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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 that states that the
continents once formed a single land
mass, 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.
Glaciers: 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 and Puzzle Theory
Wegener’s Pangaea
Oceanic Crust and
Sea-Floor
Spreading
• 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.
Sea-Floor Spreading
• 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, showing the time
of formation of the Atlantic
Ocean.
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.
•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).
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
•It is even possible to
measure the speed of
continental plates extremely
accurately, using satellite
technology.
KEY: WEGENER’S EVIDENCE
Glaciers
Rocks and Fossils
Mountain Belts