Event Name : Fisher Maar/Flood
|Start: 1500 (± 50 Years) || Years BP C-14 (raw) || |
|Pyroclastic flow, surge, or nuee ardente: ||
|Caldera lake change: ||
From Stelling and others (2014): "The caldera was, at one point, completely filled with water to a maximum depth of ~100 meters, as evidenced by beach deposits high on the inside of the caldera rim (fig. 8 in original text). An explosive phreatomagmatic eruption occurred from within the collapsed Turquoise cone area about 1,500 years ago and likely generated a tsunami in the caldera lake that spread radially (Stelling and others, 2005). Tsunami wave deposits from the collapse of Turquoise cone and subsequent phreatomagmatic eruptions are present at the north shore of the modern western caldera lake and on the isthmus between the northern lakes, along with erosional scars from the waves. The most dramatic effect waves had on the caldera was on the southern wall, where a wave overtopped the caldera rim. Cobbles from a wave-cut terrace on the southern wall have been lifted ~20 meters higher onto the top of the caldera wall. The resistant, caldera-forming eruption deposit that caps the southern caldera wall was incised, and the wave at least partly eroded the softer underlying deposits in the wall to form a canyon below lake level. The combined erosion of the tsunami and the newly-draining lake carved the modern 100-meter-deep southern outflow canyon (figs. 6D and 8 [in original text]), allowing the lake to drain catastrophically. Had the lake drained gradually through incremental erosion of the southern outflow canyon, beach deposits reflecting periods of lower lake levels would be present along the inner caldera wall. The absence of these deposits, as well as the presence of large boulders (up to 5 meters in diameter) of dense pyroclastic flow material torn from the top of the caldera wall and found more than 2 kilometers downstream in the southern outflow canyon, support a catastrophic flood event. During the lake-draining event, Turquoise cone and Mt. Finch acted as earthen dams, preserving the lakes in the northern portion of the caldera."
From Stelling and others (2005): "The collapse of Turquoise Cone was not the last activity in this region of the caldera. Thick surge deposits exposed on the eastern shore of Turquoise Lake were emplaced after collapse, and have been dated at 1500 years (Table 3 [in original text]). Several young cinder cones have developed adjacent to the remnant arcuate ridge of Turquoise Cone. Currently, hydrothermal plumes within the Turquoise Lake are stirring sediment, providing the notable water color. The intensity and distribution of this hydrothermal system is variable, as plumes in the western caldera lake appear and disappear within 1 year."
"After the collapse of Turquoise Cone, only a small arcuate ridge remained above the caldera lake surface. A relatively recent explosive eruption from this vent created a small mound of red-orange scoria that extended the southern limb of the Turquoise Cone amphitheater. Hydrothermally altered lithics associated with this event are found strewn across the caldera, suggesting a phreatic component to the eruption. Soils immediately below this deposit have been dated at 1500 (+/-50) years. We suggest this eruption generated a wave in the caldera lake that spread radially. Wash deposits from this wave are found at the north shore of the modern west lake and on the base of the small cinder cone (IC in Fig. 3C [in original text]) between the northern lakes, along with erosional scars from the wave itself. The most dramatic evidence for this wave appears on the southern caldera wall. Cobbles from the wave-cut terrace on the southern wall are found ~20 m higher, where they overtop the lowest part of the caldera rim. The densely welded pyroclastic flow from the caldera-forming eruption that caps the southern caldera wall has been deeply incised at this location. Within the southern canyon, large boulders (up to 5 m in diameter) of welded tuff are found several kilometers downstream from the nearest densely welded exposure of the flow, indicative of a flood deposit. Similar features have been found in the outflow canyons of Medicine Lake volcano in California (Donnelly-Nolan and Nolan, 1986) and Aniakchak Volcano in Alaska (Waythomas and others, 1996). Organic-rich soil directly above this flood deposit has a 14C date of 1500 (+/-50) years, matching the date of the Turquoise Cone explosive eruption. The presence of flood deposits in the southern canyon suggests that the wave broke over the southern caldera rim and quickly eroded through the welded tuff cap rock. A narrow slot canyon was carved into the caldera wall, catastrophically draining the lake. Additional wave cut terraces below the one marking the upper lake surface are lacking, suggesting that the lake drained rapidly."