Secular variation of the Earth’s magnetic field and application to paleomagnetic dating of historical lava flows in Chile

Abstract

The recent geomagnetic secular variation is mainly characterized by the large growth of the South Atlantic Magnetic Anomaly during the last three centuries, first documented by the geomagnetic model gufm1 (Jackson et al., 2000). We report new paleomagnetic results (directions and paleointensities) from several sites in two well dated lava flows in Chile, the 1835 AD eruption of the Osorno volcano and the 1751 AD eruption of the Llaima volcano. In addition, paleointensities were obtained on 14 samples from bricks of shelters built along the main road across the Andes from Santiago (Chile) to Mendoza (Argentina) in 1770±5 AD. The results confirm the high reliability of the global geomagnetic model gufm1 for the last three centuries with a large amplitude of the secular variation in inclination (∼20°) and intensity (∼25μT). Results from three 14C dated volcanic units in the time interval 1400–1750 AD indicate that more paleomagnetic results in well dated lava flows are necessary to improve the robustness of existing global geomagnetic models. At this stage, precise paleomagnetic or archeomagnetic dating in South America using global models should be restricted to the last 3 centuries.To illustrate the potential of paleomagnetic dating in region and time interval with very large geomagnetic secular variation, we report paleomagnetic data from several sites in historical lava flows (1700–1900 AD) from the Antuco, Llaima and Villarrica volcanoes that permit to refine the ages of the major historical effusive volcanic events.