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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? 1 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. 2 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! 3 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 …) 4