Wikibooks: Historical Geology/Geomagnetic reversals

• Format: Book
• Language: English
• Subjects: South Pole; North Pole; South pole
• Online Access: https://en.wikibooks.org/wiki/Historical_Geology/Geomagnetic_reversals

In this article we shall consider the evidence for reversals of the Earth s magnetic field. =Paleomagnetism= When [[igneous rocks]] are formed as the temperature of the rock falls beneath what is known as the Curie temperature (roughly speaking the temperature above which a material cannot be magnetized and below which it can) iron based [[minerals]] such as [[magnetite]] and [[hematite]] are magnetized by the Earth s magnetic field indicating the directions of the Earth s north and south magnetic poles at the time that the rocks are formed. [[Sedimentary rocks]] can also indicate these directions when sediment is deposited gently in a low energy environment such as the deep ocean floor magnetized grains of [[magnetite]] and [[hematite]] will orient themselves to the Earth s magnetic field like so many tiny compass needles indicating the directions of the north and south magnetic poles at the time the sediment was deposited. When geologists realized that this was the case it was immediately obvious that studying the magnetism of ancient rocks would tell them about the Earth s magnetic field as it existed in the deep past paleomagnetism . =Geomagnetic reversals= The study of paleomagnetism led to the discovery of magnetic field reversals. At present the Earth s magnetic field exhibits what is known as normal polarity that is it has the magnetic north pole near the geographic north pole and the magnetic south pole near the geographic south pole. But studies of the paleomagnetism of ancient rocks showed evidence that in the past the Earth has sometimes had reversed polarity with the magnetic north pole in the southern hemisphere and vice versa. Apparently then the Earth periodically undergoes geomagnetic reversals in which the north and south magnetic poles switch ends. =Geomagnetic reversals how do we know?= We can see today that magnetic [[minerals]] in [[sedimentary]] and [[igneous rocks]] align themselves with the present direction of the magnetic field and the physics of this is well understood — this is ...