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Global Geophysics > Earth outer shell, seismology, interior > Flashcards

Earth outer shell, seismology, interior Flashcards

1
Q

What is differentiation (in terms of planetary formation)?

A

The process by which random chunks of primordial matter were transformed into a body whose interior is divided into concentric layers that differ from one another both physically and chemically

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2
Q

How long ago did the Earth’s crust form?

A

4.4 billion years ago

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3
Q

How did the Earth’s crust form? maybe a bit simple

A

Lighter rocks floated to the surface of the magma ocean

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4
Q

When was the earliest evidence for water at the surface of the Earth?

A

3.8 billion years ago

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5
Q

When was the earliest evidence for primitive life?

A

3.5 billion years ago

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6
Q

What are cratons?

A

Pieces of old (>2.5Ga) and thick (>200km) continental lithosphere.
The stable infrastructure of the Earth’s continents is assembled around cratons

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7
Q

What are:?

  1. Shields
  2. Platforms
A
  1. Unsedimented cratons

2. Sedimented cratons

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8
Q

Why was Earth’s heat flow nearly 3x greater than it is today during the ARCHEAN?

A

Greater concentration of radioactive isotopes

Residual heat from Earth’s accretion

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9
Q

What did the high temperatures of the Archean result in (in terms of crust formation?)

A
  • Greater degrees of melting at oceanic spreading ridges
  • Formation of oceanic lithosphere with thick crust
  • Highly depleted mantle
  • Lithosphere would not subduct deeply because of its buoyancy
  • Removal of denser melt further lowered density of residual mantle
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10
Q

What are cratonic mantle roots probably composed of?

A

Buoyantly subducted slabs of a highly depleted oceanic lithosphere

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11
Q

How do deep mantle roots increase the stability of cratons?

A

Make them less susceptible to tectonic thickening by collisions or destruction by subduction

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12
Q

When did the first continents form?

A

During the Archean

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13
Q

What is the Juvenile crust?

A

Younger Continental Crust that has been added to the cratons since the Archean

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14
Q

What % of the worlds present continental crust formed during the Archean?

A

Only 5-40%

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15
Q

What is the thickness of juvenile crust?

A

Half that of cratons

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16
Q

How was the Juvenile crust formed?

A
  1. At continental magmatic arcs & ocean island arcs (formed within subduction zones)
  2. In lesser amounts by mantle plumes (but under lower temperature conditions)
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17
Q

Why is it surprising that continental growth has occurred in 3 major periods of time?

A

Models of monotonous secular cooling of the core and mantle suggest a smooth distribution of the ages of the juvenile crust formed as a result of partial melting from the mantle

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18
Q

What are the 3 major explanations for the episodic age distribution of the continental crust (CC)?

A
  1. Incomplete sampling of CC
  2. Uneven preservation of CC
  3. Non-uniform crustal generation
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19
Q

Theory for uneven age distribution of CC

A
  • 2 layer mantle convection
  • No transfer of material across phase boundary between upper and lower mantle (670km discontinuity)
  • Large volumes of subducted slabs accumulate at boundary at discrete periods
  • Results in massive overturn of mantle
  • Brings v. high T lower mantle to shallow depths produce voluminous melting at upper mantle depths
  • Overturns became more frequent as Earth cooled but with reducing max’ temp of upper mantle each cycle
  • Earth’s cooling resulted in change from layered to whole-mantle convection regime 1.2Ga
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20
Q 
What occurred in the following time periods?
>4.5Ga
4.5 - 4 Ga
4 - 2.5 Ga
2.5 - 1 Ga
<1 Ga
A 
Planetary embryo collisions
Magma ocean &amp; differentiation. Crust formation
Formation of cratons
Start of primitive plate tectonics
Start of modern plate tectonic system
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21
Q

How do we determine the structure of the continental crust?

A

Controlled source seismology

Seismic refraction

22
Q

What is the Moho discontinuity?

A

A strong velocity contrast at the base of the crust

From <7km/s to >8km/s

23
Q

What is the most common model of the CC

A

Three-layer model

Divides CC into upper, middle and lower crust

24
Q

What are the compositions of the upper, middle and lower crust?

A 
Upper = felsic (granite/granodiorite)
Middle = intermediate - felsic
Lower = mafic (comp' close to basalts)
25
Q

Density values of CC?

A

2600 km/m^3 at surface

3100 km/m^3 at base

26
Q

What is the average crustal thickness of the oceanic crust (OC)?

A

7km

27
Q

Characterise the OC. How does this differ from CC?

A

Uniform thickness and seismic structure.

CC is highly heterogeneous in 3Ds

28
Q

How old is the oldest OC?

A

160 million years old (v. young)

29
Q

How can we determine whole composition of OC when we are unable to drill a complete section

A

Analyses of ophiolites

Comparisons of seismic structure with determinations of seismic velocities of known rock types,

30
Q

What is the upper mantle made up of?

A

Probably garnet peridotite as petrology shows this partially melts to form basalt

31
Q

Where is the LVZ? Why does it exist?

A

Between 100-350 km.
Represents upper lithosphere
Rocks here are close to their melting point but is small degree of melt <1%

32
Q

Where is the ‘seismic lithosphere’?

A

Region above the top of the LVZ

33
Q

How do we study the structure of the LVZ

A

SURFACE WAVE DISPERSION ANALYSIS is best.

Is well defined for s-waves but not p-waves

34
Q

What marks the base of the asthenospherg?

A

660 km discontinuity

35
Q

What discontinuities exist in the mantle? What marks a discontinuity?

A

~410 and ~660 km

rapid increase in P- and S-wave velocities

36
Q

What causes the 410 and 660 km discontinuites?

A

Result of PHASE TRANSITIONS
410 - change from olivine type lattice to a β-phase spinel type lattice
660 - upper and lower mantle boundary where y-phase spinel changes to perovskite

37
Q

What is D’ or the mesosphere

A

Lower mantle
Monotonic increase in Vp and Vs
Same structure everywhere

38
Q

What is D”

A 
200km region above core-mantle boundary
Highly laterally variable
Seismically fast and slow regions
Find ULVZs due to partial melt here (source of mantle plumes?)
Slab graveyard
39
Q

Give a summary of Earths structure

A 
Seismic Lithosphere (u. mantle &amp; crust) 0-100 km
LVZ (u. Asthenosphere) 100-350 km
Asthenosphere 100-660km
Mesosphere (D') 660-2885km
D" 200 km above CMB
40
Q

How does seismic data indicate there is a (molten outer) core?

A
  • P-wave velocity drops significantly across CMB

- S-wave velocity cannot travel through it

41
Q

What is the core thought to be made up of and why?

A

Vp = 8-10 km/s
Density requirement = 10-11,000 kg/m3
Iron mixed with up to 10% Nickel

42
Q

Where is the boundary between the inner & outer core?

A

5350 km

43
Q

Give evidence for a solid inner core

A

PKIKP phase arrives early rel’ to PKP which only passes through outer core

44
Q

Why do seismic waves propagate through the inner core when travelling S-N than E-W?

A

Growth of minerals in direction of the magnetic field

45
Q

What are the 3 contributory components of the Earth’s magnetic field?

A

Main
External
Anomalies

46
Q

Main magnetic field

A

Produced in core
Wavelengths 100-1000s km
Varies 1-100 Ma

47
Q

External magnetic field

A

Interactions of the solar wind with ionised gas of earths atmosphere.
Wavelengths 100s km
Varies sec-10s yrs

48
Q

Anomalies

A

Shallow depth magnetic bodies
Wavelengths m-kms
No variation with time

49
Q

What is the Curie point?

A

The temperature at which certain materials lose their permanent magnetic properties

50
Q

What is Peclet number?

A

Pe = vd/κ
Measures relative importance of advection wit conduction
Pe&raquo_space; 1 advection dominates.

Global Geophysics (1 decks)

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