Refined seismic tomography of North American subduction

For some time relics of the Farallon plate that was subducted beneath North America during its late Mesozoic and Cenozoic westward drift have been known from seismic tomography, but only in a blurred form. Advances in computation from many seismic records are steadily improving the resolution of this revolutionary technique, and a more finely tuned picture of the mantle beneath the North American continent has now emerged (Sigloch, K. et al. 2008. Two stage subduction history under North America inferred from multiple-frequency tomography. Nature Geoscience, v. 1, p. 458-462). The American-German-French team reveal several pieces of the ‘lost’ plate in an astonishingly complex 3-D representation of the North American mantle down to 1800 km. There are two main blocks: one still active and connected to the active subduction zone between British Columbia and northern California that dips steeply to about 1500 km depth, the other inactive and stranded beneath the eastern part of the continent. The authors believe that the two separated around the end of the Mesozoic. They suggest that the break coincided with the within-plate deformation and volcanism known as the Laramide era that lasted from 70-50 Ma, which probably coincided with low-angled subduction of the Farallon plate. After the break, the flat subduction ‘rolled-back’ westwards, leaving a track on volcanism across the western part of the continent. The authors also ponder on the relationship between the changed style of subduction and the thermal event that produced the Columbia River continental flood basalt event at 17 Ma.

Geomagnetic cows

Unless you are a committed ‘towny’, you may have noticed that livestock tend to face in the same direction when feeding and lying down; so much so that a herd of grazing cows can resemble a collective harvesting machine. However, few of us country folk have bothered to see if the direction in which they face varies from day to day. In fact it does; but only a bit. Thanks to the high-resolution images provided by Google Earth, a group of German and Czech scientists have measure the alignment of almost 3000 cows and wild deer that show up on images of 241 localities on 6 continents (Begall, S. et al. 2008. Magnetic alignment in grazing and resting cattle and deer. Proceedings of the National Academy of Sciences, v. 105, p. 13451–13455). In all the populations the animals roughly align themselves north-south. More to the point, they line up parallel to the local lines of magnetic force with a remarkable degree of consistency.

Now, this is not a study aimed at the annual IgNoble Awards, but a cunning check on whether herding animals have some kind of built in compass akin to those in birds. That would have an evolutionary advantage in seasonal migration – domestic cows are derived from wild bovids of the Pleistocene temperate grassland plains. I have a made a quick check of some local cattle and sheep, again using Google Earth, and I can’t say that I am convinced. But the study is based on statistical analysis of rose diagrams of the long axes of cattle, so there may be a tendency for poleward pointing. However, the herds and flock that I examined may be independent minded beasts. Yet, if Begall et al.’s stats are correct, then geophysicists have perhaps a new means of exploration for local distortions in the magnetic field as might happen near magnetite ores; incidentally sometimes rich sources of vanadium. The method may delay disoriented ramblers lacking compass or GPS receiver, and might place them at some risk. Frankly, they would be better off looking for which side of trees the moss grows on…

See also: Callaway, E. 2008. Magnetic cows in mystery alignment. New Scientist, v. 199 30 August 2008 issue, p. 10.


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