EarthRef.org Reference Database (ERR) -- Beck 1996

Baag et al. <1995> suggest that magnetic fields produced by strongly magnetized surface lava flows may cause a strong and systematic deflection of the near-surface magnetic field. They term this the ''magnetic terrain effect.'' They further suggest that as a consequence of the magnetic terrain effect, directions of remanent magnetization of lava flows in general are suspect as indicators of past directions of the geomagnetic field. I tend to doubt the overall paleomagnetic significance of the magnetic terrain effect, on the grounds that properly conducted paleomagnetic studies of rocks of many types, including volcanics, commonly yield paleomagnetic poles that cluster. This would not be the case if volcanic rocks were conspicuously poor recorders of the direction of the ambient magnetic field. Possibly the strongly magnetized, rapidly extruded, dramatically well-exposed, essentially flat-lying Hawaiian rocks studied by Baag et al. <1995> provide a worst case scenario. However, I have no important quarrel with their observations nor with their interpretations regarding Hawaii. What I wish to comment on here is their suggestion that the magnetic terrain effect may explain the shallow magnetic inclinations observed in the Oregon-Washington Coast Range. Baag et al. <1995> appear to have seriously misunderstood the original study and in so doing have perpetuated a common and extremely pernicious misconception about the tectonic interpretation of paleomagnetic data. ¿ American Geophysical Union 1996 Baag et al. <1995> suggest that magnetic fields produced by strongly magnetized surface lava flows may cause a strong and systematic deflection of the near-surface magnetic field. They term this the ''magnetic terrain effect.'' They further suggest that as a consequence of the magnetic terrain effect, directions of remanent magnetization of lava flows in general are suspect as indicators of past directions of the geomagnetic field. I tend to doubt the overall paleomagnetic significance of the magnetic terrain effect, on the grounds that properly conducted paleomagnetic studies of rocks of many types, including volcanics, commonly yield paleomagnetic poles that cluster. This would not be the case if volcanic rocks were conspicuously poor recorders of the direction of the ambient magnetic field. Possibly the strongly magnetized, rapidly extruded, dramatically well-exposed, essentially flat-lying Hawaiian rocks studied by Baag et al. <1995> provide a worst case scenario. However, I have no important quarrel with their observations nor with their interpretations regarding Hawaii. What I wish to comment on here is their suggestion that the magnetic terrain effect may explain the shallow magnetic inclinations observed in the Oregon-Washington Coast Range. Baag et al. <1995> appear to have seriously misunderstood the original study and in s