Paleomagnetism

Paleomagnetism is both the fixed orientation of a rock's crystals based on the orientation and direction of the Earth's magnetic field at the time of the rock's formation and the study of such phenomena used as indicators of Earth's paleomagnetic history.

The Earth's magnetic field orientation as a dipole is not fixed and is continuously changing by small [Page 1804]increments. This leads to an effect known as polar wander. This continuing process is still active today and forces charts of the earth's magnetic field, known as isomagnetic maps, to be redrawn every 5 years.

The phenomenon of polar wander has been used extensively in geology as an indicator of global position and the geologic age of iron-bearing rocks known as ferromagnetic rocks. Studying paleomagnetics has, in part, established the position and intensity of the Earth's magnetic pole over geologic time. Geologists use this new sagaciousness to determine aspects of global tectonic history. Using paleomagnetic techniques it now becomes possible to determine the past relative positions of two separate continents, provided they were previously conjoined to the same plate. A paleogeographical reconstruction of the earth can then coalesce from global paleomagnetic data endowing geologists with valuable insight into Earth's past.

The ability of geologists to determine past magnetic fields is locked in the chemistry of ferromagnetic rocks. These substances have more electrons spinning in one direction than the other; thus the individual magnetic fields of the atoms in a given region tend to align in the same direction. When an igneous rock is crystallized from lava or magma in the presence of a magnetic field, the magnetic elements leave a magnetic signature frozen in the rock. During the cooling of molten rock, iron-bearing minerals become magnetized in alignment with earth's magnetic field as they descend through a critical temperature known as the Curie Temperature, which occurs at approximately500°C–600°C. The rock's magnetic properties are retained (thermal remanent magnetism) for geologically long periods unless the rock is again heated to near the Curie temperature. Rock samples have their thermal remanent magnetism determined by magnetometers in a laboratory. Around the world, thousands of formations and outcrops have been paleomagnetically analyzed and documented making accurate paleocontentinal maps of Earth a reality and furthermore giving geologists an almost clairvoyant gaze into the past.

Dramatic alterations to the polarity of the Earth's magnetic field were first noticed on the seafloor of the Atlantic Ocean. Known as the mid-Atlantic spreading ridge, this narrow rift continuously deposits new basalt separating the east coast of North America from the west coast of Europe and Northern Africa. The seafloor in the Atlantic is striped with bands of rocks magnetically trending northward and alternating parallel bands trending southward. Presuming the seafloor at the Mid-Atlantic rift spread similarly in the past as it does today, then as time progressed, the Earth's magnetic field regularly underwent a full reversal in dominant direction. Thus, magnetically, the seafloor in the Atlantic appears as linear anomalous bands. These reversals were recorded in the basalt going back approximately 100 million years. In that time, the rates of reversal have varied considerably from one reversal to the next. These magnetic reversals are recorded not only in basalt, but also in other igneous rocks and sediments. If the reversals themselves represent simultaneous cosmopolitan phenomenon, they then act as unique stratigraphic markers wherever and whenever they occur. These polarity events, therefore, provide a precise tool for chronostratigraphic correlation of marine and terrestrial sediments. Based on seafloor spreading and sediment depositional rates, the reversal process is thought to occur over a period of 1,000 to 10,000 years which includes a 60% to 80% decrease in intensity of the field some 10,000 years preceding the reversal, whereas the actual reversal itself only takes 1,000 to 2,000 years. The field then builds up in intensity returning to normal intensity. The last polar reversal occurred approximately 700,000 years ago. Polar reversals have various durations and are divided into two main groups: chrons (epochs) occur when the polarity is the same for a long period of time, and subchrons (events) occur when the polarity only maintains a single direction for a relatively short period of time.

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