Morphology of a crater-filling lava lake margin, The Peninsula tuff cone, Tule Lake National Wildlife Refuge, California: implications for formation of peperite textures

A crater-filling lava lake basalt at The Peninsula tuff cone in northeastern California has features on its base formed by interaction of the basalt with wet hydrovolcanic tuff. A spectrum of features, including elongated, streamlined, flute-shaped lobes, and irregular corrugations, are exposed on the base of the lava lake. Flute-shaped lobes range in amplitude from several centimeters to 0.5 m, plunge downdip with respect to the subjacent layered tuff toward the center of the crater, and are better defined at depths more than 3 in below the lava take surface. These may have been formed by growth of Rayleigh-Taylor instability waves caused by the density contrast between the wet tuff and basalt, combined with directed stress from flow of the lava lake radially outward from the crater. Formation of an insulating vapor film at the wet tuff-basalt interface and downward pressure exerted by the lava lake allowed the edge of the lava lake to quench gradually, and prevented formation of peperite at greater depths in the lake. Corrugations exposed on the upper 1.5 m of the lava lake margin increase in size from several millimeters to 10 cm in amplitude toward the top of the lava lake and approximately parallel the surface. Corrugations may be the remnant of fluidal peperite preserved on the edge of the lava lake, where the peperite was eroded, or may be deformation features formed within an insulating vapor film due to oscillation of the vapor film. These were formed higher in the lava lake, where the lava lake basalt was thinner, wet tuff deposits confining the lava lake were thinner, and temperature differences between the wet tuff and lava were greater. If corrugations represent deformation of the lava edge within the vapor film, rather than peperite formation, they preserve an intermediate process that is involved in the formation of peperite. This study offers a unique opportunity to evaluate some of the initial mechanisms involved in the formation of peperite based on features preserved in the lava where the host rock. in this case hydrovolcanic tuff, has been eroded but the properties of the host rock are known from adjacent exposures. (C) 2002 Elsevier Science B.V. All rights reserved.