Continental Drift, Isostasy, and Earth Structure Flashcards
Alfred Wegener
- Coined continental drift in 1912
When was Wegener’s theory accepted widely?
- After 1970
- 99.9% accepted
Why was theory of continental drift eventually accepted? (i.e. evidence)
- 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
What is a better way to see the fit of continents?
- Offshore continental shelves
Du Toit (1927)
- S. African geologist
- Reported on geological expedition to S. America
- Realized that the continents had similar assemblages and ‘looked like home’
Specific evidence from Du Toit
- 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
With what motion did Pangaea break up?
- Rotational due to curvature of Earth
Paleontology (Wegener citing DuToit)
- 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
Paleoclimatology
- Tropical coal forests in N. continents
- Areas of placation w/ known ice movement in S. Continents
What did Wegener suggest based on palaeontology and stratigraphy?
- 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
What is the oldest ocean crust in Atlantic?
- Grand Banks in Newfoundland
- Approx. 180 Ma (older likely subducted)
- (N. and S. Am only 80 - 130 Ma)
What were new contstraints for Tectonics in the Early 20th century?
- Horizontal Shortening
- Recognition of lithosphere and asthenosphere, from gravity data (isostasy)
Horizontal Shortening
- 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
Evidence for Isostasy?
- 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
Isostasy and 2 models
- Crust ‘floats’ on fluid-like mantle
- Airy
- Pratt
Airy Isostasy
- 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
Pratt Isostasy
- Density of crust is lower beneath mountain while base of crust is at same level
- Flat moho, varying density
Principle of Isostasy
- 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
What are the main differences in Airy vs. Pratt
- Airy: uniform crustal density, varying height of crust and Moho
- Pratt: varying crustal density, flat Moho (With Pratt the Moho is flat)
Bowie/Hayford (1909)
- 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
Sir Harold Jeffreys (1923)
Quoted that the work by the US Survey to put theory of isostasy on its present basis was an outstanding achievement of the time
What are the main implications of isostasy
- 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
Post-glacial rebound
- 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)
R. Daly (1923)
- ‘Our Mobile Earth’ (1926)
- Slab Pull
- Canadian Geologist
- Asthenosphere
- Oceanic Crust
- Mechanism for slab pull
- Driving force
R. Daly, Asthenosphere
- Solid (transmits S-waves), but only semi-rigid
- Deforms as a viscous fluid (e.g. glass) on long timescales
- Basaltic
R. Daly, Oceanic Crust
- Basaltic
- Cooler, more dense, unstable situation with denser material on top
R. Daly, Mechanism
- Broken crustal rind sinks, dragging along horizontal block (Slab-pull)
- eg lava flow: rigid upper layer slides over deeper melt
R. Daly, Driving Force
- Broad domes form at surface
- Continents slide down flanks
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
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
Earth Contraction
- Alternative theory to continental drift
- Earth shrunk to form topography like mountains and valleys
- Like a grape to a raisin
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)
When was the end of the active debate on drift theory in U.S.?
1926 - 1932
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
Why was continental drift rejected?
- Lack of adequate causal mechanism
- Criticism from physicists
- Wegener’s overly-zealous methodological approach
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)
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
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)
Plate Tectonic Resolution
- Old geological evidence (descriptive)
- New geophysical evidence, instrumental measurements (magnetism, seismology, plate rotations)
- Paleomagnetism
Resolution from Paleomagnetism
- Continents moved, at least in latitude
- Seafloor spreading, ‘magnetic tape recorder’
Crust/mantle Boundary
- Moho
- Change in seismic velocity, represents change in composition
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
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
Mantle crust structure
- Peridotite (olivine)
- approx. 8km/s seismic velocity (p-wave)
Ophiolites
- Oceanic crust now on continental crust (obducted, collision?)
- Thrust up
- Layers give history
- Same structure as young oceanic crust/mantle
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
Seismic velocities (p-wave, km/s) of ophiolite layers - How does seismic velocity change up the section?
- 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
What does the lithosphere include?
- Crust and upper mantle
How do we know deep Earth structure?
- From seismic waves
- Defraction/ reflection at boundaries
- Geophysics
- P-waves
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
What are the upper mantle boundaries defined by?
- Changes in rheology associated with mineralogical changes in phase
Rheology
- Study of deformation and flow of matter
- Rheos = stream
Where is the transition zone from olivine to spinel?
- Approx. 410km
- In Upper Mantle
- Rheological change
Where is the transition zone from spinel to perovskite?
- Approx 660km
- About Upper and Lower Mantle boundary
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
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
Top/base of asthenosphere
limits partial melting
410km discontinuity
Change in olivine to spinel structure
660km discontinuity
- Change to ‘post-spinel’ composite (perovskite and magnesiowustite
- Max depth of EQ’s
410 - 660km
Mantle transition zone (upper to lower)
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
LAB: Seismology
- Low velocity zone (particularly s-wave)
- High seismic attenuation
LAB: Rheology
- Low mechanical strength
- Low viscosity (convects easily)
LAB: Petrology/ Temperature
- Onset of partial melting
- High electrical conductivity (melt conducts better)
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
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
Source of magnetic field?
Outer Core conductive liquid motion generating currents that move Fe-Ni
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
Who discovered solid inner core?
- Inge Lehmann
- 1936
