The inclination of the magnetic field is horizontal at the equator, steeper at high latitudes, and vertical at the poles. The magnetic field is inclined downward in the Northern Hemisphere and upward in the Southern Hemisphere . 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.