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Warm-UpWrite a paragraph using these sentence stems:
• The title of this graphic is...
• Its purpose is to ...
• One critical observation that I made was...
• This is important because ...
• I reached a number of conclusions through my analysis of this
graphic.
• First, ....
• Second, ....
• Finally, ...
Continental Drift
Continental Drift• The first step in the scientific
method begins with making
observations
• 102 years ago, in 1912, Alfred
Wegener made observations
about the similarities of
different continents
• Based on his observations,
and using the scientific
method, he formulated a
hypothesis
Continental Drift• Alfred’s hypothesis
– Continents once formed a
single landmass called
pangea
– The supercontinent,
Pangea, began breaking up
into smaller continents 250
ma ago
– The smaller continents
drifted into their present
locations
Continental Drift• Observations and
evidence to support
his hypothesis
① The shoreline of the
east coast of South
America and the
shoreline of the west
coast of Africa looked
like they would fit
together like a jigsaw
puzzle
Continental Drift
③ Evidence from rock
formations
– The ages and types of rocks
matched on both continents
– Mountain chains that ended on
one continent continued on the
other
② Fossil evidence
– The same fossils were
found on both
continents
• Mesosaurus
Continental Drift
④ Climatic Evidence
– Debris from glaciers matched across
continents
– Tropical plant fossils were found in areas that
are now much colder
• If continents were joined and in different locations
it would explain the climatic differences
Continental Drift
• ….that dude Wegener is Crazy!
• Tons of evidence supported his theory
but….how did the continents move?
– Wegener said they plowed through the ocean
rock
– check it out
– Wegener’s mechanism for
movement was disproved
Continental Drift• What happened to crazy Alfred?
– Wegener died before he could ever prove his
hypothesis…bummer….
TO BE CONTINUED
Continental Drift• Harry Hess - 1950
– Scientist studying a mid-ocean ridge called the Mid-
Atlantic ridge
• What’s a mid-ocean ridge?
– Undersea mountain range with a narrow valley at
the center of the ridge called a rift (crack)
Continental Drift• Hess proposed a new hypothesis to
support Alfred’s theory of continental drift
– Hess said that continents move because of
sea-floor spreading that occurs at mid-ocean
ridges
Continental Drift
• Sea-floor spreading
– Molten magma from Earth’s interior rises and
weakens the lithosphere creating a rift
– Magma begins to ooze out of the rift
– The intrusion of magma pushes the plates
apart
– Magma solidifies to form new oceanic crust
– Mid-Atlantic Ridge
Continental Drift
• Hess came up with a great
hypothesis….‘hey if the ocean floor is
moving then maybe Alfred wasn’t crazy;
maybe the continents are moving too!’
• Scientists thought…’Great now we have
two crazy scientists!’
• Hypothesis for sea-floor spreading wasn’t
accepted because of lack of evidence
Continental Drift• Mid-1960’s evidence for Hess’s
hypothesis was found….paleomagnetism
– magnetic reversals and sea-floor spreading
Continental Drift
• Paleomagnetism
– Earth’s outer core is a giant magnet
– As magma solidifies into rock the magnetic
minerals (iron, nickle) align with the magnetic
poles
– Magnetic reversals
• Magnetic poles change over time as the polarity in
the outer core flips
• Magnetic reversals are recorded in the rock record
• check it out
• magnetic reversals
Continental Drift• Hess’s hypothesis is supported
by the following evidence:
1. Symmetry of magnetic patterns
a. Polarity of rocks on either side of the
ridge matched
2. Symmetry of ages of rocks
• Sediment thin by the ridge, thick by
the continent
• Dead critters (fossils) in sediment
used to date the rock
• Radiometric dating (read into your
future learning)
3. Patterns were seen on land
Continental Drift
• Wegener and Hess weren’t such crazy
dudes after all!!!
– Mechanism for continental drift = sea floor
spreading
– Mechanism for sea-floor spreading =
convection in the asthenosphere
