1. Seafloor Spreading (still continuing ch. 4.1)

2.  Underwater mountain range called the Mid-Atlantic Ridge discovered in 1947.  Part of an 80,000-km-long mid- ocean ridge system around the whole earth.  Rocks from the ocean floor were young compared to most continental rocks.

5.  Princeton geologist Harry Hess hypothesized that the mid-ocean ridges were breaks in the earth’s crust, where magma was rising up and spreading away from the ridge in both directions, forming new rock.  If the ocean floor was moving, then maybe the continents were also moving !?!

7. Paleomagnetism of the Ocean Floor  Earth acts as a giant magnet.  Compass needles align with the magnetic lines of force that extend from pole to pole.  When magma cools, iron-rich minerals line up with the magnetic field.

8.  When rock hardens, magnetic orientation of the minerals become permanent and point to north.  Geologists have discovered some rocks with magnetic orientations pointed south…  All north-pointing rocks (normal polarity) were from similar time periods.

9.  All south-pointing rocks (reverse polarity) were also from similar time periods.  This led to the discovery that Earth’s magnetic field has reversed itself many times throughout history.  Magnetic patterns on the ocean floor show alternating bands of normal and reverse polarity.

10.  Identical magnetic striped patterns on either side of the mid-ocean ridges.