Plate Tectonics Seafloor Spreading.

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Presentation transcript:

Plate Tectonics Seafloor Spreading

Technology Advances (WWII & Post WWII) Echo sounding Sonar → for ocean depth measurements Magnomometer Detects small changes in magnetic fields Ships towing magnomometer record magnetic fiels strengths of ocean floor rocks

How did this technology revive Continental Drift? Sonar -allowed ocean floor maps to be made Mountain chains found Deep sea trenches found

How did this technology revive Continental Drift? Ocean rock & sediment samples Rock ages vary along ocean floor Youngest rock closest to ridges Oldest rock farthest from ridges Ocean rock much younger (on average)than continent rock

How did this technology revive Continental Drift? Sediments Thinnest near ridges Thickest farthest from ridges Ocean sediments, in general, much thinner than expected Since continental crust erodes & much is carried to the oceans, the ocean sediments should be extremely thick

How did this technology revive Continental Drift? Magnetism Ocean floor rock Basalt Mafic High in iron Iron minerals align parallel to Earth’s magnetic field Like a compass needle Minerals freeze that way Become a record of the magnetic field Normal polarity minerals aligned same as today, causing strong reading Reverse polarity minerals aligned opposite today, causing weak reading (+, - cancel each other)

How did this technology revive Continental Drift? Study of ocean floor magnetic polarity shows a stripe pattern on the ocean floor + - + - + - + - The patterns (*age & polarities) are symmetrical on the two sides of the Mid-Atlantic Ridge *Age of reversals were compared with age of magnetic record on land

New Theory: Seafloor Spreading Harry Hess – American scientist States that new ocean crust is formed at ocean ridges & destroyed at deep sea trenches.

How is Crust Formed? Magma (less dense than solid rock) moves up to fill gap left as seafloor spreads apart. Magma cools & hardens, adding to ocean floor between continents

Missing Link Hess’s Seafloor Spreading explains how the continents move → they are pushed apart as new seafloor is added. Hess’s Seafloor Spreading + Wegener’s Continental Drift = current Theory: Plate Tectonics