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Chapter 7 Section 2
Pages 198-201
Continental Drift
Continental Drift
Scientist Alfred Wegener had a theory
called continental drift.
 His hypothesis states that the continents
once formed a single land mass, then over
a LONG PERIOD OF TIME broke apart, and
eventually drifted to their present location.
 This land mass was called Pangaea.
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Wegener’s Theory
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Wegener’s hypothesis seemed to explain several things:
Puzzle Theory: How well the continents fit together, like a
puzzle.
Fossil Pattern: Fossils of the same plant and animal species
are found on continents that are on different side of the
Atlantic.
Rocks: Rock formations and rock dating showed that these
rocks and formations were the same age, thus leading
scientists to believe that they were once connected into
similar rock formations, such as mountain chains.
Wegener’s Theory: Evidence
Fossil Evidence
Fossils
of the same plant and animal species are found on continents that are on
different side of the Atlantic.
• In Wegener's mind, the drifting of continents after the break-up of Pangaea explained not only the matching
fossil occurrences but also the evidence of dramatic climate changes on some continents. For example, 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. Other mismatches of geology and climate included distinctive fossil ferns
(Glossopteris) discovered in now-polar regions, and the occurrence of glacial deposits in present-day arid
Africa, such as the Vaal River valley of South Africa.
Glaciation
•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 glaciation is most likely if the Atlantic Ocean were missing and the
Continents were joined.
Fossil Evidence
Wegener’s Pangaea
Evidence: Map Key for Class work
Geologic Province
Glaciation/Glaciers
Mountain Belts
Rock and Fossil
Oceanic Crust
Map of Oceanic Crust
Ages in the Atlantic
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Since Wegener's day, scientists have mapped
and explored the great system of oceanic
ridges, the sites of frequent
earthquakes, where molten rock rises
from below the crust and hardens into
new crust.
We now know that the farther away you
travel from a ridge, the older the crust
is, and the older the sediments on top of the
crust are.
The clear implication is that the ridges are
the sites where plates are moving apart
(click on the picture at the left to see a map
of the age of the ocean crust).
This map shows the ages of the crustal rocks that make up
the floor of the Atlantic Ocean. Red represents the youngest
rocks; the deepest red marks the Mid-Oceanic Ridge, where
continental plates are pulling apart and new crust is being
formed. Older rocks are yellow, green, and blue: the
deepest blue rocks, along the coastlines of Europe, Africa,
and the Americas, are Jurassic in age, showing the time of
formation of the Atlantic Ocean.
•Where plates collide, great mountain ranges may be pushed up, such as the
Himalayas; or if one plate sinks below another, deep oceanic trenches and chains of
volcanoes are formed. Earthquakes are by far most common along plate
boundaries and rift zones: plotting the location of earthquakes allows seismologists
to map plate boundaries and depths (click on the picture at the right to view a map
of quake epicenters). Paleomagnetic data have allowed us to map past plate
movements much more precisely than before. It is even possible to measure the
speed of continental plates extremely accurately, using satellite technology.
Nevertheless, Wegener's basic insights remain sound, and the lines of evidence
that he used to support his theory are still actively being researched and expanded.
Mid-Ocean Ridge &
Sea-Floor Spreading
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A chain of submerged mountain ranges that
runs through the center of the Atlantic Ocean.
Sea-floor spreading
takes place along
the mid-ocean ridge
Sea- Floor Spreading
Sea-floor spreading is the
process by which new
oceanic lithosphere forms as
magma rises toward the
surface and solidifies or
hardens.
As tectonic plates move
away from one another, the
sea floor spreads apart and
magma fills in the gaps,
creating new ocean floor.