Plate Tectonics Review Tools A B Pangaea supercontinent Wegener continental drift theory author Glossopteris fern fossil Hess sea floor spreading author magnetic stripe polarity reversal evidence mid-ocean ridge spreading center of rifts oldest rocks rock age near the continent youngest rocks rock age near the mid-ocean ridge plate tectonics continents move with lithosphere continental drift only continents move asthenosphere plastic-like layer below the lithosphere mantle earth layer below the crust continental crust least dense of the two crust types oceanic crust most dense of the two crust types subduction what happens when one plate dives under the other one convergent colliding plate boundary divergent separating type plate boundary transform fault laterally moving plate boundary convection cells circular mantle areas thought to move the plates hot spots areas of mid plate volcanoes primary wave first earthquake wave to arrive secondary wave earthquake wave one half of p wave's speed Lwave last wave to arrive seismograph measures earthquake waves focus actual place where earthquake occurs epicenter located on surface above focus Tsunami large seismic water wave trench deep troughs caused by subducting plates Mt. St. Helens Cascade volcano on a convergent boundary island arc volcanic string of volcanic islands caused by convergent boundary iron and nickel composition of Earth's core magnetometer device used to measure magnetism iron element in igneous rocks which indicates polarity San Andreas Fault transform fault created by the collision of the Pacific and N. American plates Three number of seismograph stations to determine an epicenter rift valley formation running through the center of the mid-ocean ridge Hawaiian Is. & Yellowstone NP sit on hot spots earthquake seismic wave caused by plate boundaries