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VOCABULARY CH. 10
Continental Drift
Mid-Ocean Ridge
Sea Floor Spreading
Paleomagnetism
Plate Tectonics
Divergent Boundary
Convergent Boundary
Transform Boundary
Rifting
Rift
Terrane
Pangaea
Panthalassa
Asthenosphere
Lithosphere
Subduction Zone
Craton
Shields
Trench
OBJECTIVES:
Summarize Wegeners hypothesis of continental drift
Describe the process of sea-floor spreading.
Identify how paleomagnetism provides support for the idea of sea-floor
spreading.
Explain how sea-floor spreading provides a mechanism for continental
drift.
In 1912, a German meteorologist
by the name of ALFRED WEGENER,
proposed the idea of continenetal
drift.
Wegener hypothesized that the
continents once formed part of a
single landmass called a
supercontinent.
Wegener called this supercontinent
Pangaea.
Fossil evidence was found to help support Wegener’s theory.
The plant glossopteris, has been
found on the following continents.
1.
2.
3.
4.
5.
6.
South America
Africa
Madagascar
India
Antarctica
Australia
Evidence from Rock Formations
The rocks on the east coast of S. America and the west
coast of Africa match in age and in the type of rocks layers
Mountain chains that end on one continent continue on other
continents across oceans.
For example the Appalachian Mountains continue into
Greenland and match mountains in Scotland and Europe in
composition and structure.
Climatic Evidence is also found.
Glacial evidence is found in South America and Africa. Regions
that today are far to warm for glaciers.
Fossil evidence shows that tropical and subtropical swamps and
forests existed in areas like Michigan that are too cold for such
features today.
Wegener theorized that if the continents were connected and in
different positions, the climatic changes could be explained.
Wegener believed that
Pangaea began its breakup during the Mesozoic
era.
Wegener also proposed
that the collision of
continents and the
crumbling of the crust
was responsible for the
building of mountain
ranges.
Despite the evidence that Wegener collected to support his
theory, other scientist rejected his ideas because Wegener
could not come up with a feasible means for how the
continents moved.
Wegener suggested that the continents plowed through the
rocks on the oceans floor. This was easily disproved by
geologic evidence.
Wegener died in 1930 before he could discover the
mechanism that drove the continents.
MID OCEAN RIDGES
The evidence Wegener needed was discovered almost 20
years after his death.
Geologist found that the sediments nearest the ridge were
thinner than those farther away.
The rocks also got older as you went farther from the ridge.
The ocean floor is also much younger than the continental
rock. There are no rocks on the ocean floor older than 175
million years old, while rocks on land are as old as 3.8
billion
In the late 1950’s a geologist named Henry Hess proposed a
new hypothesis. He proposed that the valley at the center of
the ridge was a crack, or rift, in the earth’s crust.
Hess suggested that magma from deep inside the earth
would rise through these cracks as the ocean floor moved
away. The magma would cool and form new ocean floor.
This process became known as seafloor spreading. Hess
suggested that if the ocean floor is moving, the continents
might be moving too. Hess thought this might be the
mechanism that Wegener failed to find.
When rock on either side of the ridge hardens, the magnetic
orientation of the minerals becomes permanent. This residual
magnetism is called paleomagnetism.
This helped to
support the idea
that the ocean
floor moved.
The similar age
of the rocks on
each side of the
rift gave support
to Wegeners
theory.