Historical Geology, Plate
Tectonics, and Paleogeography
Continental Drift
• The hypothesis that the
continents have somehow
moved across the globe
through time.
• Implies major changes in
the geography of the Earth
through time.
• Originally inspired by the
“jigsaw puzzle” fit of the
continental coastlines on
either side of the Atlantic
Ocean.
1858, geographer Antonio Snider-Pellegrini
Alfred Wegener (1880-1930)
• Austrian meteorologist and
geophysicist.
• Early proponent of continental
drift.
• 1915 - Wegener publishes The
Origin of Continents and Oceans.
• South African geologist.
• Wegener’s enthusiastic supporter. Alexander du Toit (1878-1948)
• Amassed detailed evidence in
favor of drift
• Published “Our Wandering
Continents: A Hypothesis of
Continental Drift” - 1937.
Wegener’s Map
showing the
breakup of
Pangea during the
Mesozoic Era
Continental Drift according to Wegner and du Toit
Much of Wegener’s and du Toit’s evidence for the existence of
Pangea came from the rocks and fossils of the Permian (286-245
million years ago), before the breakup of Pangea.
If the history of the Earth were a single year…
December
1
formation of continents drift into their modern
Pangea
positions
5
10
coal swamps
first tetrapods
15
20
25
31
Homo
Age of Dinosaurs
largest mass
extinction
first reptiles
Permian
first birds
evolution
of modern
mammals
formation of
the Alps and
Himalayas
Permian fossils are very similar in widely separated continents
Permian rock strata are very similar on widely separated continents
India
Australia
South America
South Africa
(Antarctica)
Glacial grooves and glacial deposits from the Permian
do not make sense in the present geography.
Glacial grooves and striations
Central Park, New York
Glacial striations and
grooves cut into Permian
sands, South Africa
Glacial grooves and glacial deposits from the Permian come
together in a Pangean geography.
The study of the geography of the past and the
movement of continents due to plate tectonics is
called Paleogeography
http://www.earthbyte.org/people/dietmar/
Movies/Pangea_400.mov
How do we know the position of continents
going back thorough the past?
• Paleogeographers use much of the same data
that Wegener and DuToit did - matching up
similar rock formations, analyzing distributions
of fossil species, and looking for evidence of
climate preserved in rocks.
• More precise data on the geographic position
of continental plates can be obtained by
studying Remnant Magnetism in Rocks.
Remnant Magnetism
• 1940’s - geologists discover that the Earth’s
magnetic field is recorded by iron-bearing
minerals during the formation of rocks.
• Remnant magnetism is trapped in the rock.
• Magnetic minerals act like tiny compasses,
pointing to the former position of the
magnetic pole (declination).
• Magnetic minerals record the inclination of
the Earth’s magnetic field, which indicates
paleolatitude.
• Magnetic minerals also record the polarity of
the Earth’s magnetic field in the past.
Remnant
magnetism records
“apparent polar
wander” - the
changing
declination of a
continent through
time.
N
Remnant
magnetism records
“apparent polar
wander” - the
changing
declination of a
continent through
time.
N
Remnant
magnetism records
“apparent polar
wander” - the
changing
declination of a
continent through
time.
N
Remnant
magnetism records
“apparent polar
wander” - the
changing
declination of a
continent through
time.
N
When “polar wandering” was first discovered, there were
two competing explanations for it.
Polar wander assuming no continental drift
Europe
N.Am
Different continents show different locations for the
magnetic pole through time - impossible!
Polar wander with continental drift
Polar wander paths converge - proof that the
continents have moved over time!
Remnant magnetic particles take
on the inclination of the Earth’s
magnetic field lines in the rock as
it cools.
Measuring the inclination of the remnant
magnetic field in a rock sample gives the
approximate paleolatitude that the rock
cooled at.
0°
25°
45°
75°
90°
Magnetic field reversals
• 1940’s - geologists discover that the
Earth’s magnetic field randomly “flips” so
that the N and S magnetic poles become
reversed.
• Reversals occur randomly and last for
100’s of thousands to millions of years.
• Pattern of reversals recorded in the rock
through time is unique - like a bar code.
Supercomputer
simulation of the
Earth’s magnetic field
N
S
N
S
Magnetic field
becomes weakly
polarized and unstable
Magnetic polarity reversal
S
N
N
R
Reversals of the Earth’s magnetic field are
recorded in rocks as intervals of Normal(N)
and Reversed (R) polarity.
Pliocene
Magnetic Time Scale
Eocene
Tertiary
Pleistocene
Miocene
Oligocene
Cretaceous
Paleocene
Pattern through
time of polarity
reversals is
random and
forms a unique
‘bar code’ that
can be used to
identify different
intervals in
Earth’s past - a
third method for
dating rocks!