The mechanical disconnection of the lithosphere from the Earth’s deep mantle by a more ductile zone in the upper mantle – the asthenosphere – suggests that the lithosphere might move independently. If that were the case then points on the surface would shift relative to the axis of rotation and the magnetic poles, irrespective of plate tectonics. So it makes sense to speak of absolute and relative motions of tectonic plates. The second relates to plates’ motions relative to each other and to the ancient position of the magnetic poles, assumed to be reasonably close to that of the past pole of rotation, yet measurable from the direction of palaeomagnetism retained in rocks on this or that tectonic plate. Plotting palaeomagnetic pole positions through time for each tectonic plate gives the impression that the poles have wandered. Such apparent polar wandering has long been a key element in judging ancient plate motions. Absolute plate motion judges the direction and speed of plates relative to supposedly fixed mantle plumes beneath volcanic hot spots, the classic case being Hawaii, over which the Pacific Plate has moved to leave a chain of extinct volcanoes that become progressively older to the west. But it turns out that between about 80 to 50 Ma there are some gross misfits using the hot-spot frame of reference. An example is the 60° bend of the Hawaiian chain to become the Emperor seamount chain that some have ascribed to hot spots shifting (see http://earth-pages.co.uk/2009/05/01/the-great-bend-of-the-pacific-ocean-floor/).
Age of Pacific Ocean floor, showing the Hawaii-Emperor seamount chain in black. (credit: Wikipedia)
Ideas have shifted dramatically since it became clear that hot spots can shift, and there has been an attempt to estimate their actual motions (Doubrovine, P.V. et al. 2012. Absolute plate motions in a reference frame defined by moving hot spots in the Pacific, Atlantic, and Indian oceans. Journal of Geophysics Research: Solid Earth, v. 117, B09101, doi:10.1029/2011JB009072). It is early days for the revised view of absolute motion of the lithosphere and estimates go back only 120 Ma. However, one outcome has been a realistic examination of whether the positions of the poles have shifted through time; a possibility that is hidden in apparent polar wander paths. Since the mid-Cretaceous it seems that a slow and hesitant, but significant polar shuffle has taken place, varying between 0.1 and 1.0° Ma-1, starting in one direction and then the movement retraced its steps to achieve the current proximity of magnetic poles to the poles of rotation.
One of the interesting things about the beaver is that its obsession with civil engineering may have a profound effect upon landscape. Before Europeans set foot in North America, it is estimated that up to 400 million of them inhabited the continent. The ponds that they create by building the dams in which they live securely, encourage sedimentation. It is quite possible that this creates recognisable stratigraphic formations; but no-one really knows as active and wet beaver habitats hide what lies beneath them. It is clearly urgent to obtain this intelligence: the Geological Society of America’s monthly Geology contained in its first issue for 2012 a paper that indeed probes the legacy of large rodents long gone (Kramer, N. et al. 2012. Using ground penetrating radar to ‘unearth’ buried beaver dams. Geology, v. 40, p. 43-46).
The target for surveillance was the eponymous Beaver Meadows in Colorado, USA, and not only did the researchers from Colorado State University deploy ground-penetrating radar, but used the seismic reflection method as well, to quantify volumes of beaver-induced sedimentation. Fortunately, despite their past presence in some strength, beavers no longer frequent Beaver Meadows and no ethical lines in the sand were crossed. Beaver and elk seemingly have long competed for the meagre resources of Beaver Meadows, the rodent having finally succumbed locally to determined efforts by the elk to consume the beavers’ victuals. As disconcerted and no doubt sulking beavers failed to maintain their dams and lodges, the water table fell, further encouraging the elk. Eventually, at some time after the Beaver Survey of 1947, the last of them moved to new meadows. Their ravages (see http://animal.discovery.com/videos/fooled-by-nature-beaver-dams.html) of what would otherwise be dense woodland have, however, made it possible for geophysicists to try out their sophisticated kit on a new and thorny issue: they ran 6 km of GPR and seismic profiles.
A beaver. Image via Wikipedia
In much the same way as larger scale geophysical data are interpreted for petroleum traps, signs of hydrocarbons, mighty listric faults and zones of tectonic inversion, the beaver-oriented sections potentially yield considerable insight to the trained eye. There are indeed beaverine sedimentary aggradations of Holocene age above the local glacial tills. Beneath Beaver Meadow they amount to as much as 50% of post-glacial sediment. Apparently, the deposits have a linear element that follows the local drainages.
Japan's deep-sea Drilling Vessel "CHIKYU" Image via Wikipedia
The most important factors in attempting to assess risk from earthquakes are their frequency and the time-dependence of seismic magnitude. Historical records, although they go back more than a millennium, do not offer sufficient statistical rigor for which tens or hundreds of thousand years are needed. So the geological record is the only source of information and for most environments it is incomplete, because of erosion episodes, ambiguity of possible signs of earthquakes and difficulty in precise dating; indeed some sequences are extremely difficult to date at all with the resolution and consistency that analysis requires. One set of records that offer precise, continuous timing is that from ocean-floor sediment cores in which oxygen isotope variations related to the intricacies of climate change can be widely correlated with one another and with the records preserved in polar ice cores. For the past 50 ka they can be dated using radiocarbon methods on foraminifera shells The main difficulty lies in finding earthquake signatures in quite monotonous muds, but one kind of feature may prove crucial; evidence of sudden fracturing of otherwise gloopy ooze (Sakagusch, A. et al. 2011. Episodic seafloor mud brecciation due to great subduction zone earthquakes. Geology, v.39, p. 919-922).
The Japanese-US team scrutinised cores from the Integrated Ocean Drilling Program (IODP) that were drilled 5 years ago through the shallow sea floor above the subduction zone associated with the Nankai Trough to the SE of southern Japan. Young, upper sediments were targeted close to one of the long-lived faults associated with the formation of an accretionary wedge by the scraping action of subduction. Rather than examining the cores visually the team used X-ray tomography similar to that involved in CT scans, which produce precise 3-D images of internal structure. This showed up repeated examples of sediment disturbance in the form of angular pieces of clay set in a homogeneous mud matrix separated by undisturbed sections containing laminations. The repetitions are on a scale of centimetres to tens of centimetres and were dated using a combination of 14C and 210Pb dating (210Pb forms as a stage in the decay sequence of 238U and decays with a half-life of about 22 years, so is useful for recent events). The youngest mud breccia gave a 210Pb age of AD 1950±20, and probably formed during the 1944 Tonankai event, a great earthquake with Magnitude 8.2. Two other near-surface breccias gave 14C ages of 3512±34 and 10626±45 years before present. These too probably represent earlier great earthquakes as it can be shown that mud fracturing and brecciation by ground shaking needs accelerations of around 1G, induced by earthquakes with magnitudes greater than about 7.0. So, not all earthquakes in a particular segment of crust would show up in seafloor cores, most inducing turbidity flow of surface sediment, but knowing the frequency of the most damaging events, both by onshore seismicity and tsunamis, could be useful in risk analysis. In its favour, the method requires cores that penetrate only about 10 m, so hundreds could be systematically collected using simple piston coring rigs where a weighted tube is dropped onto the sea floor from a small craft.
The record of the Earth’s magnetic field for the most part bears more than a passing resemblance to a bar-code mark, by convention black representing normal polarity, i.e. like that at the present, and white signifies reversed polarity. The bar-code resemblance stems from long periods when the geomagnetic poles flipped on a regular, short-term basis, by geological standards. The black and white divisions subdivide time as represented by geomagnetic into chrons of the order of a million-years and subchrons that are somewhat shorter intervals. Stemming from changes in the Earth’s core, magnetostratigraphic divisions potentially occur in any sequence of sedimentary or volcanic igneous rocks anywhere on the planet and so can be used as reliable time markers; that is, if they can be defined by measurements of the remanent magnetism preserved in rock, which is not universally achievable. Yet this method of chronometry is extremely useful, for most of the Phanerozoic. However, there were periods when the geomagnetic field became unusually stable for tens of million years so the method is not so good. These have become known as superchrons, of which three occur during Phanerozoic times: the Cretaceous Normal Superchron when the field remained as it is nowadays from 120 to 83 Ma; a 50 Ma long period of stable reversed polarity (Kiaman Reverse Superchron) from 312 to 262 Ma in the Late Carboniferous and Early Permian; the Ordovician Moyero Reverse Superchron from 485 to 463 Ma.
Because the geomagnetic field is almost certainly generated by a self-exciting dynamo in the convecting liquid metallic outer core, polarity flips mark sudden changes in how heat is transferred through the outer core to pass into the lower mantle. It follows that if there are no magnetic reversals then the outer core continued in a stable form of convection; the likely condition during superchrons. But why the shifts from repeated instability to long periods of quiescence? That is one of geoscience’s ‘hard’ questions, since no-one really knows how the core works at any one time, let alone over hundreds of million years. There is however a crude correlation with events much closer to the surface. The Kiaman superchron spans a time when Alfred Wegener’s supercontinent Pangaea had finished assembling so that all continental material was in one vast chunk. The Cretaceous superchron was at a time when sea-floor spreading and the break-up of Pangaea reached a maximum. The Ordovician, Moyero superchron coincides with the unification of what are now the northern continents into Laurasia and the continued existence of the southern continents lumped in Gondwana, so that the Earth had two supercontinents. Those empirical observations may have been due to chance, but at least they provide a possible clue to linkage between lithosphere and core, despite their separation by 2800 km of convecting mantle that transfers the core heat as well as that produced by the mantle itself to dissipate at the surface. Enter the modellers.
How part of the Earth transfers heat is, not unexpectedly, very complex, depending not only on what is happening at that point but on heat-transfer processes and heat inputs both above and below it. The surface heat flow is complex in its own right ranging from less than 20 to as much as 350 mW m-2, the largest amount being through zones of sea-floor spreading and the least through continental lithosphere. Wherever heat is released in the core and mantle, willy-nilly the bulk of it leaves the solid Earth along what is today a complex series of lines; active oceanic ridge and rift systems such as the mid-Atlantic Ridge. These lines weave between six drifting continental masses and many more sites of additional heat loss – hot spots and mantle plumes. The many heat escape routes today complicate the deeper convective processes and there are many possibilities for the core to shed heat, yet they continually change pace and position. When, inevitably, all continental lithosphere unites in a supercontinent, almost by definition, the sites of heat loss simplify too, the supercontinent acting like an efficient insulating blanket. In a qualitative sense, this kind of evolving scenario is what modellers try to mimic by putting in reasonable parameters for all the dynamic aspects involved. Two physicists at the University of Colorado in Boulder, USA, Nan Zhang and Shije Zhong, have formulated 3-D spherical models of mantle convection with plate tectonics as a basis for whole Earth thermal evolution over that last 350 Ma (Zhang, N & Zhong, S. 2011. Heat fluxes at the Earth’s surface and core–mantle boundary since Pangea formation. Earth and Planetary Science Letters, v. 306, p. 205-216). The acid test is whether the model can end with a close approximation to modern variations in heat flow and distribution of ages on the sea floor; it does. A probable key to stability in the means of transfer of heat from core to lower mantle – itself a key to a constant outer-core dynamo and geomagnetic polarity – is reduced heat flow at equatorial latitudes; a sort of equatorial downflow of convection with upflows in both northern and southern hemispheres. Zhang and Zhong’s model produced minimal core-to-mantle heat flow at the Equator at 270 and 100 Ma, both within geomagnetic-field superchrons. Well, that is a good start. Superchrons seem also to have occurred from time to time during the Precambrian, one being documented at the Mesoproterozoic-Neoperoterozoic boundary about 1000 Ma ago. At that time, all continental lithosphere was assembled in a supercontinent dubbed Rodinia (‘homeland’ or ‘birthplace’ in Russian).
In 2008 a team of geophysicists from Cambridge University, UK published an astonishingly detailed picture of about 500 km2 of a land surface complete with drainage systems (Figure 3 in Rudge, J.F. et al. 2008. A plume model of transient diachronous uplift at the Earth’s surface. Earth and Planetary Science Letters, v. 267, p. 146-160). The surprise was not its Palaeogene age (~55 Ma), but that it is buried beneath the Atlantic continental shelf about 200 km west of the Shetland Isles and had been revealed by detailed, 3-D seismic reflection surveys during oil exploration. Technically it is buried landscape unconformity that resulted from uplift (by almost 500 m) and erosion (for ~1.3 Ma) that interrupted Palaeocene to Eocene marine sedimentation and was suddenly buried to preserve the details of river channels: uplift rapidly gave way to subsidence and conditions returned to marine about 0.6 Ma later. The timing and the location of such a transient crustal bulge, during the early part of opening of the North Atlantic, suggests that it stemmed from a thermal source, probably the Iceland hot spot straddled by the mid-Atlantic Ridge. The model favoured by the authors is radially horizontal spreading of a pulse of especially hot mantle outwards from the plume beneath the Iceland hot spot; a ‘plume head’. Volumetric expansion of the lithosphere causes the uplift, and movement away from the plume of the hot mantle results in an annular, outward moving ripple. Cooling once the thermal source has passed produces subsidence.
The idea clearly has ‘legs’ for a whole number of reasons, not the least being the sheer number of long-lived hot spots above mantle plumes that affect the ocean basins and parts of the continents, Africa and North America especially. Now it has been publicised more widely than in a specialised journal (Williams, C. 2011. Pulsating planet. New Scientist, v. 209 (12 March 2011), p. 41-43). One of the original authors is reported to have suggested that the ~55 Ma thermal ripple beneath the nascent North Atlantic may have destabilised gas hydrates in the sediments causing methane to belch out in its wake. That is a possible mechanism for the Palaeocene-Eocene thermal maximum and its huge associated carbon isotope ‘spike’ likely stemming from boosted atmospheric methane.
The Grand Canyon from the South Rim. Image via Wikipedia
Probably the most famous extant bulge is the one through which the Colorado River has carved the USA’s 1.8 km deep Grand Canyon: the Colorado Plateau. Long believed to have formed above hot, low-density lithosphere too, this uplift is the subject of completely new ideas that also have stemmed in part from seismic data, though not produced by artificial reflectance methods. Geophysicists in the US have developed a system that uses hundreds of transportable seismometers that are being ‘marched’ from west to east as an array that uses seismographs from natural earthquakes world-wide to perform seismic tomography –3-D mapping of varying seismic velocities and thereby rigidity and density in the mantle – with improved resolution because of the close spacing of the recording stations. Publications from the Earthscope USarray are beginning to appear from the western USA, one of which concerns the Colorado Plateau (Levander, A.et al, 2011. Continuing Colorado plateau uplift by delamination-stylee convective lithospheric downwelling. Nature, v. 472, p. 461-465). The western part of the plateau is associated with a high-velocity anomaly that extends to around 90m km beneath, which the authors ascribe to a large blob of rigid mantle that has detached from the lithosphere and is slowly sinking. This ‘drip’ is an example of delamination where mantle that becomes detached from the lithosphere causes it to thin and reduces its overall density. The overlying crust rises in response. There is a thermal effect, as warmer, less rigid asthenosphere convects upwards to fill the gap left by the drip, but it is an effect rather than a cause of the uplift.
See also: Zandt, G. & Reiners, P. 2011. Lithosphere today… Nature, v. 472, p. 420-421.
Most people are quite content with an annual holiday abroad, yet a number of geoscientists yearn for something more adventurous. The Croatian geophysicist Andrija Mohorovičić was among the first to study estimates of speeds at which seismic waves travelled through the Earth, discovering in 1909 that below a depth of about 30 km below the continental surface they moved faster than in the uppermost layer. He had discovered the boundary between the continental crust and the underlying mantle, a discontinuity that bears his name though often shortened to the ‘Moho’. Having been traced beneath most of the Earth’s surface, a group of American scientists discussed over a drink or three at a ‘wine breakfast’ in 1957 a project to drill through the Moho to find out what the mantle was made of. The brainchild of Harry Hess, one of the first to suggest plate tectonics as a driving mechanism for continental drift, was dubbed Project Mohole. With US government support, a drilling barge designed for offshore oil drilling and a system of thrusters and pre-GPS locational instrumentation to keep the barge on station the Mohole was spudded in 1961 on the seabed near Guadalupe Island off Baha California in Mexico; about the time that John F. Kennedy declared his belief that the USA could land a man on the Moon by the end of the 1960s. There was something of a thrill factor about Project Mohole, and its first attempts were reported in Life Magazine by John Steinbeck, author of The Grapes of Wrath and amateur oceanographer. It turned out that sending a drill bit to the mantle was more difficult than a manned lunar landing. Only a few metres of basaltic crust was recovered and Congress cancelled Mohole funding in 1966. Nevertheless, the project was the forerunner of the highly successful Ocean Drilling Program and its predecessors, probably the most prolific international collaboration of any kind.
The drilling barge CUSS1 used for the original Mohole Project. Image via Wikipedia
Since the 1960s research into the mantle has been continued with great success by looking at upthrust masses such as those in the Alps and in ophiolite complexes, nodules in alkaline basalts and kimberlites that form below 100 km into the mantle, samples dredged from oceanic fracture zones, and indirectly from the geochemistry of basalts that are derived by partial melting of mantle materials. Yet, there is still an air of frustration about some igneous petrologists and geophysicists; they want to touch the real thing! Now, at last, they may have their chance, for improved drilling and positioning technology developed by ODP and the petroleum industry make a hole through the Moho feasible. Indeed one is planned once drill-bits and lubricants suitable for the anticipated temperatures and pressures have been finalised. Three sites are under consideration: near the original Mohole; in the Cocos Plate off Costa Rica and the Pacific Plate near Hawaii, each combining the coolest crust, thinnest sediment cover and shallowest possible water – i.e. just off a mid-ocean ridge or hot-spot. The Costa Rica site (ODP site 1256) has the thinnest crust due to rapid sea-floor spreading by the East Pacific Rise there and is the most likely to be drilled. It already has a core the penetrates to 1.5 km in oceanic crust and a current project aimed at sampling the cumulate gabbro layer of the lower oceanic crust. That will still be 3.5 km above the local Moho.
There is an obvious question; will an ocean-floor site, however favourable, and a hole drilled through it help resolve fundamental issues regarding the mantle? Well, probably for oceanic lithospheric mantle, but that has had basaltic magma removed from it to form the crust above. Also mid-ocean ridge basalts have geochemical features that suggest that their source mantle had been a melt source previously, compared with the source mantle materials for alkaline and some other types of basalt that seem to have been less depleted in certain elements. The most important question posed by the mantle in general concerns how it originally formed during the Earth’s earliest history, accretion of debris from the solar nebula, the moon-forming event and extraction of the metallic core. A Mohole can contribute little to those issues.
Source: Teagle, D.A.H. & Ildefonse B. 2011. Journey to the mantle of the Earth. Nature, v. 471, p. 437-439.
It used to be a joke in the Geological Surveys of the Soviet Union that they employed so many thousands of geologists that the entire USSR could be mapped in a few years if they all linked hands and walked from east to west. Geophysicists are trying for something similar to map the mantle underlying the USA in 3-D. The USArray involves 400 portable seismometers, currently spread out at 10 km intervals in the western States, is intended to act like a fly’s eye in monitoring arrival times of seismic waves from worldwide earthquakes. The plan is to steadily move the array eastwards until by 2013 it has reached the Atlantic coast. From that data the geophysicist hope vastly to improve the resolution of seismic tomographic images of the deep Earth (see Kerr, R.A. 2009. Scoping out unseen forces shaping North America. Science, v. 325, p. 1620-1621). Yep, they are definitely going for a high ‘Wow factor’ rating. Yet is seems that there are other expletives floating around as the strangely knobbly and discontinuous architecture that is emerging from early data processing refuses to fit many simple hypotheses being tested.
The world-wide network of seismic recording stations was originally set up partly to improve detection of underground nuclear weapons tests. It is the source for the mapping of variations in seismic-wave speeds in the mantle by seismic tomography that is revolutionising ideas about the Earth’s internal dynamics. Nowadays nuclear explosions have been miniaturised so that detecting them and their locations and distinguishing them from small natural earthquakes has become difficult. The growing concerns about nuclear weapons proliferation have spurred an upgrade and expansion of seismic monitoring, and other means of verifying that seismic signals have indeed been produced by underground nuclear explosions, such as sensitive analysis of air sample for isotopes leaking from tests (Clery, D. 2009. Test ban monitoring: no place to hide. Science, v. 325, p. 382-385). If this enhanced source of seismic data is routinely made available to tomography researchers, it should boost resolution of seismic speed anomalies and sharpen up ideas about deep tectonics.
Supershear earthquakes
In an analogous fashion to the sonic booms made by aircraft travelling faster than sound, it seems possible that the rupture of a fault may travel faster than the seismic waves that it generates. Evidence is accumulating that such faults produce the equivalent of a sonic boom (Fisher, R. 2009. Seismic boom. New Scientist, v. 203 (1 August 2009), p. 32-35) despite mathematical suggestions that faults cannot propagate so fast. Experiments show that there is a seismic equivalent of the Mach fronts associated with sonic booms, and they amplify the shock of earthquakes that produce them. High amplitude at the Mach front causes it to travel further away from a fault line than normal seismic surface waves – those that cause most damage, and it also gives rise to ground motions different from those normally linked with earthquakes: more like a hammer blow than shaking. The net conclusion is that these ‘supershear’ earthquakes may pose hazards beyond those involved in risk assessment near active fault zones. Field evidence for supershear events are signs of disturbance by recent earthquakes that are further from an active fault zone than existing theory predicts. So far such evidence has only turned up along active strike-slip faults on continents, such as the Kun Lun Fault in Tibet and the North Anatolian Fault in Turkey. Yet, these form the longest seismically active zones, including the infamous San Andreas Fault in California.
Global warming is intensifying cyclonic storm systems, the energy retained by the greenhouse effect being redistributed to winds and in turn to ocean waves, which even have a small effect on local gravitational potential. The effects become coupled to the solid Earth and appear as the background ‘noise’ in seismograms. So historic seismograms, both digital and in paper form, potentially supply a proxy for climate change going back as far as the 1930s when seismographic stations first began to be set up. In some instances the records are continuous, and when digitised form a unique record that integrates, but one yet to be exploited fully (Bromirski, P.D. 2009. Earth vibrations. Science, v. 324, p. 1026-1027.
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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