The natural remanent magnetization of the upper Keweenawan Nonesuch Shale and Freda Sandstone has been analyzed with thermal, alternating field, and chemical demagnetization techniques. The results of this study are in good agreement with previously published works by DuBois and by Vincenz and Yaskawa, but place a tighter constraint on the North American apparent polar wander path. Fifty-eight samples, representing nearly 900 m of section, have been collected from the flanks of the Porcupine Mountain uplift. From principally thermal demagnetization analyses, a mean direction of primary magnetization has been calculated for the Nonesuch Shale, with declination 279.8°, inclination +9.8°, yielding a virtual geomagnetic pole position at 176.5° E, 10.3° N, and for the Freda Sandstone, with declination 271.3° inclination + 0.7°, yielding a virtual geomagnetic pole at 179.5° E, 1.2° N. A group of intermediate (secondary) components of magnetization is removed between temperatures of 350 °C and 550 °C, yielding well clustered directions. Its mean direction with declination 280.6°, inclination −9.5°, resulted in a virtual geomagnetic pole at 169.2° E, 3.7° N. This secondary magnetization is assumed to be of chemical origin and is most likely associated with the late Precambrian copper mineralization of the Nonesuch Shale. By thorough sampling of the stratigraphic column it is possible to infer the general direction of motion of a plate as the sediments were deposited. The motion of the North American plate as observed in the upper Keweenawan magnetizations is in agreement with the previously published polar wander paths for the late Precambrian.
The reported times of major eruptions since 1900 from the world's nonsubmarine volcanoes have been compared at each location with the phase of the various components of the solid earth tide. A correlation, significant to the 5% level, was found between eruption times and the fortnightly component of the tide for the total data set of 680 eruptions. The probability of eruption is greatest at times of maximum tidal amplitude. For individual volcanoes significant peaks in eruption probability occur also at phases other than at the fortnightly tidal maximum. The volcanoes could be subgrouped in terms of petrology, geographic location, local crustal deformation rates, and other geophysical parameters. Subpopulations of andesitic and basaltic eruptions each showed significant concentrations of events at the tidal maximum. In addition, basalt eruptions had an equally well developed concentration at the tidal minimum. In a detailed study of the Japanese region, each volcano that characteristically erupted at or near the fortnightly tidal maximum is located in an area having a negative Bouguer anomaly, a large crustal thickness, and a small rate of horizontal crustal deformation. Conversely, volcanoes in areas characterized by thinner crusts and crustal deformation rates greater than 3.0 cm/yr generally erupt at or near the fortnightly tidal maximum.
Infrasonic‐acoustic signals from five explosive eruptions of El Chichon volcano during March 29 through April 4, 1982, were recorded by a microbarograph array and Seismic Research Observatory (SRO) seismograph collocated near McKinney, Texas. Analyses of these signals for frequencies from 0.0033 to 0.025 H have demonstrated functional relationships of amplitude to frequency consistent with ω−1 for microbarometric data, ω−2 for vertical mode seismic data, and ω−3 for radial mode seismic data. These observed slopes are in agreement with those theoretically predicted by Sorrells (1971). Estimations of kinetic energy releases by the explosions of El Chichon are presented and range from 4.9×1022 ergs to 2.0×1023 for the largest eruption on April 4, 1982, to 4.2×1021 to 6.1×1021 ergs for the smallest eruption on April 3, 1982. The method of Posey and Pierce (1971) to estimate the explosivity from long‐period infrasonic signals is not likely to be applicable, strictly speaking, to the shorter‐period data. Thus it is believed that the energy release for the smallest eruption is overestimated by possibly an order of magnitude. Comparable estimations of volcanic explosivity from SRO radial seismic data and microbarometric array data strongly recommend using SRO and International Deployment Accelerometers network data for far‐field analyses of volcanic explosions and construction of a rigorous volcanic explosivity scale.
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