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EOS410PltTec > Continental Drift, Isostasy, and Earth Structure > Flashcards

Continental Drift, Isostasy, and Earth Structure Flashcards

1
Q

Alfred Wegener

A
  • Coined continental drift in 1912
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2
Q

When was Wegener’s theory accepted widely?

A
  • After 1970

- 99.9% accepted

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3
Q

Why was theory of continental drift eventually accepted? (i.e. evidence)

A
  • Fit/shape of continents
  • Marine deposits on land (alternating marine/terrestrial conditions)
  • Near identical rocks on different continents
  • Similar living and fossil assemblages in widely separated continents
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4
Q

What is a better way to see the fit of continents?

A
  • Offshore continental shelves
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5
Q

Du Toit (1927)

A
  • S. African geologist
  • Reported on geological expedition to S. America
  • Realized that the continents had similar assemblages and ‘looked like home’
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6
Q

Specific evidence from Du Toit

A
  • Gondwana beds coincident from Uruguay north to Karoo
  • Folds continue from Cap to sierras of Buenos Aires
  • Basement rocks are crystalline pre-Cambrian
  • N. American Appalachians continuous with European Caledonia fold belt
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7
Q

With what motion did Pangaea break up?

A
  • Rotational due to curvature of Earth
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8
Q

Paleontology (Wegener citing DuToit)

A
  • Same genera of Earthworms in Africa/central America and India-Ceylon/Australia (Can’t fly, swim, or be dormant)
  • Glossopteris plant fossil found in all southern continents
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9
Q

Paleoclimatology

A
  • Tropical coal forests in N. continents

- Areas of placation w/ known ice movement in S. Continents

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10
Q

What did Wegener suggest based on palaeontology and stratigraphy?

A
  • Mid atlantic opened by Jurassic (N. Am - N. Af)
  • Greater depth of seafloor in West (away from Atlantic ridge) suggest older seafloor
  • South Atlantic opened by Lower to Mid-Cretaceous (S. Am - S. Af) with rift opening gradually from South
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11
Q

What is the oldest ocean crust in Atlantic?

A
  • Grand Banks in Newfoundland
  • Approx. 180 Ma (older likely subducted)
  • (N. and S. Am only 80 - 130 Ma)
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12
Q

What were new contstraints for Tectonics in the Early 20th century?

A
  • Horizontal Shortening

- Recognition of lithosphere and asthenosphere, from gravity data (isostasy)

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13
Q

Horizontal Shortening

A
  • New constraint for tectonics in early 20th century
  • Strata in Alps collapsed to 20% of original by horizontal displacement along thrust fault
  • 600km to 120km
  • Huge thrusts also in Appalachians, Scotland and Scandinavia
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14
Q

Evidence for Isostasy?

A
  • 18th - 19th century surveyors map shape of Earth
  • Expected lateral gravity attraction of mountains but was less than expected due to mass deficiency beneath mountains
  • Compensation to support lower density root
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15
Q

Isostasy and 2 models

A
  • Crust ‘floats’ on fluid-like mantle
  • Airy
  • Pratt
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16
Q

Airy Isostasy

A
  • Height (h) balanced by root (b)
  • All crust has equal density (iceberg analogy)
  • Pressure at point 1 = Pressure at point 2
  • Pressure = density x gravity x thickness
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17
Q

Pratt Isostasy

A
  • Density of crust is lower beneath mountain while base of crust is at same level
  • Flat moho, varying density
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18
Q

Principle of Isostasy

A
  • Beneath a certain depth (compensation depth) the pressures generated by all overlying materials are everywhere equal
  • Plates float at an elevation that depends on thickness and density
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19
Q

What are the main differences in Airy vs. Pratt

A
  • Airy: uniform crustal density, varying height of crust and Moho
  • Pratt: varying crustal density, flat Moho (With Pratt the Moho is flat)
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20
Q

Bowie/Hayford (1909)

A
  • 85%-90% of gravity variations across US (105 stations) can be explained by calculating the ‘isostatic density difference’
  • Had to assume Pratt theory b/c it was computationally simpler
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21
Q

Sir Harold Jeffreys (1923)

A

Quoted that the work by the US Survey to put theory of isostasy on its present basis was an outstanding achievement of the time

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22
Q

What are the main implications of isostasy

A
  • Rigid layer rests on top of a more fluid layer
  • Vertical motions possible if thickness of rigid layer changes (e.g. glaciers disappear)
  • Airy isostasy: lateral motions in fluid layer also possible
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23
Q

Post-glacial rebound

A
  • Airy isostasy: ice is removed, rock rebounds vertically
  • Mechanism of mobile substrate
  • Mostly around Hudson Bay Canada with hotspots in N. BC due to ‘little ice age’
  • Lateral motion less than vertical (approx. 1 mm)
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24
Q

R. Daly (1923)

A
  • ‘Our Mobile Earth’ (1926)
  • Slab Pull
  • Canadian Geologist
  • Asthenosphere
  • Oceanic Crust
  • Mechanism for slab pull
  • Driving force
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25
Q

R. Daly, Asthenosphere

A
  • Solid (transmits S-waves), but only semi-rigid
  • Deforms as a viscous fluid (e.g. glass) on long timescales
  • Basaltic
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26
Q

R. Daly, Oceanic Crust

A
  • Basaltic

- Cooler, more dense, unstable situation with denser material on top

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27
Q

R. Daly, Mechanism

A
  • Broken crustal rind sinks, dragging along horizontal block (Slab-pull)
  • eg lava flow: rigid upper layer slides over deeper melt
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28
Q

R. Daly, Driving Force

A
  • Broad domes form at surface

- Continents slide down flanks

29
Q

C. Schuchert (1923)

A
  • Accepted some movement (10’s of miles)
  • But thought continental drift was a deranged theory
  • Didn’t believe the Americas had drifted so far from the rest
30
Q

A. Holmes (1928)

A
  • Father of modern geochronology
  • Mantle convection driven by radioactive decay
  • Ignored until 1960’s
  • Realized substrate cannot be wholly liquid b/c too slippery and can’t exert grip on overlying continents
  • Ascending currents would be disruptive and create new ocean while mountains would build on continental margins
31
Q

Earth Contraction

A
  • Alternative theory to continental drift
  • Earth shrunk to form topography like mountains and valleys
  • Like a grape to a raisin
32
Q

Willis/ Schuchert alternative theory 1932

A
  • Permanent ocean basins plus ‘land bridges’ (continental links)
  • Gondwana existed, internally connected by upthrust ocean ridges
  • Same mechanism as continental margin mountain chains
  • Basaltic ridges isostatically unstable (subside, disappear)
  • Antarctic Ocean hemmed in during Permian, cutting off warm water (explains glaciations)
33
Q

When was the end of the active debate on drift theory in U.S.?

A

1926 - 1932

34
Q

Problems with the contraction/Land-bridge theory

A
  • Cannot explain extensional environments (i.e. cont. rifts like E. Africa)
  • Felsic Paleozoic sediments in N. Am derived from East but clearly not oceanic based on roundness/size
  • No mechanism for up/down motions, particularly ocean ridges
35
Q

Why was continental drift rejected?

A
  • Lack of adequate causal mechanism
  • Criticism from physicists
  • Wegener’s overly-zealous methodological approach
36
Q

Rejection based on lack of adequate causal mechanism

A
  • How can continents ‘plow’ through solid oceanic rock
  • But Airy isostasy justifies for semi-fluid flow
  • Reality of Alpine thrust sheets (possibility has only been demonstrated by fact, not explained)
37
Q

Rejection based on criticism from physicists

A
  • Especially Sir Harold Jeffreys (1891 - 1989, still rejected to the end of his days) though he praised isostasy
  • Earth’s viscosity is too high to allow it to flow
  • No evidence of force to move continents
38
Q

Rejection based on Wegener’s behavior

A
  • Overly zealous
  • American’s (Willis) opposed based on Wegener being an advocate rather than impartial investigator
  • Selected facts to fit theory (Unconsciously to fit preconceived theory?)
  • Dogmatism, over generalizing, special pleading
  • Violated American standards (Multiple hypothesis, objective decision of best hypothesis)
39
Q

Plate Tectonic Resolution

A
  • Old geological evidence (descriptive)
  • New geophysical evidence, instrumental measurements (magnetism, seismology, plate rotations)
  • Paleomagnetism
40
Q

Resolution from Paleomagnetism

A
  • Continents moved, at least in latitude

- Seafloor spreading, ‘magnetic tape recorder’

41
Q

Crust/mantle Boundary

A
  • Moho

- Change in seismic velocity, represents change in composition

42
Q

Oceanic crust structure

A
  • Young (< 180 Ma)
  • Thin layered, well defined
  • Mafic
  • 5.5 - 7.2 km/s
  • Denser, 3g/cm^3, therefore higher
  • Approx. 7km thick
43
Q

Continental crust structure

A
  • Old (<4.4 Ga)
  • Thick (approx. 35km)
  • Layering poorly defined
  • Felsic
  • 5 - 6.8 km/s
  • Less dense, 2.7g/cm^3, therefore lower
44
Q

Mantle crust structure

A
  • Peridotite (olivine)

- approx. 8km/s seismic velocity (p-wave)

45
Q

Ophiolites

A
  • Oceanic crust now on continental crust (obducted, collision?)
  • Thrust up
  • Layers give history
  • Same structure as young oceanic crust/mantle
46
Q

Ophiolite layering, bottom to top

A
  • Upper mantle, peridotite,
  • Moho
  • Layer 3, ultrabasic cumulates then gabbros
  • Layer 2, sheeted dykes (basalt) then Pillow basalts
  • Layer 1, Sediments and Sea
47
Q 
Seismic velocities (p-wave, km/s) of ophiolite layers
- How does seismic velocity change up the section?
A
  • Sediments (2.0)
  • Pillow basalts (3.5 - 6.2)
  • Sheeted dykes (3.5 - 6.2)
  • Gabbros (6.5 - 7.2)
  • Cumulate-rich gabbros (6.5 - 7.2)
  • Moho, sharp change
  • Dunites, Harzburgites (8.0)
  • Seismic velocity increases up section, sharp boundary at Moho
48
Q

What does the lithosphere include?

A
  • Crust and upper mantle
49
Q

How do we know deep Earth structure?

A
  • From seismic waves
  • Defraction/ reflection at boundaries
  • Geophysics
  • P-waves
50
Q

Which seismic waves are used to define boundaries?

A
  • P-waves

- Not S-waves b/c they don’t travel through gas/liquid and cannot penetrate outer core

51
Q

What are the upper mantle boundaries defined by?

A
  • Changes in rheology associated with mineralogical changes in phase
52
Q

Rheology

A
  • Study of deformation and flow of matter

- Rheos = stream

53
Q

Where is the transition zone from olivine to spinel?

A
  • Approx. 410km
  • In Upper Mantle
  • Rheological change
54
Q

Where is the transition zone from spinel to perovskite?

A
  • Approx 660km

- About Upper and Lower Mantle boundary

55
Q

Where is the zone of partial melting in upper mantle and how do s-waves behave?

A
  • Known as Asthenosphere
  • Approx. 100 - 200km
  • Rheology change Plastic vs. brittle deformation
  • S-waves slow down, low velocity zone
56
Q

How do S-waves change in velocity from surface to lower mantle?

A
  • Slow at surface
  • Increase sharply at lower lithosphere
  • Decrease in upper asthenosphere
  • Increase with slight wobbles from Upper mantle through olivine/spinel to spinel/perovskite
57
Q

Top/base of asthenosphere

A

limits partial melting

58
Q

410km discontinuity

A

Change in olivine to spinel structure

59
Q

660km discontinuity

A
  • Change to ‘post-spinel’ composite (perovskite and magnesiowustite
  • Max depth of EQ’s
60
Q

410 - 660km

A

Mantle transition zone (upper to lower)

61
Q

What is the Lithosphere/Asthenosphere Boundary (LAB) defined by?

A
  • Seismology
  • Rheology
  • Petrology/ temperature
  • Different definitions may produce different depths but agree in general, not in detail
62
Q

LAB: Seismology

A
  • Low velocity zone (particularly s-wave)

- High seismic attenuation

63
Q

LAB: Rheology

A
  • Low mechanical strength

- Low viscosity (convects easily)

64
Q

LAB: Petrology/ Temperature

A
  • Onset of partial melting

- High electrical conductivity (melt conducts better)

65
Q

Lower Mantle

A
  • 660 - 2885km
  • Fairly uniform lithology, Perovskite
  • Solid
  • D” layer, lowest 200 - 300km of mantle
  • Often decreased seismic velocity (increased temp), due to interactions between mantle and liquid core
66
Q

Outer Core

A
  • 2885 - 5155km
  • Likely iron-nickel mixture, but also some light elements (Si, S, K, O?)
  • Density 8-15% too low for pure Fe and Ni
  • No transmission of S-waves, therefore liquid
  • Rate of fluid motion likely 10km/yr
  • Source of magnetic field, produced by motion of conductive fluid, generating currents
67
Q

Source of magnetic field?

A

Outer Core conductive liquid motion generating currents that move Fe-Ni

68
Q

Inner Core

A
  • 5155 - 6370km
  • Density and seismic velocity consistent with pure Fe
  • Solid
  • Evidence for solidity:
  • ID of p-wave that must have travelled as s-wave through inner core
  • Whole Earth oscillations after large EQ’s
69
Q

Who discovered solid inner core?

A
  • Inge Lehmann

- 1936

EOS410PltTec (12 decks)

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