

What separates crust from the underlying mantle?

A

Mohorovicic (Moho) discontinuity

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4
Q

What is the primary composition of the oceanic curst?

A

Mafic rocks enriched with oxides of iron, magnesium and calcium

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5
Q

What does elevated Fe content do to the oceanic crust?

A

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

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6
Q

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

A

its thinness

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7
Q

What are the principal rocks that comprises the OC?

A

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

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8
Q

Describe layers of the OC

A

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

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9
Q

What is the density of OC?

A

3.0 g/cc

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10
Q

Location and age of the oldest oceanic crust

A

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

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11
Q

Why is the oceanic relatively young?

A

Because old oceanic crusts are destroyed in subduction zones

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12
Q

How can fragments of old oceanic crusts be preserved

A

Through obduction of ophiolites

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13
Q

What are ophiolites?

A

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

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14
Q

Generalized composition of the continental crust?

A

Granitic/Granodioritic

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15
Q

Oxides which enrich the continental crust?

A

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

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16
Q

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

A

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

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17
Q

What makes Cont Cr more buoyant than OC?

A

Its lower density and greater thickness

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18
Q

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

A

Thinned continerntal margins and rifts

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19
Q

Age and Location Oldest documented Continental Crust?

A

4.03Ga, NW Territories of Canada

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20
Q

Age of greenstone belts in Greenland?

A

4.28 Ga

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21
Q

Age of Metamorphosed Sed Rocks in Jack Hill Craton in Australia

A

4.4 Ga

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22
Q

Relative composition of the mantle in terms of oxide content?

A

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

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23
Q

Relative composition of mantle

A

Ultrabasic

24
Q

Dominant rocks in the upper mantle

A

Ultramafic such as Peridotite

25
Q

Depth of the Upper Mantle

A

400km

26
Q

Dominant Minerals in the Mantle

A

Pyx and Olivine

27
Q

Minor minerals in the Mantle

A

plag. garnet, spinel

28
Q

What comprises the Lithosphere?

A

Crust and Uppermost mantle

29
Q

Rheology of Lithosphere

A

Brittle

30
Q

Why is the lithosphere the site for most earthquakes

A

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

31
Q

What is the Low Velocity Zone?

A

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

32
Q

Depths of the LVZ?

A

100-250km below the surface

33
Q

What marks the top of the LVZ?

A

Contact between the strong lithosphere and the weakly plastic astenosphere

34
Q

deepest part of the Asthenosphere?

A

660km

35
Q

Rehology of asthenosphere?

A

Plastic and flows slowly rather than rupturing

36
Q

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

A

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).

37
Q

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

A

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

38
Q

Oxide phase which coexist with Perovskite?

A

Periclase MgO

39
Q

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

A

Inversion to Garnet, Garnet to Ilmenite and Perovskite

40
Q

What does the base of the TZ at 660 marks?

A

The boundary between the astenosphere and the mesosphere or lower mantle

41
Q

Silica Polymorph in the Lower Mantle?

A

Stishovite

42
Q

Depth range of the mesosphere?

A

660-2900km

43
Q

How do we study the lower mantle?

A

Through Seismic Anomaly

44
Q

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

A

D’’ or D double prime Layer

45
Q

Location of D’’

A

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

46
Q

Relative Composition of the Core

A

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

47
Q

How was the liquid nature of the outer core discovered?

A

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

48
Q

How does Geophysical studies describe the outer core?

A

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

49
Q

The discontinuity between the outer and inner core and its depth

A

Lehmann Discontinuity at 5150km

50
Q

What happens at lehmann in terms of seismic waves?

A

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

51
Q

density of the inner core

A

13g/cc

52
Q

Why is the inner core seismically anisotropic?

A

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

53
Q

Upper-Lower crust discontinuity

A

Conrad

54
Q

Crust-Mantle Discontinuity

A

Mohorovicic

55
Q

Upper-Lower Mantle Discontinuity

A

Repetti Discontinuity

56
Q

Mantle-Core Discontinuity

A

Guttenberg Discontinuity

57
Q

Outer Core Inner Core Boundary

A

Lehmann Discontinuity

GLE 2024 (Day 1) (34 decks)

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