Earth’s magnetic field varies through both space and time. The planet’s magnetic poles migrate through the years and sometimes even reverse, leading the north and south magnetic poles to switch places. The strength of the field also varies. Polarity reversals occur when the field’s intensity is low. Understanding the historical variations of the field helps reveal processes in Earth’s core and sheds light on the current behavior of the magnetic field.
Magnetic minerals in igneous rocks record the magnetic field at the time of their formation. Geophysicists can trace field changes by indirectly sensing these changes recorded in newly formed seafloor, for instance. On land, scientists often seek out volcanic features, like volcanic ash flow tuffs. Terrestrial samples can be particularly useful for identifying shifts in intensity, as well as in the direction of the field.
Recently, however, Avery et al. discovered that ash flow tuffs, which should preserve the strength of the magnetic field during a volcanic eruption, may actually be a flawed material for testing magnetic variations. The researchers examined samples from Bishop Tuff, located in eastern California, a frequent site for paleomagnetic and petrologic studies. Bishop Tuff formed 767,000 years ago when, over a few days, an erupting volcano deposited approximately 200 cubic kilometers of ash and lava: enough material to fill 80 million Olympic-sized swimming pools or the whole of Lake Tahoe. The authors tested the samples for variations in magnetic mineralogy.
The results indicated that although portions of the tuff produced high-quality paleointensity estimates, the estimates varied depending on where samples were collected and their magnetic mineralogy. The authors found that rocks in the ash flow became magnetized in fundamentally different ways—from venting volcanic gas, interactions between ash and water, varying eruption temperatures, and ash solidification and compaction. In turn, portions of the tuff yielded significantly higher paleointensity estimates than neighboring sections depending on how they formed. These differences in rock formation could generate up to a 20% overestimate of paleointensity.
Furthermore, the authors found that unsuitable magnetization in some samples may not be filtered out by the selection criteria typically applied in the experimental process. The limitations in the selection criteria could further complicate the use of ash flow tuffs for paleomagnetism research.
The study carries broad implications for a large body of research into paleomagnetism. The finding that rocks with reliable magnetic signatures formed in ways outside the scope of theory means that conclusions from past research may be called into question. The study will help guide future sampling efforts to avoid the pitfalls unveiled by the analysis. (Geochemistry, Geophysics, Geosystems, https://doi.org/10.1029/2018GC007665, 2018)
—Aaron Sidder, Freelance Writer