Continental Drift, Isostasy, and Earth Structure Flashcards
1
Q
Alfred Wegener
A
- Coined continental drift in 1912
2
Q
When was Wegener’s theory accepted widely?
A
- After 1970
- 99.9% accepted
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
4
Q
What is a better way to see the fit of continents?
A
- Offshore continental shelves
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’
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
7
Q
With what motion did Pangaea break up?
A
- Rotational due to curvature of Earth
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
9
Q
Paleoclimatology
A
- Tropical coal forests in N. continents
- Areas of placation w/ known ice movement in S. Continents
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
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)
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)
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
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
15
Q
Isostasy and 2 models
A
- Crust ‘floats’ on fluid-like mantle
- Airy
- Pratt
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
17
Q
Pratt Isostasy
A
- Density of crust is lower beneath mountain while base of crust is at same level
- Flat moho, varying density
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
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)
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
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
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
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)
24
Q
R. Daly (1923)
A
- ‘Our Mobile Earth’ (1926)
- Slab Pull
- Canadian Geologist
- Asthenosphere
- Oceanic Crust
- Mechanism for slab pull
- Driving force
25
R. Daly, Asthenosphere
- Solid (transmits S-waves), but only semi-rigid
- Deforms as a viscous fluid (e.g. glass) on long timescales
- Basaltic
26
R. Daly, Oceanic Crust
- Basaltic
| - Cooler, more dense, unstable situation with denser material on top
27
R. Daly, Mechanism
- Broken crustal rind sinks, dragging along horizontal block (Slab-pull)
- eg lava flow: rigid upper layer slides over deeper melt
28
R. Daly, Driving Force
- Broad domes form at surface
| - Continents slide down flanks
29
C. Schuchert (1923)
- 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
A. Holmes (1928)
- 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
Earth Contraction
- Alternative theory to continental drift
- Earth shrunk to form topography like mountains and valleys
- Like a grape to a raisin
32
Willis/ Schuchert alternative theory 1932
- 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
When was the end of the active debate on drift theory in U.S.?
1926 - 1932
34
Problems with the contraction/Land-bridge theory
- 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
Why was continental drift rejected?
- Lack of adequate causal mechanism
- Criticism from physicists
- Wegener's overly-zealous methodological approach
36
Rejection based on lack of adequate causal mechanism
- 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
Rejection based on criticism from physicists
- 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
Rejection based on Wegener's behavior
- 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
Plate Tectonic Resolution
- Old geological evidence (descriptive)
- New geophysical evidence, instrumental measurements (magnetism, seismology, plate rotations)
- Paleomagnetism
40
Resolution from Paleomagnetism
- Continents moved, at least in latitude
| - Seafloor spreading, 'magnetic tape recorder'
41
Crust/mantle Boundary
- Moho
| - Change in seismic velocity, represents change in composition
42
Oceanic crust structure
- Young (< 180 Ma)
- Thin layered, well defined
- Mafic
- 5.5 - 7.2 km/s
- Denser, 3g/cm^3, therefore higher
- Approx. 7km thick
43
Continental crust structure
- 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
Mantle crust structure
- Peridotite (olivine)
| - approx. 8km/s seismic velocity (p-wave)
45
Ophiolites
- Oceanic crust now on continental crust (obducted, collision?)
- Thrust up
- Layers give history
- Same structure as young oceanic crust/mantle
46
Ophiolite layering, bottom to top
- Upper mantle, peridotite,
- Moho
- Layer 3, ultrabasic cumulates then gabbros
- Layer 2, sheeted dykes (basalt) then Pillow basalts
- Layer 1, Sediments and Sea
47
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Seismic velocities (p-wave, km/s) of ophiolite layers
- How does seismic velocity change up the section?
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- 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
What does the lithosphere include?
- Crust and upper mantle
49
How do we know deep Earth structure?
- From seismic waves
- Defraction/ reflection at boundaries
- Geophysics
- P-waves
50
Which seismic waves are used to define boundaries?
- P-waves
| - Not S-waves b/c they don't travel through gas/liquid and cannot penetrate outer core
51
What are the upper mantle boundaries defined by?
- Changes in rheology associated with mineralogical changes in phase
52
Rheology
- Study of deformation and flow of matter
| - Rheos = stream
53
Where is the transition zone from olivine to spinel?
- Approx. 410km
- In Upper Mantle
- Rheological change
54
Where is the transition zone from spinel to perovskite?
- Approx 660km
| - About Upper and Lower Mantle boundary
55
Where is the zone of partial melting in upper mantle and how do s-waves behave?
- Known as Asthenosphere
- Approx. 100 - 200km
- Rheology change Plastic vs. brittle deformation
- S-waves slow down, low velocity zone
56
How do S-waves change in velocity from surface to lower mantle?
- 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
Top/base of asthenosphere
limits partial melting
58
410km discontinuity
Change in olivine to spinel structure
59
660km discontinuity
- Change to 'post-spinel' composite (perovskite and magnesiowustite
- Max depth of EQ's
60
410 - 660km
Mantle transition zone (upper to lower)
61
What is the Lithosphere/Asthenosphere Boundary (LAB) defined by?
- Seismology
- Rheology
- Petrology/ temperature
- Different definitions may produce different depths but agree in general, not in detail
62
LAB: Seismology
- Low velocity zone (particularly s-wave)
| - High seismic attenuation
63
LAB: Rheology
- Low mechanical strength
| - Low viscosity (convects easily)
64
LAB: Petrology/ Temperature
- Onset of partial melting
| - High electrical conductivity (melt conducts better)
65
Lower Mantle
- 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
Outer Core
- 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
Source of magnetic field?
Outer Core conductive liquid motion generating currents that move Fe-Ni
68
Inner Core
- 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
Who discovered solid inner core?
- Inge Lehmann
| - 1936
