Abstract

The Cinder Cone quartz basalt flows in Lassen Volcanic National Park, California, are probably only a few hundred years old (Finch, 1937), and there are at least four—the old bench flow, painted dunes flow, main flow, and 1851 flow. The flows consist almost exclusively of block lava and are petrographically indistinguishable. Ubiquitous quartz xenocrysts, siliceous pumice inclusions, and a high (56 per cent) SiO2 content in the presence of 6–8 per cent olivine show that the basalt is clearly contaminated (probably by a rhyolitic tuff).

Fifty-four core samples were collected from the surface of the flows and from volcanic bombs. Magnetic measurements of the samples revealed a large unexpected scatter in magnetic directions that was not reduced by demagnetization. The cumulative experimental error from all sources is far too small to account for the large deviations in magnetic direction. It is concluded that the probable cause of the scatter is rotation of the surface lava blocks after cooling below the Curie point. Movement within the still mobile core of the flow may well have caused this rotation. Samples collected from volcanic bombs showed a similar broad scatter which is tentatively ascribed to postcooling movement of the underlying cinders, possible by downslope creep.

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