By ALLAN COX
Paleomagnetic measurements can help to determine whether isolated outcrops of lava may represent the same eruption because they accurately indicate the direction of the earth's magnetic field when the lava cooled. For example, studies of historic lava flows on Hawaii have established that the mean direction of remanent magnetization of an individual flow is nearly parallel to the earth's magnetic field of its time, as recorded at magnetic observatories close to the site where the lava solidified. These results show that the angular difference between the measured direction of magnetization of a flow and the observed direction of the earth's field, which may be regarded as the irreducible error of the paleomagnetic method, is not more than 4° if at least eight oriented samples are collected from each flow. Multiple sampling is necessary because directions of magnetization of individual samples commonly vary from 5° to 15°
Remanent magnetization is useful for stratigraphic purposes, because the geomagnetic field that controls the magnetization changes direction with time. If plotted on a sphere, or on a spherical projection, the changing pole of the earth's magnetic field determined at a given locality appears as a point that traces out an irregular path around some mean direction. Observatory records show that the velocity of motion and the amplitude of the swing of the magnetic pole vary widely from place to place. A shift in pole position as great as 40° has occurred during a mere 200 years at some observatories, whereas the change has been less than 2° at others. On the basis of such data, it is obvious that the path traced out by the shifting magnetic pole may cross and recross itself many times during an interval as long as 100,000 years. This conclusion is of considerable importance in applying paleomagnetism to stratigraphy.
Two outcrops of volcanic rock that differ in direction of magnetization by several tens of degrees, for instance, must have cooled several hundred years apart, if we assume that the ancient geomagnetic field changed at the maximum rate observed today. A time interval of 10,000 years or 100,000 years would also be consistent with such measurements.
On the other hand, because the geomagnetic field can coincide with a former position, two outcrops of volcanic rock that have the same direction of magnetization may have cooled at different times, but such a coincidence is very unlikely. It is more probable that the outcrops solidified simultaneously.
The directions of magnetization of several basalt flows in the Snake River Group are plotted in figure 13. Each point is the direction of magnetization of an oriented sample after partial demagnetization in an alternating magnetic field to remove the effects of possible lightning strikes (Cox, 1960; Graham, 1961). The samples for a particular flow represent several outcrops and show that the direction of magnetization is the same throughout an individual basalt unit. For example, the samples from the Sand Springs Basalt are from three outcrops that span a distance of 22 miles. At the 95-percent confidence level, the mean directions of magnetization of these three outcrops are the same. On the other hand, samples from different basalts (compare Sand Springs and Thousand Springs) are magnetically distinct.
For the Bancroft Springs Basalt of former usage and the McKinney Basalt, however, both of which were sampled at three outcrops (fig. 3), the directions of magnetization overlap; at the 95-percent confidence level, their mean directions are not significantly different. From the foregoing remarks about the meaning of magnetic data, this result suggests that the Bancroft Springs and McKinney are almost surely the same flow. However, coincidence in direction of magnetization, as already explained, does not rule out the remote possibility that the Bancroft Springs and McKinney are different. Moreover, the measured direction of magnetization is close to the direction of an axial dipole field (Cox and Doell, 1960) that occurs more commonly than any other direction in young rocks, as determined in many regions. Even in the Snake River Plain, two other basalts (not shown in fig. 13 because they are remote from the area described here) have nearly the same direction of magnetization as the Bancroft Springs and McKinney. Coincidence in the direction of magnetization is clearly a necessary condition, but not a sufficient condition, for establishing equivalent age. Nonetheless, when these paleomagnetic results are combined with the knowledge of geology given earlier in this report, the conclusion is virtually inescapable that the Bancroft Springs Basalt and the McKinney Basalt are identical.
Last Updated: 28-Mar-2006