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

• 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.• Amassed detailed evidence in

favor of drift• Published “Our Wandering

Continents: A Hypothesis of Continental Drift” - 1937.

Alfred Wegener (1880-1930)

Alexander du Toit (1878-1948)

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.

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December

first tetrapodscoal swamps

formation of Pangea

Age of Dinosaurs

first birds

continents drift into their modernpositions

evolutionof modernmammals

formation of the Alps and Himalayas

first reptiles

largest mass extinction

If the history of the Earth were a single year…

Homo

Permian

Permian fossils are very similar in widely separated continents

Permian rock strata are very similar on widely separated continents

IndiaAustraliaSouth AmericaSouth Africa(Antarctica)

Glacial grooves and glacial deposits from the Permian do not make sense in the present geography.

Glacial grooves and striationsCentral 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.

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Remnant magnetism records “apparent polar wander” - the changing declination of a continent through time.

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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

N.Am

Europe

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 takeon the inclination of the Earth’s

magnetic field lines in the rock asit cools.

0° 25° 45° 75° 90°

Measuring the inclination of the remnant magnetic field in a rock sample gives the approximate paleolatitude that the rock

cooled at.

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.

N

S

Supercomputer simulation of the Earth’s magnetic field

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S

Magnetic field becomes weakly polarized and unstable

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SMagnetic polarity reversal

Reversals of the Earth’s magnetic field are recorded in rocks as intervals of Normal(N) and Reversed (R) polarity.

N

R

Cret

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rtiar

y

Paleocene

Eocene

Oligocene

Miocene

PliocenePleistocene

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!

Magnetic Time Scale