Download PLATE TECTONICS: BIRTH OF A THEORY

1. BIRTH OF A THEORY
Earth Revealed Classroom Adaptation
PLATE TECTONICS:
BIRTH OF A THEORY
OBJECTIVES
This episode describes the historical development of one of the most important scientific theories of
all time: plate tectonics. After watching the video, reading the text, and doing the associated exercises,
you should be able to:
• explain the theory of plate tectonics and describe the history of its development
• understand why the theory of plate tectonics was not widely accepted until the 1960s
• describe the relationship between seafloor spreading and plate tectonics
• explain marine magnetic anomalies and describe how they are formed at mid-ocean spreading
centers.
SUMMARY OF CONTENT
In particular, the major points you should learn from this unit are the following:
• Plate tectonics is a theory that has developed since the 1960s. This theory has revolutionized the
way that geologists view the evolution of our planet.
• The earth is dynamic; plate tectonics helps to explain the origin and evolution of both continents
and ocean basins.
• Scientists can not accept a theory unless the evidence from all avenues of research consistently
support that theory and, in this case, that it sensibly adds to our understanding of how our planet
has evolved.
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Earth Revealed Classroom Adaptation
1. BIRTH OF A THEORY
PREP
ARA
TION FOR THE UNIT
PREPARA
ARATION
The concepts in this unit could easily be explored without any prior knowledge of geology. This unit is
an excellent illustration of the scientific method at work. A good way to begin this unit is with a careful
examination of the classroom map; notice the apparent fit of Africa and South America. Many people
are under the mistaken impression that early scientists did not even notice the obvious similarity of
these coastlines. Although the apparent fit was clear, it is important to stress that the framework of
how the world works could not accommodate moving continents until the mid-1960s. Thus it was
easier to explain this fit as coincidence than to invoke a theory that required moving of enormous
masses of continental material through rigid oceanic seafloor crust.
If your class has studied plate tectonics, continental growth, or physiography of the seafloor,
review of this material aid help student understanding of Birth of a Theory.
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1. BIRTH OF A THEORY
Earth Revealed Classroom Adaptation
STUDENT REVIEW SHEET FOR
BIRTH OF A THEORY
Outline the history of the development of the theory of plate tectonics.
Student Review Sheet Answer Guide
Outline the history of the development of the theory of plate tectonics.
pre-1900s
— Natural historians noticed the apparent fit of many of the continents.
— Data collected on fossil distribution, mountain ranges, and rock types.
early 1900s
— Alfred Wegener proposes his theory of Continental Drift
continents move through the solid sea floor
— His theory is not accepted because there is no conceivable mechanism.
World War II
— Extensive data on the ocean basins and the seafloor collected.
early 1960s
— Harry Hess suggests the idea that becomes plate tectonics but he does not present
enough evidence for it to be accepted.
mid 1960s
— Marine magnetic anomalies discovered.
— These provide strong evidence for seafloor spreading.
since 1960s
— All aspects of earth science re-evaluated within the framework of plate tectonics.
Important T
erms
Terms
Continental Drift – A theory that suggested that continents which had been connected at one time
drifted away from one another into their present positions. The weakness of this theory was that
it did not propose a physically reasonable mechanism by which the continents were suggested
to move.
Crust – The outermost layer of the earth. These solid rocks can be either oceanic crust (mostly basalt
and gabbro) or continental crust (various rock types roughly granitic in composition) and vary in
thickness from 5 to 75 kilometers.
Guyots – Flat topped submarine peaks with tops that are now between 1,000 and 1,500 meters
below sea level. Guyots were once volcanic islands and they are topped by coral and evidence
of wave erosion which indicate that they were once found at sea level. The oceanic crust
subsides as it moves away from the mid-ocean ridge and the extinct volcanic structure settles
below sea level.
Magnetic Stripes – Alternating bands of strong and weak magnetic fields on the sea floor. The
magnetic stripes record the episodes of changing magnetic orientation during spreading of the
sea floor.
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Earth Revealed Classroom Adaptation
1. BIRTH OF A THEORY
Mantle – The layer of the Earth that lies immediately beneath the crust. It is solid material. The
uppermost mantle is rigid and moves with the crust in plate tectonics. The lower portions of the
mantle are plastic and can flow very, very slowly allowing the plates to move on top of the
moving mantle.
Paleomagnetism – The ancient magnetic fields of the Earth. We can infer the orientation of ancient
magnetic fields because some types of rocks record this information at the time of their formation.
Pangaea – The supercontinent that broke up about 200 million years ago to form the various
continents that we see today.
Plate Boundary – The edge of a tectonic plate. Most volcanism, earthquakes, and mountain building
occurs along the boundaries of the tectonic plates.
Plate TTectonics
ectonics – The theory that the surface of the earth is divided into about a dozen major plates
that are slowly moving relative to one another. The plates are about 100 kilometers in thickness
and can contain continents, ocean basins, or both. The motion causes the plates to shift position
and change in size with time.
Plate – A large, moving slab of rock at the earth’s surface. Plates are comprised of the uppermost
portions of the mantle, oceanic crust, and continental crust.
Tectonics – The study of the origin and development of the broad structural features of Earth. The
term originates from the Greek work tecton, meaning “to build.”
Birth of a Theory
An Overview of Plate TTectonics
ectonics
The theory of plate tectonics has revolutionized our understanding of Earth and how it works. Based
on the concept that the outer layer of Earth is composed of separate plates, plate tectonics enables
earth scientists to explain such diverse phenomena as earthquakes, volcanoes, mountain building,
the shapes of the continents, the age of the seafloor, and the distribution of fossils and some rock
types. The plates are rigid, about 100 kilometers thick, and vary tremendously in size. Each tectonic
plate includes oceanic and continental crust and the uppermost mantle. Plate tectonics states that
these plates are moving relative to one another; they can pull apart, collide, or slide past one
another.
Historical Context
The emergence of the theory of plate tectonics brought together diverse scientific ideas that had been
developing for several centuries. The voyages and discoveries of the 16th century allowed early
mapmakers to observe that the outline of the west coast of Africa seemed to match that of the east
coast of South America. In 1620, Francis Bacon commented on this remarkable fit. During the late
19th century, explorers and naturalists made numerous discoveries regarding the plant and animal
life of different lands, and the rocks types that could be found as well. For example, a similarity was
found between the fauna of the island of Madagascar, off the east coast of Africa, and that of India,
separated by nearly 4,000 kilometers of ocean. Another puzzling discovery was the similarity of
certain rock formations and fossils on Africa, South America, Australia, Antarctic, and India. The
great expedition of the H.M.S. Challenger between 1872 and 1876 discovered the mid-Atlantic
Ridge.
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1. BIRTH OF A THEORY
Earth Revealed Classroom Adaptation
Alfred W
egener and Continental Drift
Wegener
Alfred Wegener, a German meteorologist, studied the fit of the continental margins and the distribution
of rock types. He was intrigued by the jigsaw-puzzle fit of continents now separated by ocean basins.
Not only did widely separated continents appear to fit together, but he found remarkable geologic
similarities as well: rock types, fossil assemblages, and mountain belts often lined up across enormous
ocean basins. As a result of his research, in 1915 Wegener proposed that all the continents had once
been joined together but had broken apart into the distribution of the continents we see today.
Wegener called his theory continental drift. Continental drift was not universally accepted because
geophysicists of that time could not accept that the massive continents could move through oceanic
crust.
Harry Hess, P
aleomagnetism, and Seafloor Spreading
Paleomagnetism,
During the late 1950s and 1960s evidence from studies of the sea floor and paleomagnetism
renewed interest in continental movements. In 1962, Harry Hess suggested that mid-ocean ridges
were narrow zones where hot mantle was rising into the ocean crust and creating new sea floor.
From his observations, Hess developed a hypothesis that contrasted with Wegener’s idea of continents
pushing through the ocean crust and suggested instead that the continents rode atop a moving
mantle. The continents were passive passengers on the moving plates. Hess also recognized that the
earth was not increasing in size and thus seafloor must be consumed at oceanic trenches to balance
out its creation at spreading centers. As intriguing as Harry Hess’s suggestions were to other geoscientists,
there was still no viable mechanism to cause all of this moving around of plates.
The strongest evidence that crust is created at spreading centers was found through studies of
the magnetic properties of rocks. The strength and direction of Earth’s magnetic field at the time of a
rock’s formation is preserved in the orientation of tiny magnetic crystals in some rock types. By towing
instruments that detect the magnetic field behind oceanographic research ships, scientists discovered
striking patterns in the magnetic characteristics of the seafloor rocks. They found linear magnetic
anomaly patterns that preserved evidence of the reversals of Earth’s magnetic polarity as well as the
spreading of the sea floor in both directions from the mid-ocean ridge. These magnetic band show
periods of normal and reversed magnetism on one side of the crest that are mirror images of those
on the opposite side. As the sea floor spreads away from mid-ocean ridge, half moves to one side
and the other half moves to the other side, forming two symmetrical magnetized bands. The process
is repeated over and over again as the sea floor spreading continues during periodic reversals of the
earth’s magnetic field.
The magnetic striping of the sea floor rocks could be correlated in all of the major ocean
basins providing important evidence that oceanic crust is created at the mid-ocean ridges. Harry
Hess’ ideas of seafloor spreading became the theory plate tectonics. The past thirty years have seen
a blossoming of the earth sciences as geologists and oceanographers have worked to understand the
earth within the context of this new and exciting theory.
Further Study and Suggested Activities
1.
Closely examine a physiographic map of the seafloor and either discuss or explain the
following:
a. Where would you expected to find the youngest oceanic crust?
b. Where would you expect to find the oldest oceanic crust?
c. Where would you expect seafloor sediments to be thickest? thinnest?
Plate tectonics states that crust is created at mid-ocean ridges. Thus it is at these ridges
that oceanic crust is the youngest. Sediment accumulation on this young crust should be
negligible. As crust moves away from the ridge it gets progressively older until the oldest
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Earth Revealed Classroom Adaptation
1. BIRTH OF A THEORY
oceanic crust is found along the continental margin. The oldest oceanic crust on Earth is
of Jurassic age and is found in the western Pacific Ocean. The older the seafloor the
greater the time for accumulation of seafloor sediments so, in general, the thickest
sedimentary sequences are found at great distance from the mid-ocean ridges.
2.
Make some algebraic calculations to develop an understanding of the rates of sea floor
spreading and continental separation. Average spreading rates for use in your calculations are the following:
Mid-Atlantic Ridge
San Andreas Fault
East Pacific Rise
2 cm/year
6 cm/year
10 cm/year
Calculate when Africa and North America began separating. Carefully measure the
distance between the two continents, convert to centimeters, and determine how many
years have gone by since spreading began. Do not forget to divide by two because the
ridge creates crust on both plates! These continents actually began spreading about 150
million years ago. Account for differences in your answers by discussing the accuracy of
your measurements on the map, whether or not the spreading rate has been constant
during this time, etc.
Calculate when Los Angeles will become a suburb of San Francisco. These two cities
in California are on opposite sides of the San Andreas Fault so they are moving closer to
each other at a rate of approximately 6 cm/year. Use a map to determine the distance by
which they are separated. The truly great earthquakes on the San Andreas Fault are
capable of producing an offset of 10 meters. Determine how many major (>magnitude 8)
earthquakes it will take to move Los Angeles all the way to San Francisco.
Use the spreading rates given above to predict where you would find the oldest
seafloor crust. You may assume that spreading occurs perpendicular to the ridge crest so
you will want to make calculations on several spots that are at the greatest distance from
the ridge crest.
3.
Cut the continents out of a map of the world and reorganize these puzzle pieces to create
the supercontinent of Pangaea. Discuss the likely outcome if you had cut your continents
along the edge of the continental shelf rather than at the present shoreline. Discuss the
likelihood that the continental outline has remained exactly the same over the past 200
million years.
4.
This question is probably best as a discussion question (although it could be used on an
exam as well). How would you go about looking for ancient plate boundaries (assuming
there is not tectonic activity at the present time)? On a map of the world identify as many
ancient plate boundaries as you can.
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1. BIRTH OF A THEORY
Earth Revealed Classroom Adaptation
Test Questions
Multiple Choice
1.
The major objection to Wegener’s continental drift theory was
a. the lack of correspondence of the continental coast lines
b. the absence of an obvious mechanism capable of moving continents
c. the lack of fossil evidence indicating that landmasses were once joined
d. lack of rock continuity from one separated area to another
answer: b
2.
According to the theory of plate tectonics,
a. the seafloor is approximately the same age everywhere in the Pacific
b. only in the Atlantic is new sea floor formed at the deep sea trenches
c. old sea floor in all oceans is subducted in the mid ocean ridges
d. new sea floor is formed at the mid ocean ridges
answer: d
3.
Evidence that the continents were capable of moving came in the form of:
a. fossil assemblages that matched across large ocean basins
b. a solid crust lies atop a plastic mantle
c. magnetic characteristics of the seafloor indicate that crust is continuously being
created
d. coral fossils in high latitudes must have formed in warm, tropical waters
answer: c
4.
Wegener’s theory of continental drift was not widely accepted because
a. other scientists did not believe that rock types and fossil assemblages were similar
across major ocean basins
b. magnetic profiles of seafloor crust did not suggest magnetic anomalies
c. at the time there was no plausible mechanism by which continents could move
d. the field of geology was in its infancy and not much was known about the distribution
of different rock types.
answer: c
5.
Magnetic anomaly patterns
a. are linear and extend for hundreds of kilometers
b. are oriented perpendicular to mid-ocean ridges
c. record the rotation of the earth’s orientation in space
d. occur only in the Atlantic ocean
answer: a
6.
Which of the following fields of study was not important in developing the theory of plate
tectonics?
a. oceanography
b. geology
c. paleomagnetism
d. meteorology
answer: d
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Earth Revealed Classroom Adaptation
1. BIRTH OF A THEORY
Essay Questions
1.
Briefly describe the fundamental difference between continental drift and plate tectonics.
Although both theories are centered around continents moving in a dynamic earth, only
plate tectonics suggests a mechanism by which the continents can move. The creation of
oceanic crust at mid ocean ridges results in the creation and enlargement of an ocean
basin and the subsequent spreading of continents as well.
2.
What was Wegener’s evidence for continental drift?
In addition to the apparent fit of the west coast of South America and the east coast of
Africa, Wegener was impressed by the continuity of mountain belts and rock types across
ocean basins, the similarity of fossil assemblages on widely separated continents, and the
evidence for ancient glaciation in mild climates on several continents.
3.
What are marine magnetic anomalies and why were they so important in the development of plate tectonics?
Marine magnetic anomalies are the pattern of “stripes” on the seafloor which correspond
to seafloor rocks with different magnetic characteristics. These stripes run parallel to the
mid-ocean ridges and the pattern is symmetrical about the ridge crest and similar in all of
the major ocean basins. The discovery of these patterns in the seafloor were critical in the
development of plate tectonics because they indicated that oceanic crust is created at the
mid-ocean ridge and it moves away from the ridge crest as it ages. This creation of
oceanic crust by seafloor spreading provided a mechanism that could also move
continents as parts of the tectonic plates.
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