SEA FLOOR SPREADING
The Mid Atlantic ridge neatly divides the Atlantic Ocean into two equal halves, and the geography of the ridge is almost exactly parallel to the coastlines of Africa and South America and well as Europe and North America. Additionally, the age distribution of the ocean floor is symmetrical about the axis of the ridge. The youngest rocks on the floor of the Atlantic are at the mid ocean ridge. In fact, the mid-ocean ridges are the site of active volcanic eruptions--this will be seen when we overlay the positions of active volcanoes on our map--and thus the youngest oceanic rocks can be considered to have an age of "0". The oceanic rocks become older on either side of the ridge, increasing in age to a maximum of approximately 175 million years in the North Atlantic near the coast of Europe and also the coast of North America. If we accept the Sea Floor spreading hypothesis of Harry Hess, proposed in 1962, we can "undo" the spreading of the Mid Atlantic ridge and reconstruct Africa and South America to the position that Wegener mapped them in his reconstruction of Pangea. We can do the same in the northern hemisphere, placing Europe adjacent to North America, and recreating Pangea as Wegener envisioned.  

MAGNETIC ANOMALIES AND THE AGE OF THE EARTH

On land, many rock formations can be sampled and dated radiometrically or with fossils, but sampling rocks from the bottom of the ocean would be much more difficult. Fortunately, it is possible to figure out the age of the ocean floor simply by towing a magnetometer behind a ship and recording the magnetic field. How does this work?

For hundreds of years, since people began using compasses, we have relied on them to point North. However, a million years ago, compasses would have pointed South; before that, North, and so on, because the earth's magnetic field flips its direction from time to time. It does not flip at regular intervals. For example, the field was "normal" (the same direction as now) for 200,000 years, "reversed" (the opposite direction from now) for 300,000 years, normal for 50,000 years, reversed for 190,000 years, and has now been normal for 730,000 years. These reversals are recorded in rocks that contain iron (particularly basaltic volcanic rocks, because when the volcanic flows cool, the iron contained within them is aligned parallel to the prevailing magnetic field at that time). This means that basalts which erupted at the midocean ridges preserve the record of magnetic field reversals; rocks that cooled under "normal" magnetic fields are normally polarized and rocks that cooled under "reversed" conditions are reversely polarized. As the hot basalt emerges from the ridge, it is pushed away from the ridge in both directions by more emerging basalt. This pushing out to both sides causes magnetic "stripes", or anomalies, that are symmetrical about the ridges. Because the reversal pattern is irregular, but the same all over the world, it can be used like tree rings to date rocks by examining the pattern of magnetization that they preserve. This "magnetostratigraphy" has been verified by direct sampling of sea floor rocks and age determinations.

Discovery Topics > Plate Tectonics > Sea Floor Spreading <