RUI: Rapid geomagnetic change recorded by a partially remagnetized lava flow

Occidental College, Los Angeles CA

Investigators

Abstract

It is well established that the geomagnetic field, which is generated by fluid motions in the Earth's core, has reversed polarity many times in the geological past, most recently about 800,000 years ago. The geological record shows that this global-scale change, during which compasses everywhere go from pointing north to pointing south (or visa versa), takes only a few thousand years to occur. An especially interesting aspect of polarity reversals is the speed at which the field can change direction during the event. Controversial evidence from 16 million year old volcanic rocks in Oregon suggests that the ancient field changed as rapidly as 6 degrees per day, a rate that difficult to reconcile with standard estimates of several key deep earth properties. The goal of this study is to critically test an interpretation of new data representing just the second example of rapid transitional field change from the geologic record. This study focuses on stack of 15.2 million year old lava flows exposed in the Sheep Creek Range (North Central Nevada) that happened to erupt, cool, and became permanently magnetized parallel to earth's magnetic field during a reverse-to-normal polarity switch. One flow from high in the stack has an unusual composite magnetization recording two geomagnetic field direction over 50 degrees apart. We interpret this composite magnetization to be the result of a two-stage cooling process during which the geomagnetic field changed rapidly (at least 1 degree/week) to produce the two magnetization components. To confirm our interpretation, better constrain the rate of field change, and more fully understand its geomagnetic context, we are investigating how the composite magnetization of this flow varies laterally and will determine how strong the geomagnetic field was before, during, and after the interval of rapid directional change. The outcome of the study will sharpen our understanding of deep earth processes and properties as well as provide a glimpse of the environment that future generations will experience when the earth's magnetic field next reverses polarity. In addition to the scientific goals of the project, the research is supporting the training of undergraduate students and providing opportunities for them to conduct research projects, is contributing to the broadening of participation for underrepresented groups in the earth sciences, and is contributing to research infrastructure at Occidental University. The research is being supported by the EAR Geophysics Program and the EAR Education and Human Resources Program.