Magnetic field variations

Magnetic field variations

The distortion was recognized by Icelandic mariners as early as the late 18th century. More important, because the presence of magnetite gives the basalt measurable magnetic properties, these magnetic variations have provided another means to study the deep ocean floor. When newly formed rock cools, such magnetic materials record the Earth's magnetic field.

The currents in the core of the Earth that create its magnetic field started up at least million years ago

detect minute deviations in the Earth's magnetic field caused by iron  kilns, some types of stone structures, and even ditches and  in  Using magnetic instruments adapted from airbornedetectors developed during World War II to detect submarines, the magnetic variations across the ocean floor have been mapped. The the iron-rich, volcanic rock making up the ocean floor — contains a strongly magnetic mineral  and can locally distort compass readings.