BEGIN:VCALENDAR VERSION:2.0 CALSCALE:GREGORIAN PRODID:UW-Madison-Physics-Events BEGIN:VEVENT SEQUENCE:0 UID:UW-Physics-Event-1461 DTSTART:20090414T170500Z DURATION:PT1H0M0S DTSTAMP:20260413T054659Z LAST-MODIFIED:20090216T144240Z LOCATION: 4274 Chamberlin SUMMARY:Taking the pulse of the geodynamo\, Chaos & Complex Systems Se minar\, Brad S. Singer\, UW Department of Geoscience DESCRIPTION:During the past 2.6 million years Earth's outer core geody namo has produced at least 18 geomagnetic excursions and 5 full polari ty reversals. This record has been compiled from terrestrial volcanic rocks\, including mainly basaltic lava flow sequences\, but also two silicic ash beds\, that have been analyzed using modern paleomagnetic techniques and dated using the 40Ar/39Ar variant of the K-Ar radio-iso topic clock. Several brief periods of field instability associated wi th excursions correlate with lows in paleointensity or directional cha nges recorded globally in marine sediments that are dated using astron omically-forced oxygen isotope signals or ice layer-counting. However\ , the lack of correlation of several excursions between marine and ter restrial records indicates that neither sediments\, nor lava flows\, a re ideal recording media. Another factor complicating correlation is that some excursions may be geographically localized and not expressed globally. Despite decades of observation\, these records remain frag mentary\, especially when periods of millions of years are considered. Recent 40Ar/39Ar dating in our laboratory\, that includes age determ inations for the Mono Lake\, Laschamp\, Blake\, Pringle Falls\, Big Lo st\, West Eifel excursions\, as well as the Halawa (C2r.2r-1) cryptoch ron\, prompt us to critically review the terrestrial record of geodyna mo instability and propose a Geomagnetic Instability Time Scale (GITS) for the Quaternary period. Both the ca. 4:1 ratio of excursions to r eversals during the past 2.6 Ma as well as the temporal pattern of occ urrence of these events provide fundamental input as to the long‑ \;term behavior and\, possibly\, the structure of the core dynamo. On the one hand\, intervals of significant temporal clustering of excurs ions have highlighted a relatively stable period of high field strengt h lasting >250\,000 years in the middle of the Brunhes chron during wh ich time few\, or no\, excursions took place. On the other hand\, suc cessive paleomagnetic excursion records may be critical in regard to u nderstanding the behaviors and interactions between the mantle‑\; influenced field source in the shallow core (the hypothesized SCORR 09\;field) and the deeper‑\;held source of the axial dipole. If in fact a successful reversal attempt requires the axial dipole field source to be weakened below some threshold strength for substantial du ration (ca. 10\,000 ‑\;20\,000 years)\, times of grouped excursio ns may also be the most probable times for a change in polarity. URL:https://www.physics.wisc.edu/events/?id=1461 END:VEVENT END:VCALENDAR