KEY QUESTIONS
How long has plate tectonics been operating on Earth?
What is the driving mechanism for plate tectonics?
Historical Overview
Francis Bacon (1620) - continents "fit" together.
Alfred Wegener (1880-1930) - Evidence for drift of continents
a) Shape of continents --- Pangea (200 million years ago) had 40% of
continental landmass in S. hemisphere.
b) Fossil evidence: Mesosaurus fossils found only in Brazil and South
Africa which are 3000 miles apart today.
c) Habitats of Living Animals: hippos only found in Africa and Madagascar
which are now 300 miles apart.
d) Rock type and structural similarities.
e) Glacial Deposits; record of past climates.
-Wegener was ridiculed because he could not postulate a plausible driving
force. Wegener suggested that tidal influence of the moon drove the continents.
Holmes (1928) suggested that thermal convection was the driving force.
Exploration of the sea floor after WWII revealed the following:
1) mapping of mid-ocean ridges and the rift running down their central
axis indicated tensional forces.
2) high heat flow and volcanism characterize ridges.
3) age of oceanic basalt progressively increases as one travels away
from the ridge crest.
4) oceanic crust is all younger than 200 million years.
5) seamounts strung out in long chains show progressively older ages
away from hot spots.
6) there is a consistent pattern of magnetic stripes on opposite sides
of ridge crusts.
7) age and thickness of sediments deposited on oceanic crust increases
with increasing distance from ridge crests.
These observations proved that new oceanic crust was continually being
created at a rate of 1-9 cm/yr and led to the hypothesis of sea-floor spreading.
Plate tectonics = continental drift + sea-floor spreading.
Examination of N.A. continent shows that its core, the Canadian Shield,
is composed of very old (2.5 billion) cratons (protocontinents) separated
by younger (1.9-1.8 billion) mountain chains now eroded to a flat surface.
We hypothesize that plate tectonics has been operating for at least
2 billion years.
The Canadian Shield is covered by a 1 mile thick sequence of platform
sediments in U.S. and is flanked by a Paleozoic orogenic belt (Appalachians)
on the east and a Mesozoic orogenic belt (Cordillera) on the west.
Plate tectonics can be used to explain "geologic action" at plate boundaries
but falls short in explaining continental scale vertical motions far from
plate boundaries.
Recent crustal warping (downwarping in Maine) in many locations cannot
be explained by Plate Tectonics.
Plate Tectonics:
1) Plate Divergence (1-9 cm yr-1).
a) heat from below - heating and crustal stretching - tensional cracks
(rifts) - fragmentation of continents (rift valleys).
b) if process continues, linear seas develop and ultimately ocean basins
get created.
2) Plate convergence (15-20 cm yr-1) - subduction
a) oceanic plate hits continental plate - volcanic arc (Cascades and
Andes).
b) oceanic plate hits oceanic plate - volcanic island are (Japan, Alaskan
Peninsula, Phillipines etc.).
c) continental crust hits continental crust - fold mountain belts such
as the Appalachians, Alps and Urals.
Modern example: India hits Asia - Himalayas. In order for this to occur,
must completely destroy oceanic crust between the two continental landmasses.
3) Transform Fault Boundaries
a) plates slide by one another without creating or destroying crust.
b) relative motion of plates in changed (or transformed) along a transform
fault boundary.
c) the active part of the fault lies between the two ridge segments.
Plate Tectonics is consistent with seismic data which show that deep focus earthquakes are associated with subduction zones.
Convection in the mantle (a consequence of the Earth's internal heat)
is most probable cause of plate motion.
a) Seismic tomography suggests flow in mantle is far more complex than
the simple convection cells depicted on slides shown in class.
b) Alternative hypotheses:
- hot spots
- push-pull model
None of the driving mechanisms yet proposed can account for all of
the major facets of plate motion.
Development of the Appalachians
a) Late Precambrian (700-800 mya): NA split from Africa - ancestral
Atlantic.
b) Early Paleozoic (~ 600 mya): Atlantic begins to close - subduction,
volcanism, folding etc.
c) Late Ordovician: first collision (orogenic pulse) as microcontinent
thrust over N.A.; plutonism in New England.
d) Devonian: 2nd orogeny - European continent collides with N.A. in
northern Appalachians; extensive metamorphism and faulting.
e) Permo-Carboniferous (300-250 mya): Africa collides with Southern
Appalachians; thrust faulting.
f) Soon after the supercontinent was assembled it began to break up
by rifting.
g) Triassic - Atlantic coastal plan and continental shelf begin to
develop; block faulting produces series of long narrow troughs (Newark
basin, Conn river valley; Bay of Fundy) which begin to fill with red, clastic
deposits.