Magnetic Anomolies on the Sea Floor
A theoretical model of the formation of magnetic striping. New oceanic crust forming continuously at the crest of the mid-ocean ridge cools and becomes increasingly older as it moves away from the ridge crest with seafloor spreading (see text): a. the spreading ridge about 5 million years ago; b. about 2 to 3 million years ago; and c. present-day.
Early in the 20th century, paleomagnetists (those who study the Earth's ancient magnetic field) -- such as Bernard Brunhes in France (in 1906) and Motonari Matuyama in Japan (in the 1920s) -- recognized that rocks generally belong to two groups according to their magnetic properties. One group has so-called normal polarity, characterized by the magnetic minerals in the rock having the same polarity as that of the Earth's present magnetic field. This would result in the north end of the rock's "compass needle" pointing toward magnetic north. The other group, however, has reversed polarity, indicated by a polarity alignment opposite to that of the Earth's present magnetic field. In this case, the north end of the rock's compass needle would point south. How could this be? This answer lies in the magnetite in volcanic rock. Grains of magnetite -- behaving like little magnets -- can align themselves with the orientation of the Earth's magnetic field. When magma (molten rock containing minerals and gases) cools to form solid volcanic rock, the alignment of the magnetite grains is "locked in," recording the Earth's magnetic orientation or polarity (normal or reversed) at the time of cooling.
(From USGS's Dynamic Planet)
An observed magnetic profile (blue) for the ocean floor across the East Pacific Rise is matched quite well by a calculated profile (red) based on the Earth's magnetic reversals for the past 4 million years and an assumed constant rate of movement of ocean floor away from a hypothetical spreading center (bottom). The remarkable similarity of these two profiles provided one of the clinching arguments in support of the seafloor spreading hypothesis
(From USGS's Dynamic Planet)
Updated 1.20.2002 (Kluge)