Description

From Hildreth and others (2007) ^[1] : "The Gas Rocks cluster consists of three dacite domes and one stratified fragmental mafic cone. A second mafic cone (Cone 245) is exposed in a lakeshore bluff 4 km due west of The Gas Rocks. Viewed from a distance, the three high knobs that make up The Gas Rocks include two of the domes and mafic Cone 431 (the southeasternmost knob).
"The third dacite dome is much lower, standing only ~45 m above the lake on the north point of The Gas Rocks peninsula. Separated by a linear trough from the much higher adjacent dome, North Point Dome appears to be an independent extrusion rather than a thick coulee issuing from its neighbor, an inference supported by its slightly more evolved composition. The larger domes, Domes 600 and 700, have 180 and 210 m relief, respectively, above the lake. Dome 600 appears to be a single simple extrusion, as shown by the steep jointing that flares slightly outward toward the top of its northeast face. Nonetheless, a pair of lava flows, each 10 to 15 m thick, extends to the shoreline from the foot of that face. In common with a lava flow associated with North Point Dome, these flows have been truncated and thinned by the ice that flowed northwestward down the lake basin. Dome 700 likewise has steep jointing on its sheer east face, but the moderately dipping sheeting on its west slope suggests serial extrusion of a few exogenous lobes. Little or no glassy or pumiceous carapace remains on any of the domes, which were all completely submerged and scoured by glacial ice. The deep interior exposure on the steep northeast faces of the two highest domes owes to enhanced erosion by the principal glacial lobe that flowed northwestward toward Bristol Bay.
"All three domes consist of phenocryst-rich dacite. * * * We determined two 40Ar/39Ar ages on groundmass concentrates from samples of The Gas Rocks dacite. Sample U-1, from the small North Point Dome, yielded a weighted-mean plateau age of 25.7 +/- 1.4 ka, and sample U-2, from Dome 600, 23.3 +/- 1.2 ka. At their extremes, the two error envelopes just overlap, although nothing on the ground is known to contradict a younger age for the larger dome. Because of their petrographic and compositional similarity, it cannot be excluded that the three domes are essentially contemporaneous, plausibly fed by a common dike, the northwesterly strike of which would be similar to the present-day plate-convergence direction (which might in turn influence the direction of maximum horizontal compression)."