Geomagnetic polarity dating
4.2.1 Polarity Dating of Lava Flows 0-6 Ma Mercanton (1926) first realized that if rocks containing reverse magnetizations were due to reversals of the Earth's magnetic field, then this should be registered in rocks worldwide and so he obtained samples from Spitsbergen, Greenland, Iceland, the Faroe Islands, Mull, Jan Mayen Land and Australia as a test.He found that some were magnetized in the same sense as the present Earth's field and others were roughly reversed from it.In both cases there is positive (downward pointing) inclination (normal polarity) in the upper part of the core corresponding to the Brunhes chron.Results of a geochronological and paleomagnetic investigation of the volcanic island of Mauritius are reported.This concept of polarity bias persists in the literature today (e.g., Johnson et al., 1995; Algeo, 1996). Geomagnetic polarity time scale for the past 6 Myr based mainly on 3'Ar/40Ar and paleomagnetic data on igneous rocks. Volcanic activity is intermittent so the study of lava successions does not produce a continuous sequence of polarity information.As discussed in §4.5.4, the individual normal and reverse intervals (whether they be intervals of the chron polarity or subchrons of the opposite polarity) are equivalent, independent intervals drawn from a random process. Black represents normal polarity, and white represents reverse polarity. In contrast, continuous sequences can be obtained using ocean-bottom cores from deep-sea sediments, providing an independent method of determining the polarity time scale.
However, as new data appeared in the literature (Cox et al., 1963b, 1964a; Mc Dougall and Tarling, 1963, 1964) it rapidly became apparent that there was no simple periodicity; the lengths of successive polarity intervals varied haphazardly, some being long (~1 Myr) and others short (-0.1 Myr). (1964b) proposed that within intervals of predominantly one polarity lasting of the order of 1 Myr, there were short intervals of opposite polarity of the order of 0.1 Myr.These observations suggested that an ordered sequence of polarity inversions might exist in the geological record.This conclusion was further emphasized by the evidence that almost all rocks of Permian age are reversely magnetized (Irving and Parry, 1963).The longer intervals were termed magnetic polarity epochs and the shorter intervals were called events.The epochs were named after pioneering scientists in geomagnetism (Brunhes, Matuyama, Gauss, and Gilbert) whereas the events were labeled from the location of their discovery (e.g., Jaramillo, Olduvai, Kaena, and Sidufjall).
Two examples from the North Pacific are given in Fig.