Evidence for Seafloor Spreading

Another technological advance that aided the development of plate tectonics was the magnetometer, a device that measures small changes in magnetic fields. Scientists studying the seafloor with magnetometers began recognizing odd magnetic variations across the ocean floor. This finding, though unexpected, was not entirely surprising because it was known that basalt -- the iron-rich, volcanic rock making up the ocean floor-- contains a strongly magnetic mineral (magnetite) and can locally distort compass readings. More important, because the presence of magnetite gives the basalt measurable magnetic properties, these newly discovered magnetic variations provided another means to study the deep ocean floor.

Early in the 20th century, paleomagnetists (those who study the Earth's ancient magnetic field) 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-day 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.

This is important to the theory of plate tectonics because it provided scientists with evidence of plate movement. When scientists studied the ocean floor with magnetometers, they discovered that the regions with normal and reverse polarity formed a series of stripes across the seafloor, parallel to the mid-ocean ridge. They also discovered that the ages and width of those stripes matched from one side of the ridge to the other. This symmetry showed scientists that new ocean crust was being created at mid-ocean ridges as two plates were forced apart at divergent plate boundaries.


Source: http://pubs.usgs.gov/gip/dynamic/developing.html (public domain)