Big History Plate Tectonics/Continental Drift Name:_________________________________________
DOCUMENT 1
ALFRED WEGENER’S CONTINENTAL-DRIFT HYPOTHESIS
In 1915, the German meteorologist Alfred Wegener published the
hypothesis of “continental drift” in his book The Origin of the Continents
and Oceans. The continental-drift hypothesis proposed that Earth’s
continents had moved slowly across Earth’s surface and that this
movement required millions of years to happen. The idea that
continents were able to move meant that continents would have been
in very different places on Earth’s surface in the past from they are
today. In Wegener’s own words:
This is the starting point of displacement or drift theory. The basic
“obvious” supposition…that the relative position of the
continents…has never altered…must be wrong. The continents must
have shifted. South America must have lain alongside Africa and
formed a unified block which was split in two;…the two parts must
have then become increasingly separated over a period of millions of
years.
Wegener’s continental-drift hypothesis was based on:
1. Matching coastlines of continents, and
2. Similar fossils and rock types on continents that are
separated by oceans.
Wegener was not the first to observe that certain continental coastlines
fit together like pieces of a puzzle, but it was not until Wegener
published his continental-drift hypothesis that the idea of moving
continents was seriously considered as a full-blown scientific theory.
Document Questions:
1. What is the theory of Continental Drift?
2. What two pieces of evidence did Wegener have for the theory?
3. What is missing from Wegener’s claim about Continental Drift?
Write a summary sentence for
each paragraph:
Big History Plate Tectonics/Continental Drift Name:_________________________________________
DOCUMENT 2
IMPORTANCE OF CONTINENTAL DRIFT
Prior to Wegener’s continental-drift hypothesis, the common view
among most physicists and geologists was that mountains and other
features on Earth’s surface were created because the Earth was
shrinking through time. James Dwight Dana, a 19th-century American
geologist, proposed the contracting-Earth theory, which said that as the
Earth shrank, the rigid outer crust would also shrink and wrinkle,
producing mountain ranges and other features on Earth’s surface.
Wegener’s ideas proposed a very new way of thinking…
According to the United States Geological Survey (USGS)
“The Dynamic Earth” website:
“The theory of continental drift would become the spark that ignited a
new way of viewing the Earth. But at the time Wegener introduced his
theory, the scientific community firmly believed the continents and
oceans to be permanent features on the Earth’s surface. Not
surprisingly, his proposal was not well received….”
Although many Earth scientists did not initially believe Wegener’s ideas,
they did realize that if Wegener was correct, there would be some very
important consequences:
From Nature (109 (1922): 202–03):
“The revolution in thought, if the theory is substantiated, may be
expected to resemble the change in astronomical ideas at the time of
Copernicus.”
Document Questions:
1. Before Wegener, what did people think caused the mountains?
2. How did Continental Drift change the previously held beliefs?
3. How could this change be compared to the changes that
happened with Copernicus’s model of the universe?
Write a summary sentence for
each paragraph:
Big History Plate Tectonics/Continental Drift Name:_________________________________________
DOCUMENT 3
IMPORTANCE OF PALEOMAGNETISM
Paleomagnetism, developed in the 1950s, is the study of the Earth’s
magnetic field in the rock record. Some rocks contain minerals that
behave like small magnets when they are in the presence of Earth’s
magnetic field. This means that when these minerals form today they
act like little compass needles and point to the north. Paleomagnetism
was important to confirming continental drift because it showed that
certain continents had been in very different positions on Earth’s
surface in the past. Paleomagnetists were able to determine these past
positions because magnetic minerals in rocks are oriented in different
ways depending on where they formed on Earth’s surface. In many
cases, the locations where rocks formed in the past were very different
from where the rocks are located today. This suggested that continents
had moved across Earth’s surface through time, just as Alfred Wegener
proposed.
Write a summary sentence for
each paragraph and one sentence
describing what you see in the
image:
The Earth acts like a bar magnet with two magnetic poles and magnetic
field lines circle around the Earth with different orientations at different
latitudes.
According to Henry Frankel
“Palaeomagnetism rekindled interest in continental drift, but it
changed few minds…”
Document Questions:
1. What is Palaeomagnetism?
2. How did Palaeomagnetism help support the theory of Continental Drift?
Big History Plate Tectonics/Continental Drift Name:_________________________________________
DOCUMENT 4
IMPORTANCE OF SEAFLOOR SPREADING
Write a summary sentence for each
paragraph and one sentence
describing what you see in the image:
From Frankel (1988): “A stronger case for continental drift was
needed. It came from marine geology, another relatively young
field. By the early 1960s,…special attention was directed towards
the origin of ocean ridges. In retrospect, the explanation having the
greatest influence was that proposed by Harry Hess of Princeton
University. His 1960 solution…was labeled ‘seafloor spreading.’”
The figure above illustrates Hess’s model of seafloor spreading. New
ocean crust forms at mid-ocean ridges (“Ridge” in figure). As this new
ocean floor is created, the older ocean floor is moved away from the
ridge until it is finally removed from Earth’s surface by being pushed
[subducting] below continental plates at ocean trenches (“Trench” in
figure). New ocean floor created at ridges moves continents away from
one another. Destroying ocean crust at trenches moves continents
towards one another. Seafloor spreading explained how the continents
move, which finally made continental drift a plausible idea.
Document Questions:
1. What is seafloor spreading? How does the earth’s surface make new crust?
2. Who first proposed the idea of sea-floor spreading?
3. How does the idea of sea-floor spreading help support the idea of continental drift?
Big History Plate Tectonics/Continental Drift Name:_________________________________________
DOCUMENT 5
IMPORTANCE OF THE VINE-MATTHEWS-MORLEY HYPOTHESIS
The Vine-Matthews-Morley hypothesis confirmed Hess’s seafloorspreading hypothesis and, as a result, also helped to validate Wegener’s
continental-drift hypothesis. It was also important to the development
of plate tectonics theory. The Vine-Matthews-Morley hypothesis made
predictions about magnetic field orientations in ocean-floor rocks based
on the idea that Earth’s magnetic field flipped its orientation through
time. A flip, or “reversal,” of the magnetic field meant that at times in
the past a compass needle would have pointed to the south instead of
the north. As a result of these reversals, there should be parts of the
ocean floor that had magnetic orientations similar to modern
(“normal”) whereas other parts should have “reversed” orientations.
According to the Vine-Matthews-Morley hypothesis, since new oceanic
plate material is continually being created at mid-ocean ridges, oceanfloor rocks should record past reversals of the magnetic field. As the
figure below shows, if we think of strips of the seafloor with “normal”
magnetic orientations (like today) as a black stripe and the parts of the
seafloor that formed when the magnetic field was reversed as a white
stripe, then we should expect to find “zebra-stripe” patterns (black and
white stripes repeating) on the seafloor that are the same on both sides
of the mid-ocean ridge (symmetrical). Of course, seafloor rocks do not
actually change color from black to white during times of magnetic field
change, but it is easier to visualize the changes in this way.
In other words, Pittman found the “zebra-stripe” pattern proposed by
the Vine-Matthews-Morley hypothesis. The confirmation of the VineMatthews-Morley hypothesis also provided additional evidence
supporting Hess’s seafloor-spreading model. Together, (1)
paleomagnetism, (2) seafloor spreading, and (3) the Vine-MatthewsMorley hypothesis were the ingredients of plate tectonics theory that
proved that Wegener was right about the movement of continents.
Write a summary sentence for each
paragraph and one sentence
describing what you see in the image:
Big History Plate Tectonics/Continental Drift Name:_________________________________________
Document Questions:
1. What is the Vine-Matthews-Morley hypothesis?
2. Why does the image show a zebra pattern?
3. How does this hypothesis help support the theory of Continental Drift?