PLATE TECTONIC THEORY
One Theories Journey
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Continental drift -- One theory's journey
Today, you might be laughed out of a geology course
for questioning whether continents move through
geologic time. But 75 years ago, only skeptics believed
that continents could take a hike. Talk about
conventional unwisdom: W.B. Scott, former president of
the American Philosophical Society, even called the
theory of continental drift "utter damned rot." (!)
What's changed? Not the actual movement of
continents, but our understanding of geology itself. Let's
take a look at how the theory developed, and how the
evidence began to favor it.
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Antonio Snider-Pellegrini suggests that continents
were linked during the Pennsylvanian period
(325 million to 286 million years ago), because
Pennsylvanian plant fossils from Europe and
North America were similar. (See "carboniferous"
on this time line.
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Australian geologist Edward Seuss sees similarities
between plant fossils from South America, India,
Australia, Africa and Antarctica, and coins
"Gondwanaland" for a proposed ancient supercontinent with these land masses.
F.B.Taylor
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1910
American physicist F.B. Taylor proposes concept
of continental drift to explain formation of
mountain belts.
Unfortunately no one remembers him or has his
photo because he did not support his theory with
emperical evidence!
Alfred Wegner 1912
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German meteorologist Alfred Wegener proposes
theory of continental drift, based on evidence
from geology, climatology and paleontology.
Wegener names one of the ancient super-continents
"Pangea," and draws maps showing how the
continents moved to today's positions.
How’d they move again?
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Assorted arguments are used to debunk continental
drift, most importantly the lack of a mechanism
strong enough to move continents across or through
ocean basins.
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South African geologist Alexander du Toit maps
out a northern super-continent, "Laurasia," to
explain coal deposits, which presumably indicate
the remains of equatorial plants, in the Northern
Hemisphere.
Paleomagnetism
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Paleomagnetism: British scientists find that magnetic
fields recorded in rocks from Europe and North
America indicate the rocks were formed in far
different locations than their present positions.
The pattern of continental drift recorded by rocks
show Europe and North America have drifted away
from each other for more than 100 million years.
This movement opened the Atlantic Ocean.
Paleomagnetism
Gondwana
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Fossils of the plant genus Glossopteris occur on all
five Gondwana continents. The seeds were too
heavy to be carried by wind, and would have
died quickly in salt water, indicating that the
continents were once joined. Similarly, fossils of
several reptiles occur on several continents.
Gondwana
Appalachian Mountain Chain
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The Appalachian Mountain chain can be linked
with mountains in Greenland, the United
Kingdom and Norway, indicating that these land
masses were joined at the time the mountain chain
formed
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Marks left by glaciers on rocks in Africa, India,
South America and Australia make no sense -unless these continents were joined and arrayed
around the South Pole. Then, the glacial scars
would have all pointed away from the Pole when
they were made. Today, glaciers form near the
poles and move away as they travel and eventually
melt.
Ocean Floor
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The oldest rocks on the ocean floor are younger
than 220 million years, while the oldest terrestrial
rocks are about 4 billion years old, indicating that
the ocean floor is recycled back into the Earth.
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Earth's magnetic field periodically changes
polarity (your compass would point to the South
Pole). When magma solidifies at the mid-oceanic
ridges, it records the polarity of the Earth's
magnetic field. Bands of seafloor basaltic rocks
paralleling the mid-oceanic ridges carry a record
of this alternating polarity.
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Geologists think convection cells in the mantle
(the hot, plastic rock under the crust) power
continental movements, overcoming an early
objection to continental drift.
Convection Cells
Plate Boundries
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Three kinds of boundaries
Continental drift -- or plate tectonics -- involves a lot of
complicated motion at the plate boundaries. The tamest
version generally occurs beneath the oceans, when plates
move away from each other. At these divergent plate
boundaries, molten magma rises and solidifies into solid
rock, filling the gap formed as the two plates move apart.
These spreading centers (see figure below or the graphic
at the top of this page) form ridges on the seafloor. As
spreading continues and the rocks move away from the
ridge, they cool and contract as they age. The movement
usually occurs at a relatively constant rate of a few
centimeters per year and tends not to produce large
earthquakes.
What?
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Either of the other two types of boundaries may be
associated with large earthquakes. At transform
plate boundaries one plate moves laterally
against and past another. Some transform
boundaries are also described as strike slip faults.
At a convergent boundary, one plate must either
slip beneath the other (a subduction zone) or the
two plates must collide (a collision zone). A
classic subduction zone has a denser oceanic plate
diving, or "subducting", beneath a less dense
continental plate.
San Andres Fault in California
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But when a giant rock hits an immovable object -when one tectonic plate moves suddenly against
another -- the havoc of a major earthquake can
result. Lacking the stress relief of regular, minor
earthquakes, strong rock gets stuck at the fault
zone, allowing strain to build up.
Earthquake in Haiti
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When the strain gets too great, it is relieved by
the sudden movement causing a major
earthquake. The greater the strain, the larger the
earthquake. Thus in a sense, earthquakes should be
predictable if we know the strain and the strength
of the rocks.
Seizmograph
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Unfortunately, despite that simple equation, precise
predictions of quakes are not possible now.
Beyond problems measuring the strength of rocks,
we have only a foggy picture of the triggering
mechanism. "How the slip on a fault starts is a
fundamental problem in seismology," says Clifford
Thurber, a geophysicist at the University of
Wisconsin-Madison. "We don't really know the
conditions and state that a fault is in when it starts
moving."
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The problem, simply, is inaccessibility. Earthquakes
start underground -- sometimes dozens or
hundreds of kilometers deep, and "there is no
direct way to detect conditions," as Thurber says.
Indirect measurements may offer a guideline, but
direct observations would be preferable.
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That's one reason for a recent proposal to drill 3.5
kilometers into the San Andreas Fault to obtain
direct measurements from that active region.
Thurber notes that cores removed from the hole
would be examined for strength and fluid content.
Instruments in the hole itself would look at fluid
pressure, which may be implicated in initiating
quakes. "We want to get direct measurements of
the physical properties for the first time," Thurber
says
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The project would be a step toward the eventual -and we stress eventual prediction of earthquakes.
We'll get to that prospect shortly.
References
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Earth Quakes @ Geology wise. Edu
http://www.geology.wisc.edu/courses/g115/quake
/5.html
The information in this presentation was taken
almost in its entirety from Geology Wisconson.edu