Download Evidence Supporting Continental Drift

Name
Continental Drift:
Date
Period
Supporting Evidence
The Earth's crust is constantly moving, both vertically and horizontally, at rates of up to
several inches a year. A widely-held theory that explains these movements is called "plate
tectonics." It was developed in the mid 1960s by geophysicists. The term "plate" refers to
large rigid blocks of the Earth's surface which appear to move as a unit. These plates may
include both oceans and continents. When the plates move, the continents and ocean floor
above them move as well. Continental Drift occurs when the continents change position in
relation to each other.
While plate tectonics is a relatively new idea, scientists have been gathering data in
support of the Continental Drift theory for a very long time. In 1912, Alfred Wegener and
Frank Taylor first proposed the theory that 200 million years ago the Earth had only one
giant continent, from which today's continents broke apart and drifted into their current
locations. Wegener used the fit of the continents, the distribution of fossils, a similar
sequence of rocks at numerous locations, ancient climates, and the apparent wandering of
the Earth's polar regions to support his idea.
The Shapes Match
The continents look as if they were pieces of a giant jigsaw puzzle that could fit together
to make one giant super-continent. The bulge of Africa fits the shape of the coast of
North America while Brazil fits along the coast of Africa beneath the bulge.
The Plants and Animals Match
Wegener noted that plant fossils of late Paleozoic age found on several different
continents were quite similar. This suggests that they evolved together on a single large
land mass. He was intrigued by the occurrences of plant and animal fossils found on the
matching coastlines of South America and Africa, which are now widely separated by the
Atlantic Ocean. He reasoned that it was physically impossible for most of these organisms
to have traveled or have been transported across the vast ocean. To him, the presence of
identical fossil species along the coastal parts of Africa and South America was the most
compelling evidence that the two continents were once joined.
The Rocks Match
Broad belts of rocks in Africa and South America are the same type. These broad belts
then match when the ends of the continents are joined.
The Ice Matches
Wegener was aware that a continental ice sheet covered parts of South America, southern
Africa, India, and southern Australia about 300 million years ago. Glacial striations on
rocks show that glaciers moved from Africa toward the Atlantic Ocean and from the
Atlantic Ocean onto South America. Such glaciations are most likely if the Atlantic Ocean
were missing and the continents joined.
The Positions Don't Match
If the continents were cold enough so that ice covered the southern continents, why is no
evidence found for ice in the northern continents? Simple! The present northern
continents were at the equator at 300 million years ago. The discovery of fossils of
tropical plants (in the form of coal deposits) in Antarctica led to the conclusion that this
frozen land previously must have been situated closer to the equator, in a more temperate
climate where lush, swampy vegetation could grow.
Why Few People Believed
Wegener's Continental Drift theory was not readily accepted by the science community of
his day. It was difficult to conceive of large continents plowing through the sea floor to
move to new locations. What kind of forces could be strong enough to move such large
Alfred Wegener (18801930), a German
meteorologist and
geologist, was the first
person to propose the
theory of continental
drift. In his book,
"Origin of Continents
and Oceans," he
calculated that 200
million years ago the
continents were
originally joined
together, forming a
large supercontinent. He
named this
supercontinent Pangaea,
meaning "All-earth". He
was also known for his
papers on lunar craters,
which he correctly
believed were the result
of impacts rather than
vulcanism.
Scientists believe the
Himalaya mountain
range in Asia was
formed as a result of the
collision of the IndianAustralian and Eurasian
plates. In the same way,
the Andes Mountains in
South America are a
result of the convergence
of the Nazca and South
American plates. Did
you know that the coal
that is mined in
Pennsylvania was
actually formed from
tropical plant life near
the Equator? How did it
masses of solid rock over such great distances? Wegener suggested that the continents
simply plowed through the ocean floor, but Harold Jeffreys, a noted English geophysicist,
argued correctly that it was physically impossible for a large mass of solid rock to plow
through the ocean floor without breaking up.
travel northward to
Pennsylvania? Scientists
believe that 200 million
years ago, when the
dinosaurs dined upon
tropical ferns and tall
tropical vegetation, what
is now Pennsylvania was
at a different location,
namely the equatorial
region.
Continental drift was hotly debated off and on for decades following Wegener's death
before it was largely dismissed as being eccentric, preposterous, and improbable. However,
beginning in the 1950s, a wealth of new evidence emerged to revive the debate about
Wegener's ideas and their implications. In particular, four major scientific developments
spurred the formulation of the current plate-tectonics theory: (1) demonstration of the
ruggedness and youth of the ocean floor; (2) confirmation of repeated reversals of the
Earth magnetic field in the geologic past; (3) emergence of the seafloor-spreading hypothesis and associated
recycling of oceanic crust at convergent boundaries; and (4)precise documentation that the world's earthquake and
volcanic activity is concentrated along oceanic trenches and submarine mountain ranges.
Why Should Continental Drift Matter to Me?
One of the more difficult ideas for people of all ages to comprehend is the immensity of time over which the Earth
has formed and evolved. While most people have some sense that Biology has an evolving history, the physical Earth
has one, too, and they are inextricably linked together.
Scientists think they have a good understanding of how the plates have moved since Pangaea broke up, but the
motion is less clear in the time before Pangaea. Oceanic crust has an average age of only 55 million years (in
geological terms, quite young). It is totally recycled into the Earth's interior every 150 million years or so. On the
other hand, the age of continental crust averages about 2.3 BILLION years, with the oldest known rocks dating back
3.56 billion years (old by anyone's standard).
Some scientists studying these rocks suspect that the Earth has had several supercontinents throughout time.
These supercontinents all went through a cycle similar to Pangaea’s. Perhaps 200 million years in the future, people in
America won't need to cross the Pacific Ocean to reach Asia.
What can scientists predict about some of the features of the Earth existing today? Quite a lot. The
Atlantic Ocean will continue to expand, while the Pacific Ocean (originally Panthalassa Ocean) will shrink. Eventually,
the Mediterranean Sea (remnant from the Tethys Sea) will disappear, connecting Africa with Europe. India will
continue to push into the southern Asian continent, pushing the Himalayas higher. Meanwhile the city of Los Angles
will continue its journey north to join with the city of San Francisco. This will take several million years to occur.
Maybe in another 200 million years there will be no need for a transoceanic trip from America to Asia.
Continental Drift Puzzle
Objective: To understand Alfred Wegener’s reasoning behind the theory of Continental Drift by looking at some of
the data which he used. Students will complete the continent jigsaw puzzle calling attention to the fossil and
continent shape data that Wegener used to formulate his theory of Continental Drift.
Materials: Continent handout, scissors, glue stick, construction paper
Procedure:
1. Cut out the continents
2. You have the continents of the Southern hemisphere of the Earth. Below is a map of the what they look like
today. But long ago they were part of one big continent called Pangea (about 225 mya). Match up the
continents to form one continent, by fitting the continents together like a jigsaw puzzle and by using the
fossil evidence.
3. Glue the continents to the construction paper.
4. Write you name, a title, and a legend key on your construction paper (Each continent labeled with a color).