Other
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmgp33a..04k&link_type=abstract
American Geophysical Union, Spring Meeting 2008, abstract #GP33A-04
Other
1520 Magnetostratigraphy, 1522 Paleomagnetic Secular Variation, 1535 Reversals: Process, Timescale, Magnetostratigraphy
Scientific paper
In the history of paleomagnetic research, sequences of stably-magnetized undisturbed lava flows have been among the best sources of reliable consistent information about the behavior of the geomagnetic field through time. Such sequences occur in scattered locations around the world, not all offering favorable sampling conditions. Iceland's basalt lavas cover more or less continuously the last 15 million years. In the last two million years or so, eruptions here often took place under water or ice, causing stratigraphic complexities. The older subaerially erupted lavas which are on average 10 m thick and separated by thin clastic sediments, form quite regular and accessible series. The lava pile is gently tilted, generally towards the active volcanic zone. Research on these lavas in the 1950's to 1970's, especially by J. Hospers, T. Sigurgeirsson, R.L. Wilson and N.D. Watkins, contributed to several steps in the development of paleomagnetic methods and understanding of variations in the geomagnetic field. Their contributions concerned for instance statistical concepts, stratigraphic correlation, alternating-field demagnetization, the discovery of transitional directions, stability of remanence in lavas, and delineation of short reversal events. As in some of the projects of Wilson and Watkins, subsequent research by the present author has mostly been done in collaboration with geologists interested in mapping composite sections (of order 300 lava flows) through parts of the lava pile. Preference has been given to locations with little hydrothermal alteration or tectonic movements. These sections are pieced together from hillside profiles partly overlapping in age, commonly with 20-60 successive flows in each profile. Single-polarity zones which have very variable thicknesses but on average 15-20 flows, are often useful in correlation; for this however, distances between profiles should be 2-3 km or less rather than, say, 5-10 km. The stratigraphic mapping projects have generated a body of characteristic remanence directions and intensities from several thousand lava flows with excellent directional stability and within-flow agreement. Simple processing of these data can yield valuable insights into various properties of the paleo- geomagnetic field. Examples of such aspects include the frequency distribution of virtual geomagnetic poles (VGPs) in latitude and longitude, and the relative average local field strength as a function of VGP latitude which can thus be estimated without difficult paleointensity studies. Yet another aspect of the field revealed in the large uniform data set from Icelandic lavas, is a long-term decrease in the amplitude of the secular variation and in excursions since 15 million years ago. The potential of Icelandic volcanics for worthwhile paleo- and rock-magnetic studies is inexhaustible, but few foreign groups have published results from here in the last 2-3 decades; see the author's website for a bibliography. Many areas of the country remain to be stratigraphically mapped, and the coverage of radiometric age determinations is still rather inadequate e.g. for accurate dating of polarity-zone boundaries. Finally, it may be noted that paleomagnetic work in Iceland has aided in the interpretation of magnetic anomalies over the island and its surroundings, which are more complex than anomalies over the mid-ocean ridges in general.
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