Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

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sured ground now occupies nearly twothirds of the former lake's surface. It is very difficult to traverse this ground because of its uneven surface and the thicket of mountain mahogany, which grows in profusion wherever deep fissures provide opportunity for roots to reach water. The strong limbs of this vigorous tree-like shrub make an almost impenetrable tangle. Prior to the opening of the drain around Red Butte, another larger drain at a higher elevation may have stabilized the molten lake level for a long period of time. This outlet was connected to the now-collapsed upstream continuation of Bearpaw Cave (see map 10, pl. 4), and may even have changed its position and depth within the tangled expanse of north-sloping lava lobes between Hippo and Bearpaw Buttes before the outlet became entrenched in the loose ash and cinders along the east side of Bearpaw Butte (fig. 4). Before the lava lake filled to its highest level, much lava from Mammoth Crater had escaped to the northwest through the gap between Bearpaw and Eagle Nest Buttes. These flows rounded the west base of Bearpaw Butte and followed the swale east of Gillem fault (fig. 4) to the shoreline of former Tule Lake. A major lava-tube system developed within these flows, most of it older than the Bearpaw-Skull system whose flows overlap it. As shown on map 19, plate 6, the shallow basins with slumped edges and the surface gutters labeled "Heppe-Modoc system" lie on the top of a ridge that topped the near-source part of this major lava-tube system late in its development. Direct connection downstream, however, is lost in a steeply sloping field of schollendomes and tangled lava lobes and gutters, which lies west of Modoc Crater and northwest of Bearpaw Butte. That the west shoreline of the lava lake once abutted Heppe Modoc ridge indicates that the ridge predated the highest level of the lava lake. The Bearpaw-Skull line of breakdowns is in vertically stacked tubes that began in a lava lake fed by eruptions rising in or near Mammoth Crater. The lava that flowed into Tule Lake at Canby Bay was also delivered via lava tubes originating in Mammoth Crater. Earlier observers (and more recently, Donnelly Nolan and Champion, 1987) thought the Bearpaw-Skullline of lava tubes was fed from Modoc Crater. However, Modoc Crater is apparently surrounded by a sea Figure 68. Oblique aerial view (westward) of former lava lake (outlined), now a maze of rugged collapsed lava. Distance from Bearpaw Butte to Red Butte is about 2 mi. Drain for lake is at Y-shaped intersection of breakdowns west of Red Butte. See fig. 4 for location of these features in Lava Beds National Monument. 94 Selected Caves and Lava-Tube Systems, Lava Beds National Monument, California
of Mammoth lava that flowed around it, united on its north side, and impinged against the southeast part of Bearpaw Butte. The collapse trench upstream from Bearpaw Cave began within this Mammoth lava. No feeder tube from Bearpaw Butte intersects this tube or its breakdowns; instead, the tube cut into and displaced the loose ash forming the east slope of Bearpaw Butte. A small cone on the north flank of Bearpaw Butte did erupt after Bearpaw and Merrill Ice Caves were formed. From this vent a small lava tube, built entirely upon the surface, coursed down the north slope of the satellite cone and stopped at the edge of a large collapse pit in Mammoth lava that flowed around the base of Bearpaw Butte. Shoreline of the Lava Lake To support the interpretation that both the Bearpaw-Skull and Cave Loop Road lava-tube systems originated from a former lava lake fed from Mammoth Crater additional details are needed to document the position of the shoreline of this lake during its most recent stand. Evidence of the shoreline can be observed on the sides of all of the buttes that helped to contain the lake on its north and east sides (fig. 68). Red Butte is closest to the outlet that drained the lake. Remnants of the black Mammoth lava, joined to the reddish pyroclastics of Red Butte, are well exposed where the paved road rounds the northwest side of the butte. Here the elevation of the lava shoreline is 5, 160± 10 ft. The next butte to the north is Hippo. On its southwest flank, parts of a bench of basalt marking the highest shoreline adhere to the side of the butte at 5, 180 ft elevation. Sighting with a hand level from this point to Modoc Crater and to Bearpaw Butte, roughly a mile to the west, one can pick out reference points at the same elevation. These points, and others identified on the ground as the highstand of Mammoth lava against both Modoc Crater and the southeast side of Bearpaw Butte, all fall between the 5, 160 and 5,200 ft contours on the U.S. Geological Survey Medicine Lake 15-minute quadrangle map. Considering the rough margins typical of lava lakes, this is good agreement. On the southwest side of the lava lake, no older buttes display a definite shoreline. However, the fissured and schollendomed area thins and eventually disappears against the higher topography of the preexisting Heppe-Modoc ridge formed by the Heppe-Modoc lava tube (fig. 68) below the 5,200-ft contour (see U.S. Geological Survey Medicine Lake 15-minute quadrangle topographic map). A lava lake should not be thought of as similar to a lake of water in a temperate climate. An arctic lake (or sea) with its crust of ice is comparable, but notable differences remain. Even during the course of a major eruption, a lava lake would be partly covered with congealed blocks as seen during the slow transit of an almost complete cover of floating blocks across the Mauna Ulu lava lake in Hawaii during major eruptive activity. Such blocks of congealed crust will tend to jam together and block any surface outlet, a process which causes constant shift in the position of the outlets and corresponding fluctuations in the elevation of the lake level. Evidence that tangled surface lobes of block-rich lava escaped from many points on the perimeter of the lava lake north of Mammoth Crater can be seen where irregular skeins and lobes of lava poured out and solidified with steep slopes facing to the east between Red Butte and Hippo Butte, to the north between Hippo Butte and Bearpaw Butte, and to the northwest in the area west of Modoc Crater and southwest of Bearpaw Butte. Underground outlets (lava tubes) have a better chance of being preserved when they tap the molten lake beneath its solidified crust. With waning volcanism, however, they become blocked with lava jams and drain out as the lake level drops and its crust collapses into an expanse of schollendomes. Upper Ice Cave Upper Ice Cave is a small lava tube filled with ice that extends beneath the edge of the younger Callahan lava flow just south of Eagle Nest Butte (see map 19, pl. 6). Upstream from the cave, a mile-long chain of shallow breakdowns, small lava gutters, and short lava tubes can be traced toward the two pit craters north of Mammoth Crater. The place where they join the lava tubes from Mammoth Crater is lost beneath a blanket of younger pyroclastic debris and lava lobes from the crater. It seems likely that the Upper Ice Cave line of gutters and shallow breakdowns is a minor distributary developed in the last stages of building the Heppe-Modoc lava-tube system, which extended from Mammoth Crater through the gap between Eagle Nest and Bearpaw Buttes and then on to Canby Bay (fig. 4). Two similar but smaller lava gutter systems drained off this ridge to the west, and each of them originated in small sag basins that cap this ridge north of the Upper Ice Cave lava gutters. Callahan Flow The Callahan flow composed mostly of aa and block lava (fig. 69) is the youngest geologic feature in this area. Donnelly-Nolan and Champion (1987) report a radiocarbon age of 1110±60 yr B. P. Much of this large lava flow poured from a boca (the Spanish word for mouth) on the side of Cinder Butte, a steep-sided cinder cone with a small undissected crater at its top. Cinder Butte is located 0.5 mi south of the southwest comer of Lava Beds National Monument (fig. 4). Collapse Trenches Between Skull Cave and Three Bridges Area More than 40 small caves, and a few that can be traversed for more than 300 ft, lie downstream along the same lavatube system that contains Skull Cave (map 20, pl. 6). Collapse has obliterated most of the primary features of many of these caves, as well as segmenting the tubes into short sections with floors so cluttered with rubble that they are difficult to traverse. Collapse Trenches Between Skull Cave and Three Bridges Area 95
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Lava cascade in Thunderbolt Distrib
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Frontispiece. Two visitors explore
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DEPARTMENT OF THE INTERIOR MANUEL L
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Caves easily accessible from Cave L
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PLATES [In pocket] 1. Caves of the
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Many of the feature names used in t
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Tulelake LOWER KLAMATH LAKE Prisone
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EXPLANATION ................... Lav
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flow, then the lava may pool behind
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"pull outs" where the dripping plas
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Mushpot, Catacombs, Ovis, and Merri
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Waters agreed to do the cave mappin
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into the distributary in the genera
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crawled 180 ft from the Mushpot tub
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es of plaster clog its continuation
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level of the tube. The southern ent
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surface collapse and only one comer
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dead-end passages on different vert
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and a roof covered with lavacicles
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puzzles abound in the Catacombs; do
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Ovis Cave Apparently, E.L. Hopkins
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thus forms a bridge over the broken
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pit is located along the southwest
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lava tubes, and the upper parts of
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occupies the lava pool just upstrea
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underway. Collapse to the surface h
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one located just below ceiling heig
Page 56 and 57: high, but it immediately widens to
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Page 82 and 83: etween the benches. The levees are
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Page 102 and 103: Red Ice Room (fig. 63), which lies
Page 104 and 105: tation that this tube system was fe
Page 108 and 109: The larger and more interesting cav
Page 110 and 111: part talus was reversed. Moreover,
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Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

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