Paleo-intensities of the Earth's magnetic field determined from Tertiary and Quaternary rocks

A variety of rock types from eighteen volcanic units of the western United States were studied by Thellier's method. Ninety-five NRM-TRM curves were determined, and paleo-intensities are estimated from twelve of the units. Each paleo-intensity, on the average, represents the mean of values derived from five separate samples. Two paleo-intensities have standard deviations of the mean less than 5%, two between 5 and 10% and eight between 10 and 20%; no estimate is made for the remaining six units because of great internal inconsistency of the data or insufficient work. The largest ratio of paleo-intensity to the present field intensity at the same location is 1.1, the lowest 0.2. The 0.2 value is for a Miocene transition zone, supporting the hypothesis that the intensity decreases during a field reversal. Low ratios were found for a few other units. One such Pliocene unit also has an anomalous direction of magnetization, suggesting the field may have been in the act of reversing when the unit was magnetized 7.2±0.3 m.y. ago. The direction is normal for the other units with low ratios; one of these is probably less than 10,000 years old and thus cannot be associated with a reversal. Samples of dacite from the 1915 eruption of Mt. Lassen behave in a suprising and probably atypical manner; simple comparison of the NRM to the total TRM yields an experimental value much closer to the known intensity than that determined by Thellier's method.