The inclination of the Earth's magnetic field varies with latitude. The field is horizontal at the equator, steeper at high latitudes, and vertical at the poles. Iron-rich minerals are aligned parallel to Earth's magnetic field when they form during the cooling of magma. Ancient lava flows record the latitude of cooling magma. The magnetic field is said to have normal polarity when the magnetic north pole and the geographic north pole coincide.
Compass needles point toward the magnetic north pole during periods of normal polarity.
Compass needles would point toward the south pole during intervals of reverse polarity. Measurement of magnetism in rocks of the oceanic crust revealed stripes of high and low magnetic intensity corresponding with areas of normal and reverse polarity respectively. These stripes are oriented parallel to adjacent oceanic ridges. These patterns were interpreted to suggest that new crust was divided in half as it formed along the oceanic ridges and each half moved in opposite directions away from the ridge. Relationship between the orientation of the earth's magnetic field and the polarity of igneous rocks making up the ocean floor.