 The study of the past magnetic field of the Earth is known as paleomagnetism.The polarity of the Earth's magnetic field is recorded in igneous rocks, and reversals of the field are thus detectable as "stripes" centered on mid-ocean ridges where the sea floor is spreading, while the stability of the geomagnetic poles between reversals has allowed paleomagnetists to track the past motion of continents.Reversals also provide the basis for magnetostratigraphy, a way of dating rocks and sediments.