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Continental Drift &
Seafloor Spreading
Chapter 7: Plate Tectonics
Restless Continents
• In the early 1900’s, a German meteorologist named
Alfred Wegener (VEG e nuhr) suggested there was a
supercontinent, called Pangaea (meaning "all land"), that
broke up millions of years ago, slowly moved to their
current positions, and continue to move today.
Evidence for Continental Drift
• When Wegener first
noticed the similarities in
the shoreline of continents
on both sides of the
Atlantic Ocean, he then
began fitting them
together. He particularly
liked the way in which
South America and Africa
fit.
• Wegener used documented
cases of fossil organisms
that had been found on
different landmasses that
could not have crossed the
current oceans.
More Evidence…
• Matching mountain ranges
on each side of the
Atlantic Ocean: The
Appalachian Mountains that
disappear off the coast of
Newfoundland match
mountains in the British
Isles and Scandinavia which
are comparable in age and
structure.
• Ancient climates: When the
continents are put together
to form Pangaea the
remains of glacial materials
fit together to form a
pattern like the large ice
sheets that cover our poles
today.
Seafloor Spreading
• Harry Hess, professor of geology at Princeton and Navy
captain during WWII, was able to conduct echo-sounding
surveys in the Pacific while cruising from one battle to
the next.
• He said that new ocean floor was being created at midocean ridges and destroyed at subduction zones. He
called his theory SEAFLOOR SPREADING!
• One of his biggest pieces of evidence was being able to
calculate the age of the seafloor at the ridges and
subduction zones. Hess died before he could prove it.
Magnetic Striping
• The volcanic rocks which make up the sea floor have magnetism
because, as they cool, magnetic minerals within the rock align to the
Earth's magnetic field.
• Scientists noticed that some rocks were aligned to the “positive” or
north end of the Earth and some were aligned to the “negative” or
south end. Also, the ages of the rocks at the ridge were much
younger than those found at the subduction zones.
• A hypothesis was presented in 1963 by Fred Vine and Drummond
Matthews to explain this pattern. They proposed that lava erupted
at different times along the rift at the crest of the mid-ocean
ridges preserved different magnetic properties.
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• Vine and Matthews proposed
that lava erupted on the sea
floor on both sides of the rift,
solidified, and moved away
before more lava was erupted.
• If the Earth's magnetic field
had reversed (changed from one
geographic pole to the other)
between the two eruptions, the
lava flows would preserve a set
of parallel bands with different
magnetic properties.
• This provided strong support
for sea floor spreading.
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