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Continental Drift
Where did those magnificent
ocean basins come from
anyway?
In 1915, Alfred Wegener, a German
meteorologist (and apparently a jigsaw
puzzle enthusiast), proposed the bold
new hypothesis of continental drift.
He surmised that the continents had
been joined together as a supercontinent called Pangaea (“all land”)
about 200-300 Ma, and later drifted
apart.
(Note: Ma = “Mega-annums,” or
millions of years ago),
This met with great resistance in the
scientific community…
As a University of Colorado student
wrote on an Our Changing Earth exam
in 2000, “At first, people were like,
‘Huge slabs of rock moving around the
Earth? No way!’ ”
Way.
Continental Drift
Alfred Wegener
Continental Drift
evidence for it
(catching the drift)
evidence for it
(catching the drift)
fossil
Today
fit
of continents
rocks and mountain ranges of
similar types and ages on both
sides of the Atlantic Ocean
remains of dinosaurs and
ancient plants
same species found in apparently
discontinuous and remote areas
widespread distribution in India
and the Southern continents but
not found anywhere else
Did
250 Ma
these organisms find a way to
cross thousands of kilometers of
open ocean?
Today
250 Ma
Or
is there a more logical
explanation?
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Continental Drift
evidence for it
(catching the drift)
reconstruction
(~250 Ma) glacial deposits
found in equatorial regions –
showed there must have been ice
sheets located there
remains of large tropical swamps
of the same age found in high
latitude areas
Continental Drift
Today (ancient evidence)
paleoclimate
arguments against it
(drifting off …)
Wegener
ancient
250 Ma
Were
the tropics frigid and the
poles warm 250 Ma?
suggested that
the continents moved by
actually plowing through
the ocean crust,
BUT …
A prominent physicist,
Harold Jeffreys, showed
that ocean crust (basalt) is
too strong to allow the
continental crust (granite)
to pass through it.
Or
is there a more logical
explanation?
Earth’s Magnetic Field
A
bar magnet has a north and south
magnetic pole, and it is surrounded by
a magnetic field which can be seen in
the pattern of the iron filings around it.
Likewise,
Earth has a magnetic field
with a north and a south magnetic pole
(close to, but not the same as, the
geographic poles).
proposed that
Pangaea split along a
system of faults and
became a set of drifting
slabs of continental crust,
BUT …
He
failed to provide a
viable explanation for the
mechanism that could
cause this to occur.
These flaws caused a ~50-year delay before
Wegener’s continental drift hypothesis was reconsidered.
Just
Earth’s Magnetic Field
like a compass needle, any
freely-floating magnetic object will
align with Earth’s magnetic field.
When lava cools to a temperature
called the Curie point, magnetic
minerals in the lava are frozen in a
direction determined by Earth’s
magnetic poles.
– At the poles, the orientation of
the minerals will be vertical
– At the equator, the orientation
will be horizontal
– Anywhere in-between, the
orientation will be at some angle
Earth’s
magnetic field is thought to be
generated by convective circulation in
the liquid iron of the outer core.
floating magnetic materials on
Earth (including certain minerals in
lava) align with the magnetic field.
Wegener
Freely
In
this way, volcanic rocks record
the position of the magnetic poles.
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Apparent Polar Wandering
The
orientation of magnetic
minerals in ancient volcanic rocks
indicates different positions for
the Earth’s north magnetic pole
over time.
The original thought was that the
pole actually wandered through
these different positions.
Rocks from other continents
showed different paths.
MidMid-ocean Ridges
Apparent polar wandering
rekindled interest in continental
drift:
Then,
in the 1950’s and 1960’s,
people began mapping the sea
floor with sonar technology.
Were
there two magnetic poles
that migrated side-by-side and
then converged?
Or
is there a more logical
explanation? (Continents moved!)
MidMid-ocean Ridges
The centers of mid-ocean
ridges were found to be rift
valley zones, indicating that
tremendous tensional
(pulling-apart) forces are at
work there.
These are also areas of
intense heat and volcanic
activity.
Scientists surmised that
magma must be rising from
the mantle through the rift
valleys and creating new ocean
crust in these areas, with the
top surfaces consisting of
pillow basalts.
Magnetic anomalies
Oceanographers used
magnetometers to measure the
intensity of Earth’s magnetic
field while at sea.
They discovered symmetrical
“stripes” of seafloor basalt that
gave anomalously low magnetic
strength readings, perpendicular
to mid-ocean ridges.
Eventually these were
determined to be areas where
orientation of the iron-rich
minerals in the basalt were
oriented opposite to today’s
polarity, showing the north and
south poles had switched!
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Seafloor Spreading
Harry Hess, a geologist from
Princeton University, developed
the seafloor spreading hypothesis in
the early 1960’s.
He put together the mid-ocean
ridge rift valley observations with
the concept of magnetic anomalies.
The tensional forces at the midocean ridges are thought to be
caused by convection cells in the
mantle.
As new ocean crust is produced at
the mid-ocean ridge rift valleys, it
is consumed at deep-sea trenches.
(But that’s for next time …)
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