UW Geology
ESS 101
Plate Tectonics:
Independent Study: Continental drift and plate tectonics
100 points
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.
The basis of the theory of plate tectonics 1 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 plates 2 (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 From the Greek tecktonikos, which means “of a builder.” The term refers to the building of the crust.
2 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) 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
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.”
2 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.
Plate tectonics was a revolutionary theory when it was formulated in the mid-
1960s. 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.
(3) (14)
(4) (15)
3 From the Latin sub, which means “below,” and ductare, which means “to draw or pull along.” Taken together they mean “to pull down.”

Illustrations courtesy of USGS http://pubs.usgs.gov/publications/text/historical.html

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 pres ent 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 re quired 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

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 minerals 4 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, called a negative anomaly .
positive anomaly
(5) (86)
3. Positive anomalies result in a paleomagnetic signature in the rocks that is 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 .
(6)
negative anomaly
(87)
(7) (88)
normal and reversed polarities
4 There are only a few magnetic minerals. The most common is magnetite, an Fe-oxide mineral.
5 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
Cenozoic 6 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 Most Paleozoic and older rocks have been altered and do not give reliable signatures.
7 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.

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.

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?