Proceedings of the 2nd Columbia River Basalt Symposium: Maar volcanoes

摘要

Examination of maar-type volcanic cones, including tuff rings, from more than 40 localities in western North America indicates that water had access to volcano orifices during their activity. The most convincing evidence is the abundance of sideromelane (chilled basaltic glass) or its palagonitic decomposition products in the ejecta. Moreover, the volcanoes which were examined erupted in basins that either contained surface water, or else they grew above highly permeable aquifers at shallow dept. Characteristic features of maar ejecta are continuous thin beds, undulations and antidunes characteristic of base surge stratification, abundant accretionary lapilli or mud-armored rock particles, bedding sags that show soft sediment deformation, and in the subaqueous parts of the maar ramparts, great piles of subtly graded thin lenses of hyaloclastic debris.