Download PLATE TECTONICS

PLATE TECTONICS
Continental Drift
Alfred Wegener first proposes Continental Drift in his book published in 1915.
Suggests that 200 million years ago there existed one large supercontinent which
he called Pangaea (All Land)(Figure). This was not really a new idea, but
Wegener offered several lines of evidence in support of his proposal.
1. Fit of the Continents - Noted the similarity in the coastlines of North and
South America and Europe and Africa. Today the fit is done at the
continental shelf and it is nearly a perfect match.
2. Fossil Similarities - Mesosaurus, (Figure) reptile similar to modern
alligator which lived in shallow waters of South America and Africa.
3. Rock Similarities
a. Rocks of same age juxtaposed across ocean basins. (Figure)
b. Termination of mountain chains. (Figure)
4. Paleoclimatic Evidence
a. Glacial deposits at equator
b. Coral reefs in Antarctica
Idea was rejected by North American geologists because Wegener couldn't come
up with a mechanism for continental drift. Suggested tidal forces, but physicists
showed this to be impossible. Wegener dies in 1930 and his idea dies with him.
Magnetism and Paleomagnetism
Earth is a bar magnet with a magnetic north and south. At poles a compass
needle dips vertically. Downward at the north pole, upward at the south pole and
horizontal at the equator. Magnetic poles do not correspond with geographic
poles. Variation is termed the magnetic declination. It is 16 degrees east in
California. However, it has been found that even though the magnetic and
geographic poles do not correspond today when the location of the magnetic
north pole is averaged over a 5,000 year period it does correspond with
geographic north. Magnetic pole moves as much as 25 km per year.
Causes of Earth's Magnetism
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First thought to be the result of a permanently magnetized core. However,
it has been shown that when any substance is heated above 500 degrees
C it looses its permanent magnetism.
Earth is a Dynamo - Outer core is a fluid consisting largely of iron, so it is
an excellent conductor. Electromagnetic currents are generated and
amplified by motion within the liquid caused by convection. Rotation of the
Earth unifies the random convective movements generating the magnetic
field.
Paleomagnetism
In the 1950's scientists discover how to measure paleomagnetism (magnetism
frozen in the rock at the time it formed). With this knowledge scientists could tell
the direction and latitude of geomagnetic pole at the time the rock formed.
Europeans were the first to extensively study paleomagnetic pole locations and
found that by 500 MY ago magnetic north was located near Hawaii. At first it was
assumed the poles were free to wander (Apparent Polar Wandering). North
American geologists attempted similar studies largely to disprove the Europeans
and found that 500 my ago North American rocks showed the magnetic north
pole to be in the East Pacific, 3000 miles to the west of the European magnetic
north at that time. (Figure)
At the same time a series of bathymetric surveys of the ocean basins revealed a
system of ridges and trenches with high heat flow over the ridges. H. Hess
(1962) rushes to print with the idea of Sea Floor Spreading. Postulates
convection cells beneath ocean basins to drive the spreading.(Figure)
Fred Vines supports Hess with his explanation of symmetrical magnetic stripes
on either side of the Atlantic Mid-ocean ridge (Figure).
Plate Tectonics
Theory of Plate Tectonics is born. Plate - is a rigid piece of lithosphere floating
on a partially plastic substrate (asthenosphere).
Seven Major Plates (See Figure in your Text)
1.
2.
3.
4.
5.
6.
7.
Pacific
North American
South American
African
Eurasian
Antarctic
Indo-Australian
Types of Plate Boundaries (Figure)
A. Divergent (Figure)- Spreading Center - Constructive Margin.
Characterized by ocean ridges and sea floor spreading.
B. Convergent - (Figure) Characterized by trenches and island arcs
1. Ocean - Ocean (Japanese Islands)
2. Ocean - Continental (Cascade Mountains)
3. Continent - Continent (Himalayas)
C. Transform - Plates moving past one another along strike- slip faults.
(Figure)
Additional Evidence in Support of Plate Tectonics
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Distribution of earthquakes along plate margins
Location of earthquake foci along steeply-dipping subduction zones
Age dating sediments on either side of the ridge indicates the sediments
get progressively older away from the mid-ocean ridge axis
Thickness of sediments also increases away from ridge
Driving Force (Figure)
1. Convection Cells
2. Hot Spots