ERS120H5 Lecture Notes - Lecture 2: Seafloor Spreading, Mid-Ocean Ridge, Continental Drift
Document Summary
Alfred wegener proposed that continents had once been joined together to form a single huge supercontinent (pangaea) and had subsequently drifted apart. Rocks retain a record of the earth"s magnetic field that existed at the time the rocks formed. By measuring paleomagnetism in successively older rocks, geologists discovered apparent polar- wander paths. Apparent polar-wander paths are different for different con- tinents, because continents move with respect to each other, while the earth"s magnetic poles remain roughly fixed. Around 1960, harry hess proposed the hypothesis of sea- floor spreading. According to this hypothesis, new sea floor forms at mid-ocean ridges, then spreads symmetrically away from the ridge axis. Eventually, the ocean floor sinks back into the mantle at deep-ocean trenches. Geologists documented that the earth"s magnetic field re- verses polarity every now and then. The record of reversals is called the magnetic-reversal chronology.
