paleomagnetism, study of the intensity and orientation of the earth's magnetic field as preserved in the magnetic orientation of certain minerals found in rocks formed throughout geologic time. Paleomagnetic studies of rocks and ocean sediment have demonstrated that the orientation of the earth's magnetic field has frequently alternated over geologic time. Periods of "normal" polarity (i.e., when the north-seeking end of the compass needle points toward the present north magnetic pole, as it does today) have alternated with periods of "reversed" polarity (when the north-seeking end of the compass needle points southward). The cause of these magnetic "flip-flops" is not clearly understood. Ideas of paleomagnetism began in the late 1920s, when French physicist Mercanton, suggested that because today's magnetic field is close to the earth's rotational axis, continental drift could be tested by ascertaining the magnetic characteristics of ancient rocks; however, it was not until after World War II that rock paleomagnetism data was gathered. Paleomagnetism is possible because some of the minerals that make up rocks—notably magnetite—become permanently magnetized parallel to the earth's magnetic field at the time of their formation. Rocks from hot liquid magma (see lava), or even minerals made up of crystals that grow at low temperatures, can acquire magnetization. Also, when magnetized minerals become disaggregated from their parent rocks by erosion and are carried into a basin, they will tend to align themselves parallel to the earth's magnetic field as they settle in still water. When the deposit into which they settle hardens into rock, the magnetization will be fixed. Geophysicists have been able to trace changes in the orientation of the earth's magnetic field through geologic time by carefully collecting rock specimens of different ages and determining the alignment of their magnetic fields. That technique has provided a timetable for periods of normal and reversed polarity, showing 171 reversals in the earth's magnetic field in the past 76 million years. Paleomagnetic studies of the ocean floor have been of decisive importance in establishing the modern theories of continental drift and seafloor spreading.