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Where Where diddid those magnificent those magnificent ocean basins come from ocean basins come from

anyway?anyway?

Continental DriftContinental Drift� 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 super-continent 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 Earthexam in 2000, “At first, people were like, ‘Huge slabs of rock moving around the Earth? No way!’ ”

� Way.

Alfred Wegener

Continental DriftContinental Driftevidence for it

(catching the drift)

� fit of continents� rocks and mountain rangesof

similar types and ages on both sides of the Atlantic Ocean

Today

250 Ma

Continental DriftContinental Drift

� fossil remainsof 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 these organisms find a way to cross thousands of kilometers of open ocean?

� Or is there a more logical explanation?

evidence for it (catching the drift)

Today

250 Ma

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Continental DriftContinental Drift

� paleoclimate reconstruction

� ancient (~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

Today (ancient evidence)

250 Ma

� Were the tropics frigid and the poles warm 250 Ma?

� Or is there a more logical explanation?

evidence for it (catching the drift)

Continental DriftContinental Drift

� Wegener 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.

� Wegener 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.

arguments against it (drifting off … )

These flaws caused a ~50-year delay before Wegener’s continental drift hypothesis was reconsidered.

Earth’s Magnetic FieldEarth’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).

� Earth’s magnetic field is thought to be generated by convective circulation in the liquid iron of the outer core.

� Freely floating magnetic materials on Earth (including certain minerals in lava) align with the magnetic field.

� Just 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

� In this way, volcanic rocks record the position of the magnetic poles.

Earth’s Magnetic FieldEarth’s Magnetic Field

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Apparent Polar WanderingApparent 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.

� Were there two magnetic poles that migrated side-by-side and then converged?

� Or is there a more logical explanation? (Continents moved!)

MidMid --ocean Ridgesocean 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.

MidMid --ocean Ridgesocean 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 anomaliesMagnetic 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 SpreadingSeafloor Spreading� Harry Hess, a geologist from

Princeton University, developed the seafloor spreading hypothesisin 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 mid-ocean 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 …)