Sci Exper Wk 15 - Plate Tectonics - Example of a Theory's Evolution
Intro –
 Until recently, continents thought to have been in same positions over
time
 Much new data in last few decades
 Nothing short of a sci revolution
 Orig theory of “cont drift” expanded to encompass more than just
continents;
“plate tectonics” involves ocean floor develoment, mt. building, eqk
explanations, etc
I.
Cont drift hypoth
 Idea around a long time, since better world maps showed edges of Afr
& S. Amer…Da Vinci, among others, wrote about it
 Alfred Wegener formally proposed cont drift hypoth in 1915, in his book
- supercontinent (Pangaea “pan-gee-uh”) once existed, broke apart
200 million years (“m.y. ago”) Fig 5.1
He saw several lines of evidence to support this idea
A. Continental edge-matching across S. Atlantic
- opponents argued that erosion has modified shorelines
- more recent data due to undersea profiling shows edges of
continental shelves to be the match points, not shorelines Fig 5.2
- overlap points due to big deltas, etc where shelves have built out
over 200 m.y.
B. Fossil assemblages match
- mesosaurus (dinosaur) fossils limited to Afr & S. Amer Fig 5.3
…some argued for a land bridge, but modern data shows no bridge
under sea
- glossopteris fern from warm, swampy environment, had large
seeds that couldn’t be distributed very far by wind, now distributed
in S. Amer, Afr, India, Australia, even Antarctica…distrib makes
much more sense when continents put back together
C. Rock types and structures match
- Appalachian mts in US with Caledonian mts in Scandinavia..these
mts formed when “proto-Atlantic” closed up to form Pangaea, EurAfrica collided with Greenland-N Amer-S Amer Fig 5.4
-
Like ripping apart a newsapaper and then putting it back
together…mts are like the lines of newsprint
D. Ancient climate patterns make more sense
-
-
Glaciation approx 250 m.y. ago, these deposits now up near
equator
Move continents back south Fig 5.5 and the deposits are both more
polar and more continuous
Earth Sci wk 4 p.2
Another advantage with conts in this position - coal swamps in
Appalachians in Pennsylvanian time are right over the equator,
good conditions for swamps, rather than northern position they’re in
NOW
II.
The Early Debate
 Things get hostile in 1924 – book translated from German to English
 TC Chamberlin – “less bound by awkward, ugly facts….than rival
theories; there are few restrictive rules and no sharply drawn code of
conduct” …so Wegener was hammered by his contemporaries
 Big problem was in driving mechanism…Wegener couldn’t convince
colleagues of either of two ideas:
1. tidally-driven currents in Earth push continents westward
2. larger continents push through ocean crust (like icebreakes cut
through ice)
 Wegener died in disgrace, often openly ridiculed, but a faithful few
continued to pursue the idea of drift
III.
Plate Tectonics
 By 1968, much new data had been acquired
- seismic activity (eqks)
- magnetic field reversal documented through Earth history
- sonar, sea-floor mapping

IV.
essentials of theory- outer shell (“lithosphere”) of earth rigid, comprises about 20 diff
plates, some big , some small Fig 5.6
- most plates actually contain both oceanic and continental crust,
unlike Wegener idea that continents r emain separate from ocean
- all major interactions occur at plate boundaries
Plate boundaries are of 3 types Fig 5.7
1. Divergent (pull-apart) boundaries
 sea-floor spreading
- usu at crest of ocean ridge Fig 5.8
-plates pull away, fresh magma wells into fracture (here, magnetite
needles line up in direc of Earth magn field at that instant)
-new “slivers” of sea floor are added over time as plates continue to
diverge
-rates vary…avg is ???? per year

Mid-Atlantic Ridge well-studied Fig 5.8
- submerged mt chain 8 - 10,000 feet high
- rises above sea surface WHERE?
Earth Sci wk 4 p.3
- upwelling hot magma buoys up ocean ridge
- as magma is extruded, it cools and contracts to become oceanic
crust, becomes more dense
-ocean crust sinks deeper into the asthenosphere as it cools and
moves away from ridge, thus ocean floor becomes progressively
deeper further away from ridge
- end result is a sea floor with a high central ridge and deeper
flanking basins

Spreading often begins under a continent..Red Sea / E. African rift Fig
5.9
- probably similar to how Atlantic broke apart in the Newark Basin &
Atlantic margin
- transition from continental rift to oceanic rift Fig 5.10
- stretching of granitic continental crust, fractures form
- rift valley filled with basalt develops from fracture system (East
African Rift, old Midcontinent Rift in US)
- narrow sea develops (Red Sea)
- ultimately pulls apart to become large ocean (Atlantic)
2. Convergent (colliding) boundaries
 Where lithosphere is consumed, returns to the asthenosphere
below
 Several types of convergence involve different material, yield
different features Fig 5.11 but a shared element is a “subduction
zone”
A. ocean-continent convergence
- continental granitic crust is less dense (= 2.65 g/cc) than ocean
basaltic crust ( = 3.00 g/cc), so ocean slab tends to dive
underneath, cont rides up over it
- deep ocean trench usu at edge of contin shelf (Peru-Chile trench
along W. side S. Amer is good example)
- at about 100 km depth, subducting slab and seds it carries start to
partially melt, magma starts rising up inland of the trench,
eventually breaks through to form a volcanic chain of mts known as
an arc Fig 5.11a
- Andes
- Cascades ( incl. Mt. St. Helens) still active, explosive
-
Sierra Nevada sli. older, inactive, but magma chambers
exposed
B. ocean-ocean convergence Fig 5.11b
- similar to ocean-cont in many respects
- somewhat of a toss-up as to which slab goes down, since density
same
(occasional geol. articles about “reversing” subduc zones)
- volcanic arc behind trench known as “island arc” because arc builds
up from sea floor to form backbone of series of islands (Japan good
example, Aleutian islands off Alaska also…)
Earth Sci wk 4 p.4
C. continent-continent convergence Fig 5.11c
- both plates stay buoyed up, big collisions develop (Appalachians
450-250 m.y. ago, Himalayas 45 m.y. ago Fig 5.13 )
- often see an island arc sequence developed and destroyed too as
ocean separating the continents closes
3. Transform fault boundaries Fig 5.14
- lateral slippage
- no new crust made
- several types, incl. those that offset ocean ridge segments
- most famous is San Andreas
- these faults “transform” relative motion of plates, connect divergent
and convergent plate boundaries
V.
Evidence for plate tectonics theory
A. Paleomagnetism
 Magnetite frozen in polarity, points to N pole of mag field at the time
of extrusion of lava flows
 Can age-date the flows, know when they were formed
B. Polar wandering
 Lava flows studied in Europe in 1950s
 Plotted the position of N pole for a given age, found that the N pole
“wandered” from near present day Hawaii 500 m.y. ago to it’s
present position Fig 5.15
 Either magnetic N has really wandered, or perhaps Europe has
been drifting, and lava flows record different positions of a drifting
Europe relative to a stationary pole
 Answer came when N. Amer apparent polar wandering curve was
constructed; it is parallel to Europe curve, but 24o longitude apart
 Close up the Atlantic, the curves track, showing that a
supercontinent was drifting relative to a stationary pole Fig 5.15b
C. Magnetic reversals & seafloor spreading
 Magn N flip-flops through time, as documented by age-dated lava
flows Fig 5.16
 Stripes of normal and reverse polarity mapped symmetrically away
from spreading centers, recording the eruptions sequentially Fig
5.17
 Can close up the spreading centers to show “pre-drift” configuration
 Can also determine avg spreading rates
- 1-2 cm/yr for N. Atlantic
- 3-8 cm/yr for S. Pacific
 This perhaps most compelling evidence supporting plate tects theory
Earth Sci wk 4 p.5
D. Earthquake patterns
 Eqks very localized along plate boundaries Fig 5.18
 Show different depths of seismicity
 Relate diff depths of seismicity to subducting slabs
 Few eqks below 435 mi below surface, prob due to plasticity of rx at
great depth Fig 5.19
E. Ocean drilling
 Glomar Challenger drill ship
 Drilled holes and collected drill cores of ocean seds
 Seds get older w/ distance from ridge
 No seds older than 160 m.y. found
 Implication is that all oceanic crust currently on Earth is very young,
while continents are generally very old
F. “Hot spots”
 ocean crust migrates over stable hot spots, linear series of
volanoes develop through time Fig 5.21
VI. Driving Mechanism of Plates Fig 5.22
A. Convection current hypothesis (heat-driven)
 Convec currents carry heat
 Cyclic loops of magma rise and fall
B. Slab-pull, slab-push hypotheses (gravity-driven)
 More a gravity drive than convection drive
 Slab-pull
Gravity pulls down dense ocean crust at trenches
 Slab-push
Gravity pushes dense crust laterally away from elevated
mid-ocean ridges
C. Hot plumes hypothesis
 Variation on the convec current hypoth
 Convection in narrow zones
D. Summary
 Gravity and heat may both be important
 Mechanism still not completely understood