EarthRef.org Reference Database (ERR) -- Shaw et al. 1980

A method of age calculation based on the concept of common azimuths of volcanic loci, first applied by us in 1975 to the Hawaiian chain, is derived in detail and applied to the entire Hawaiian-Emperor chain. Age calculations using this method, and expressed in terms of a fictive 'collapsed distance' from Kilauea, are compared with the data and calculations available to 1975 and with the currently best available radiometric ages (new and revised). This method of age calculation represents the data for age versus fictive distance along the chain with significantly greater precision than any of several possible choices of linear correlations of age versus map distance from Kilauea. It is concluded that concepts of volcanic propagation rates relative to concepts of plate motions should be viewed in terms of the calculated age versus 'collapsed distance' relation illustrated in this paper rather than in terms of any of the subsets of linear age-distance relations proposed in the literature. Calculated propagation rates of volcanic loci relative to the Earth's surface are shown to vary episodically from incremental values as low as 2.5 cm/yr to values exceeding 100 cm/yr. On the other hand, the propagation rate of the Hawaiian chain relative to the inferred position of a melting anomaly in the mantle has a calculated mean value of 5.9 cm/yr that was constant from 73.6 to 1.4 m.y.B.P. and then changed to a mean value of 17.8 cm/yr during the latest 1.3 m.y. The rate change at 1.3 to 1.4 m.y.B.P. is considered real, barring evidence for an as-yet-undiscovered volcanic locus to the east of Hawaii that has coexisted during this time and that could now represent the future location of growth of the chain. The calculated ages are used to generate data for variations in volume rates of volcanic evolution versus age and magnitudes of volcanic volumes per unit length of chain versus age. The former rates range from zero to a maximum of 0.5 km (super 3) /yr (compared to a present-day yearly average of 0.1 km (super 3) /yr), and the latter range from zero to a maximum of about 103 km (super 3) /km. The ratios of these values also give estimates of implied propagation rates. Categorical agreement between this set of propagation rates and the directly calculated incremental values supports the contention that there is an overall systematic relation among age, distance along the chain, and volumes of magma produced in the mantle. Graphs of these data document the episodic nature of the evolution of the Hawaiian-Emperor chain and identify that there is coherence in the time intervals between episodes. The average peak-to-peak interval in data for both volume rate versus age and volume versus age is about 3 m.y.; the average peak-to-peak interval for propagation rates is about 4 m.y. Swings in the instantaneous propagation directions of the chain relative to the mean propagation directions (N 70 degrees W for the Hawaiian chain; N 10 degrees W for the Emperor chain) have undergone reversals of the same sense at average intervals of about 8 m.y. Although there are significant variations from the average intervals in each set of data, there are not major differences in patterns of behavior for the Hawaiian and Emperor chains other than the changes in mean propagation directions and in the senses of deviation of volcanic loci from the mean directions. In the Emperor chain, volcanic loci deviate from the mean trend in a dominantly counterclockwise geometric sense, and in the Hawaiian chain the geometric sense of deviation is predominantly clockwise. These observations are consistent with the idea that the transition from the Emperor chain to the Hawaiian chain involved a reorientation of mean directions of lithosphere-asthenosphere motions relative to a reorientation of principal stress directions in the lithosphere without a major change in the dynamic variations of plate tectonic regimes in the Pacific.