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UW Geology
ESS 101
Plate Tectonics:
Independent Study: Continental drift and plate tectonics
100 points
Introduction
Earth is in constant motion; our planet rotates on its axis and revolves around the
sun. The crust of Earth is also in constant motion. New crust is constantly being
generated and later recycled. Large chunks of crust are broken into segments
and “float” around the surface, driven by convection currents in the mantle. In
this lesson, we will examine how the theory of continental drift and the theory of
plate tectonics revolutionized the science of geology by explaining earthquakes,
volcanoes, and other phenomena.
Plate Tectonics Theory
The basis of the theory of plate tectonics1 is that the crust is less dense than the
mantle, and it “floats” on the more dense but plastic mantle, somewhat like
marshmallows on top of hot chocolate. The crust itself consists of two parts of
different densities, the oceanic and the continental crust. The continental crust is
less dense because the rocks contain relatively little iron, and it is much thicker
than the oceanic crust (20–90 km). Mountainous regions rise higher above the
continental crust, and extend deeper into the mantle than the thin oceanic crust.
The oceanic crust is denser because it is relatively iron-rich and lies like a thin,
heavy blanket on top of the mantle. In fact, the name “ocean crust” is an
unfortunate holdover from pre-plate tectonics times. The rocks actually have
nothing to do with the oceans. They are simply heavy, and bow downward into
the more pliable mantle causing depressions on the earth’s surface. Water runs
downhill (don’t ever lose sight of this simple fact) and it collects in these basins.
The boundary between oceanic and continental crust is approximately the edge
of the continental shelf. At this time in geologic history the continental shelf is
covered by seawater. But in previous geologic periods, like the ice ages, sea
level was lower and the shelves were exposed.
The crust (oceanic + continental) is broken up into a number of units called
plates2 (figure 4.4 p.72). There are seven major plates and a number of smaller
ones. Some are entirely oceanic crust, some consist of both continental and
oceanic crust, and a couple consists almost entirely of continental material. (Can
you find two of each kind?) Each plate moves as an independent unit over
geologic time. Each plate margin either diverges (spreads apart) from an
1
2
From the Greek tecktonikos, which means “of a builder.” The term refers to the building of the crust.
Imagine them on a spherical globe (not a flat map), they are more or less round in outline, thin, and flat.
divergent and convergent
margin
oceanic crust
theory of plate tectonics
continental crust
(1)
(10)
(2) (12)
 (see text
adjacent plate, or converges (collides) with another adjacent plate

 away from
figure 12.7 p.336). On the divergent boundary, as the plates move
each other, hot molten rock from the asthenosphere pours out
(because hot stuff
 
rises). It hardens into solid rock, thus building up the edge of each plate. This
happens periodically over millions and millions of years.
Because the earth isn’t expanding like a balloon being blown up, some part of the
plates must be breaking down. The convergent boundary of the plate, the part of
the plate that has the oldest rock, is very cold. Cold material is dense and drops.
The cold edge of the plate drops down into the asthenosphere, generally at an
angle of 45 degrees. This is called subduction,3 with the down-dropping edge of
the plate eventually becoming assimilated into the asthenosphere.
subduction
(3) (14)
The plates move on average 5 cm/year, about the same rate your fingernails
grow. These motions result in almost all the major features of the earth’s crust.
Geology happens on a plate margin. The mechanism consists of the brittle
lithosphere sliding on the warm mobile asthenosphere. The motion is generated
by the upwelling of mantle material that is heated near the core. It flows upward,
cooling near the surface and dropping down again, to become reheated. This
motion is called a convection cell (text figure 12-34)
convection cell
From the Greek lithos, which means “a stone.”
From the Greek asthenes, which means “weak.” It is the layer that is not brittle rock.
3 From the Greek tecktonikos, which means “of a builder.” The term refers to the building of the
crust.
2
The Politics of Science
Plate tectonics was a revolutionary theory when it was formulated in the mid1960s. This theory is the foundation on which we now base all our geological
hypotheses. Prior to this, it was very difficult to explain many global patterns,
such as why 80% of the active volcanoes are located around the Pacific rim and
why some areas of the earth were in a mountain-building phase, when other
areas had not formed mountains for more than a hundred million years.
From the Latin sub, which means “below,” and ductare, which means “to draw or pull along.” Taken together
they mean “to pull down.”
3
(4) (15)
Illustrations courtesy of USGS
http://pubs.usgs.gov/publications/text/historical.html
Alfred Wegener and Continental Drift
The idea of the continents being in motion was first developed under the name
“continental drift” in 1912 by Alfred Wegener, a much-traveled German
geographer. The first two pages of chapter 4 present Wegener’s theory, with his
substantial supporting evidence that was published from 1915 to 1932. His work
was ridiculed by the American and English geologic community at that time.
Geophysicists (scientists who study the inner workings of the planet) totally
rejected the concept of moving continents. There seemed to be no mechanism
that allowed solid continents to move over the rocks of the ocean floor.
(Remember, this was before geologists knew much about the ocean basins.)
There was another factor that contributed to the overwhelming rejection of
Wegener’s theory: he was German. This was a period in history in which Europe
was bitterly divided, with the United States allied with England against Germany
in World War I. Wegener’s only support was from southern hemisphere
geologists (who had helped him amass his data). This included Alexander Du
Toit, a distinguished geologist in South Africa. Du Toit believed that if he
continued to document evidence to substantiate this theory, the northern
hemisphere geologic community would be convinced. Unfortunately, here too,
politics obstructed science. South Africa had beaten the British forces in the Boer
War, and many South Africans sided with Germany during World War I. Due to
these political realities, the theory of continental drift was not taken seriously..
Testable Hypotheses for Plate Motion: Harry Hess
The release in the 1950s of sophisticated technology developed during World
War II, and the era of cooperation amongst Western Hemisphere countries, led
to an explosion in the acquisition of geologic knowledge. In 1962, Harry Hess of
Princeton University first proposed the basics of the theory of plate tectonics by
providing acceptable, testable hypotheses for plate motion. He made numerous
(educated) assumptions, but did not have much evidence at hand. In the
introduction to this paper, Hess said, “I shall consider this paper an essay in
geopoetry. I shall hold as closely as possible to a uniformitarian approach; even
so, at least one great catastrophe will be required early in the earth’s history.” His
hypothesis resulted in successful studies on the mechanisms of plate movement
and plate boundary relationships. By the mid 1970s, Hess’ theory had explained
so many previously puzzling aspects of geology that it was accepted by almost
all earth scientists. In recent years, satellite measurements of the continents
have proven that they move away from mid-ocean ridges and in the directions
proposed by the theory of plate tectonics.
Key Terms
 magnetic anomalies
 positive anomaly
 negative anomaly
 magnetic declination and
inclination
 normal and reversed
polarities
 paleomagnetism
Paleomagnetism
figure 1.3.2
diagram courtesy of USGS
http://pubs.usgs.gov/publications/text/developing.html
Measurements of the earth’s magnetic field provided one of the first advances in
plate tectonic studies. In a sense, our planet is like a large magnet with Earth’s
north and south poles being the positive and negative ends of a magnet (all
magnets have a positive end and a negative end.) This is how a compass works;
Earth’s magnetic field causes the metallic needle of the compass to point north.
However, approximately every 750,000 years, Earth’s magnetic field reverses.
Here are some points to consider:.
1. The global magnetic field forces the alignment of magnetic minerals4 to lie
parallel to the lines of magnetic force as they crystallize or as they slowly
settle out of seawater.
2. The magnetic field has changed direction repeatedly over geologic time.
Fortunately for us, there are only two directions:
a.
with the magnetic north pole located near the geographic north pole,
called a positive anomaly,5 and
magnetic anomalies
b.
with the magnetic north pole located near the geographic south pole,
(5) (86)
called a negative anomaly.
positive anomaly
3. Positive anomalies result in a paleomagnetic signature in the rocks that is
(6) (87)
called normal polarity, because this is where it is now. Negative anomalies
results in a paleomagnetic signature in the rock that is called reversed
polarity.
negative anomaly
(7) (88)
normal and reversed
polarities
4
5
There are only a few magnetic minerals. The most common is magnetite, an Fe-oxide mineral.
The word anomaly refers to the magnetic readings being either above or below an average number that has
been computed for the global magnetic field.
4. These “magnetic signatures” are locked into the rocks when they form, and
they can be measured by taking oriented cores of unaltered rocks and
putting them in a magnetometer. The results are called the paleomagnetism
of the rocks. The sequences of normal and reversed polarities determined by
thousands and thousands of paleomagnetic studies on Mesozoic and
Cenozoic6 rocks has resulted in a pattern, illustrated in figure 12.6 p.335 the
text, that shows unequal periods of reversals.
paleomagnetism
(8) (1)
5. From obtaining radiometric ages on rocks of known polarity, geologists have
established the paleomagnetic time scale. There is now a reliable one
extending back to the early Mesozoic, around 200 million years ago. This
timescale can be used as a third option for placing rocks into the geologic
time scale. All you are doing, however, is matching up the patterns of normal
and reversed polarity, so you must have some idea about the absolute age.
6. The measurement of the magnetic inclination and declination of rocks gives
their geographic position at the time they were formed (when they cooled, or
became lithified). These measurements can give us the approximate latitude
of the paleo-position,7 but not the longitude.
magnetic inclination and
declination
(9) (2)
6
7
Most Paleozoic and older rocks have been altered and do not give reliable signatures.
The geographic position at which they were formed, way back in geologic time.
illustration courtesy of USGS
The period during which the global magnetic field reverses is very short by
geological standards—approximately 100,000 years. During this time, the dipolar
field breaks up and then rebuilds in the opposite direction. Physicists have shown
that the magnetic field insulates the earth from the bulk of the sun’s ultraviolet
light. We have not yet figured out what happens to life on Earth when the field is
eliminated, but we do know that these periods do not correlate with mass
extinction events. Perhaps the period is too short to have a major biologic effect.
Evidence for Plate Tectonic Theory
The evidence for both sea floor spreading from mid-ocean ridges and subduction
at convergent margins was already available when Hess proposed his theory.
Most lines of evidence are on plate boundaries. But these were not recognized
as significant to an all-encompassing geologic theory. In many cases, all that was
required was for geologists to reassess critical data in the light of this new model.
Patterns of Positive and Negative Anomalies
The repetitive pattern of positive and negative anomalies recorded in stripes
parallel to the mid-ocean ridges (illustrated in figure 1.3.2) was already available
from studies of the seafloor by oceanographic research ships off the coast of
Oregon (above the Juan de Fuca Ridge) and south of Iceland (the mid-Atlantic
ridge). The patterns were supplied by magnetometers that trailed from the ships
as they sailed back and forth across the ridge. Of course, these patterns were
not nearly as neat and defined as the illustration above. A great deal of “noise”
overprinted the picture, and the anomaly boundaries are not straight. In 1963,
two labs independently arrived at the significance of these stripes and
hypothesized that they would be progressively older the farther one moved away
from the ridge. This was evidence of continuous upwelling of magma through
alternating periods of normal and reversed polarity and the constant building of
new plate material.
It is very expensive to obtain rocks samples from the bottom of the ocean, so
many of the ages have been determined by obtaining magnetic signatures and
dates from land-based rock units. Even on continental rocks, radiometric dates
are hard to secure sometimes. Therefore, fossils have been used to place the
rocks in the time scale. Faunal sequences of microfossils from deep-sea
sediment cores have been dated by matching magnetic anomaly patterns.
The Benioff Zones
Evidence for subduction lay in the model of the Benioff Zones that were defined
by deep earthquakes. Most the Benioff Zones were known to be around the
perimeter of the Pacific Ocean, at angles of approximately 45degrees to the
surface. The zones were mapped as dipping away from the ocean floor and
going under the adjacent continent. These deep earthquakes actually reflect the
dense, dipping lithosphere, because earthquakes can only occur in cold, brittle
rock. The Benioff Zones do not continue below about 670 km from the surface;
therefore, it is inferred that this is the depth at which the plate is heated up
enough to assimilate into the asthenosphere. Geologists now had evidence and
geographic localities of spreading margins and of subduction margins. All that
was needed was to determine the plate boundaries and to figure out the
mechanism of plate motion. The boundaries are defined by the convergent and
divergent margins and the recognition of transform boundaries like the San
Andreas fault.
Stationary Hot Spots
The recognition of stationary hot spots that produce magma from deep in the
mantle on a geologically continuous basis explained mid-plate volcanism, such
as that found in both the Hawaiian Islands and the Emperor Seamounts. If you
look at a good map of the Pacific Ocean, you will see numerous, similar midplate volcanic island chains in the south Pacific. The part of the plate that is
immediately over the hotspot is bowed upward by the heat and lies high up on
the asthenosphere. The active volcano builds up from the seafloor to above sea
level. As the plate moves over the hotspot, that volcano is cut off from its lava
source. Two geologic processes result in the volcano being reduced in size and
eventually dropping below sea level to form a seamount:
7. the basalt weathers rapidly because the mafic minerals are unstable at low
temperatures and because most islands are subjected to a lot of rain; and
8. as the plate moves away from the magma source, it cools, and the increased
density causes it to sink back down on the asthenosphere.
Plate Movement and Motions
Hot spots provide another line of evidence of plate motion. On the assumption
that hot spots are globally stationary, measurements of the volcanic traces of a
hotspot as the plate moved over it indicate how fast, and in what direction, a
plate is moving. Like numerous Pacific hotspots, the Yellowstone hotspot that
now lies under Wyoming, has shown us the path of the North American Plate
since the mid-Cenozoic.
There are still many questions regarding the application of the plate tectonics
model to every geologic phenomenon, but over the last 15 years it has been
substantiated by almost all the data and has yet to be refuted.
Further Reading and Viewing
Recommended Reading
I recommend highly the following article as optional reading:
“Our Restless Planet: Earth.” in National Geographic volume 169, August
1985: 162–81. And old one but still good.
Recommended Viewing
“The Living Machine.” Segment 1 of Planet Earth. Produced by
WQED/Pittsburgh in association with the National Academy of Sciences. 60
min. Distributed by Films Inc., 1986. Videocassette.
This video is relatively old now, but the graphics are very good, and it touches on
many of the topics you have studied in this course.
Scoring
This activity 100 points of your course score. Your product for this work will need
to include:
1.A set of process notes showing that you understand the details providing
evidence for tectonic theory.
2.A list of key terms with definitions
3.Intelligent responses to the following questions:
Study Questions:
1. Question 1:What was the evidence supporting Wegener’s hypothesis?
2. Question 2:Consider some of the reasons scientists may have rejected
Wegener’s hypothesis, despite the evidence supporting it.
3. Question 3:How did research on the Mid-Atlantic Ridge finally support
Wegener’s hypothesis?