1/7/2023 0 Comments Magnetic fieldlines on a hor![]() The use of magnetostratigraphy is obviously helpful in volcanic and igneous rocks that cannot contain fossils and which cannot readily be grouped according to a stratigraphic or depositional scheme like sedimentary rocks. The ocean floor polarity striping may be regarded as a special kind of stratigraphic column, laid out sideways and symmetrically on both sides of the MOR. Most Physical Geology textbooks carry good color maps of the ocean floor magnetic striping its origin and nature have been confirmed where ocean floor rocks are rarely exposed on land as ophiolites. This process has fuzzy boundaries and extends much further from the MOR than the “hot” dikes. Instead, the magnetization is due to crystallization and recrystallization following hydrothermal ocean floor metamorphism. The stripes are neither as sharp nor as closely spaced as one might hope since the magnetization is not “thermal” following igneous cooling. Since the Earth’s field changes polarity almost every million years, the ocean floor is magnetized with alternating polarity (north-seeking versus south-seeking magnetism) about every kilometer or so, in stripes parallel to the MOR. Each part of the ocean spreads from the MOR at a rate commonly 0≥5 cm/year or 0≥500 m/Ma. These dikes magnetize shortly after sea floor hydrothermal metamorphism, which follows igneous cooling at this time the dikes magnetize in the direction of the contemporary geomagnetic field. It was this military application that first lead to the discovery of ocean floor spreading and thus of plate tectonics.) As the ocean floor spreads by gravitational gliding from the mid-ocean ridge (MOR), batches of nearly vertical dikes intrude parallel to the rift, following the center of the MOR. (Knowledge of the background ocean floor magnetism if one wishes to detect the presence of submarines from the air by magnetometry. Their remanent magnetism is so strong that its magnitude and direction may be measured from aircraft. Magnetostratigraphy is also prominently displayed by igneous dikes intruded at the spreading centers of oceans. For example, in NW Europe, the migration of early man has dated, by comparing the magnetic directions in strata bearing evidence of hominids with the known magnetic polarity time scale. #MAGNETIC FIELDLINES ON A HOR SERIES#The duration of these polarity intervals has been documented by palaeontology and geochronology so that their time series patterns not only provide a correlation tool their calibration with absolute ages permits them to be used to date rocks that are difficult to date absolutely by other methods. ![]() The fact that reversals exist shows that the magnetism is primary. The pattern of reversals forms a magnetostratigraphy, represented by a modified “stratigraphic column” that shows alternating normal polarity (indicated by black) and reversed polarity (white). This verifies that their magnetism is primary. However, in thick sequences of many lava flows, especially in places like Iceland, Northern Ireland, or Hawaii, the stratigraphic column shows groups of flows with alternating magnetic polarity. ![]() Clearly, if all the rocks of the World had very short magnetic memories, we would not know that there had ever been field reversals all the magnetism in rocks would point northward rather like the average of all compasses. ![]() With the exception of some quiet intervals, and ancient parts of the geological column with insufficient data, we now know that reversals have occurred approximately every million years or so. Moreover, the geomagnetic field has alternated between normal polarity (as now) and reversed polarity throughout geological history. Before that, compasses pointed toward the south part of the Earth. The present north-seeking behavior of compasses has persisted for ∼780,000 years (The Brunhes’ Epoch of Normal Polarity). The actual magnetic poles never coincide with the geographic poles (=where Earth rotation axis pierces Earth) but this is only a major problem for the use of a magnetic compass when working at very high latitudes. The magnetic field lines of the present geomagnetic field point toward the magnetic north pole which is currently located in northern Canada. Graham Borradaile, in Understanding Geology Through Maps, 2014 Reversal Test ![]()
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