subject: Minerals That Reverse Themselves [print this page] Over the last 170 million years, for still unexplained reasons, the Earths magnetic field has reversed polarity some 300 times; the last reversal occurred about 780,000 years ago. During a reversal, the Earth temporarily lets down its magnetic shield, which protects it from cosmic radiation from outer space. Proof that the magnetic poles periodically change places is found on the ocean floor near spreading centers, where new oceanic crust is being generated. As layers of basalt cool, they become slightly magnetized and acquire the polarity of the magnetic poles at the time of their deposition. Furthermore, one set of alternating magnetic bands of basalt is the mirror image of the opposite set on the other side of the spreading ridge. This property became the conclusive proof for seafloor spreading because in order for the magnetic strips to form in such a manner the seafloor had to be spreading apart from a common center. In addition, the magnetic stripes provide a means of dating virtually the entire ocean floor because the magnetic reversals occur randomly, and any set of patterns is unique in Earth history.
Magnetite, which faithfully records the Earths magnetic field, was thought to be the dominant magnetic mineral in rocks. But in the early 1950s, a rare mineral called titanohematite, composed of iron, titanium, and oxygen, was found to have the odd ability to become magnetized in the opposite direction from the Earths magnetic field. This unusual behaviour could have played havoc with scientists trying to prove the theory of magnetic pole reversal. Their existence could also complicate the analysis of the magnetic orientations in rocks used to date lava flows. Although once thought to be exceedingly rare, self-reversing minerals more recently have been found in sedimentary basins and lava fields of western North America, where, in some places, such as the Bighorn Basin of Wyoming and the San Juan Basin in Mexico, they are the dominant magnetic minerals. Relatively abundant titanohematites were also found in 10,000-year-old lava flows of Californias Mount Shasta volcano.
The mineral is associated with explosive eruptions common in the Cascade Range, as witnessed by the huge lateral blast of Mount Saint Helens in 1980. Because self-reversing titanohematites form under special conditions, their presence in volcanic rock might help volcanologists understand the nature of magma that rises up through volcanoes. Normally, when rocks are imprinted with a magnetic field, the magnetic fields of their atoms line up with the Earths magnetic field as they cool past their Curie point, the temperature at which a magnetic field becomes permanent. Below that point, the field freezes until such a time as the rock is reheated, destroying the magnetic field. Thus, the magnetic fields can be thought of as tiny fossil compasses, pointing in whatever direction the Earths magnetic field happened to be during deposition. Self-reversing minerals, however, have two Curie points, one occurring at a higher temperature than the other. As the mineral cools, it develops two magnetic regions. The first has its magnetic field aligned with the Earths, and the second has a much stronger field aligned in the opposite direction. Thus, the final polarity of the rocks magnetic field is reversed.
