Event Name : Aniakchak Tuff Cones
|Start: 900 (± 80 Years) || Years BP C-14 (raw) || |
|Maar, tuff cone, tuff ring: ||
|Eruption Product: || basaltic andesite ||
From Bacon and others (2014): "The three tuff cones on the eastern caldera floor were constructed by alternating Surtseyan and hydrovolcanic explosions. Eruption of the basaltic andesite Windy and Breezy Cones on the ring-fracture system, and of andesitic Surprise Cone inboard from them, may be related to unloading of the magmatic system by sudden lowering of the intracaldera lake by ~200 m ca. 1,860 14C yr B.P."
"Three breached tuff cones form subdued arcuate ridges of bedded phreatomagmatic tephra on the eastern part of the caldera floor (fig. 10A [in original text], unit Qtc). Embraced and nearly surrounded by younger Vent Mountain lava flows, Surprise, Windy, and Breezy Cones rise ~120 m above the surrounding terrain to define 500-1,000-m-diameter explosion craters (figs. 12A and B [in original text]). Two samples of juvenile scoria from Surprise Cone have 57.9 and 58.4 weight percent SiO2, while single analyses of scoria from Windy and Breezy Cones yield 55.3 and 52.3 weight percent SiO2, respectively. Based on the similar appearances of the tuff cones, we infer that they all are roughly similar in age. Radiocarbon dating of a soil above lapilli tephra of Surprise Cone-like chemistry (NA97-6A) returned a weighted mean age of 900+/-80 yr B.P. (tables 1 and 2 [in original text]), giving a minimum age for Surprise Cone. Superposition relationships are unclear, although Windy Cone tephra appear to overlap Breezy Cone. The tuff cones have not been dated directly but are thought to postdate catastrophic draining of the caldera lake to approximately their elevation. Lacustrine clayey silt and sand are present in the breach of Surprise Cone and on a wave-cut terrace north of Surprise Lake at 1,226 ft (374 m) asl (McGimsey and others, 1994). Eruptions responsible for the tuff cones likely took place in shallow water, as suggested by the sediments within Surprise Cone. The basaltic andesite to andesite eruptions may well have been facilitated by unloading that resulted from rapid draining of the ancestral caldera lake to about the ~1,400-ft (430 m) elevation of Surprise and Windy Cone craters, although we cannot rule out the possibility that the tuff cone eruptions occurred earlier, during filling of the lake. Two well-preserved maar craters southeast of Windy cone demonstrate that phreatic or phreatomagmatic explosions occurred in the recent past, presumably under hydrologic conditions similar to present time. A third explosion crater, which also evidently did not produce juvenile ejecta and probably is unrelated to ascent of mafic magma, is approximately midway between Vent Mountain and Half Cone (fig. 10A [in original text])."