Geological Survey Bulletin 1673 Selected Caves and Lava Tube Systems in and near Lava Beds National Monument, California

Crystal Cave

Crystal Cave consists of collapse remnants of three major levels and of a few smaller lava tubes superposed upon one another (map 18, pl. 6). The cave extends beneath the floor of the large collapse trench that marks the course along which molten lava was delivered from Mammoth Crater through lava tubes that extend far beyond the Cave Loop Road area. The upstream termination of the mapped part of Crystal Cave is below a point 50 ft upstream from the lower end of the deep collapse trench that lies between Natural Bridge and Ovis Bridge. Ovis Bridge and Ovis Cave are uncollapsed sections of a large lava tube whose course lies almost directly above the lava tubes that compose Crystal Cave. To prevent a clutter of heavy lines on the map of Crystal Cave, only a small part of the overlying collapse trench near the cave entrance is shown, but details of the geometry of Crystal Cave in relation to the collapse trench, Ovis Cave, and Ovis Bridge can be seen by visually superimposing the Crystal Cave map (map 18, pl. 6) upon the Ovis Cave and Paradise Alleys map (map 4, pl. 2).

The only entrance to Crystal Cave is covered by a locked grating. Once across the ice slope at the foot of the entrance ladder, it is fairly easy to visit three levels, which have a combined length of 2,890 ft (see map 18, pl. 6). From the Blue Glacier Room, where continuation of the lower level is closed upstream by ice and a roof collapse, one can traverse an ascending ledge to a higher passageway (the Overpass level), which gives access to 200 ft of a lava tube directly above the upstream continuation of the lower level. From the Overpass two additional parts of the cave can be reached. One is Fantasy Passage, a segment of a lava tube lying 35 ft above the Overpass level; the other is Crystal Grotto, a segment of the lower level that extends upstream from the foot of a steep ice cascade descending from the upstream end of the Overpass.

Crystal Cave surpasses all other caves in the monument in the beauty and variety of its ice deposits. J.D. Howard named the cave from large (as much as 2 in. in diameter) ice crystals that occur in the hoarfrost, which forms a rime over parts of the cave's walls and roof. Beautiful draperies of large clear icicles hang where springs enter the levels from high ledges and water drips from cracks in the roof. Pools of clear ice accumulate in hollows on the floor. Some sections of cave floor are carpeted with a litter of tumbled frost crystals and broken icicles. Such litter is also welded onto rock blocks and crystal-clear ice stalagmites.

Upper Level

A combination of ice and collapse debris blocks the upper level downstream from the foot of the entrance ladder; but upstream the cave opens into a large chamber called Entry Hall, which is 18-25 ft high, 25-35 ft wide, and 100 ft long. In the southwest part of this chamber the ice floor gives way to a hummocky deposit of thick collapse rubble. Upstream 80 ft from the ladder the left (southeast) wall of the cavern contains a semicircular alcove 30 ft across. Here the rubble on the floor of the cavern slopes steeply into a small breakdown beneath the south wall of the alcove. This opening provides the only entrance into the middle and lower levels of Crystal Cave.

Beyond this alcove the upper level narrows abruptly to 8 ft, but it maintains ceiling heights of 18-25 ft that characterize the entrance cavern downstream. Shelf-like projections a little more than halfway up the walls are the broken edges of parts of the floor, clear evidence that this narrow but high level once consisted of two superposed tubes that have merged into a single passageway by collapse of the upper tube. Molten lava must have filled the tube after collapse because in places vertical lava plaster coats the surface of the broken shelves and remnants of the floor of these upper tubes. Above a few such floors are openings 2-3 ft high, sufficiently large to enter and explore, but because of their low ceilings and inaccessibility, these small flat openings were not mapped.

This composite tube extends upstream for 110 ft and then abruptly doubles in width. Remnants of the upper tube are still present in this section but are less abundant than in the narrow part of the upper level. The wider section is blocked 90 ft farther upstream by collapse rubble encased in ice. A fine display of icicles 5-15 ft long forms a drapery along the south wall adjacent to the collapse. A narrow ice cascade decorated with ice columns occupies a bottomless roof tube down the center of the ceiling at a point 80 ft downstream.

Middle and Lower Levels

The middle and lower levels are described as a composite unit because one is superposed exactly above the other and the two have merged completely, due to collapse, for over 250 ft downstream from the Red Ice Room. Moreover, the two levels are connected by five additional collapse openings farther downstream. On the composite map view (map 18, pl. 6), the lower level is offset to avoid the lines of the other levels. The middle level is shown with a dashed line on this composite view. Match lines (see map views of each level on map 18, pl. 6) denote the five breakdowns connecting the two levels. One of these match lines marks the connection and provides the only easily traversable route between the middle and lower levels. The other four openings can be negotiated only with ladders or ropes.

It might appear that the middle and lower levels were originally one lava tube, which later became segmented into two tubes by balcony building. This hypothesis would imply half-filling of the original tube with molten lava, which ponded long enough at the height of the middle level's present floor to allow solidification of a thick crust over the top of the lava pool. Then, perhaps by collapse of an obstruction downstream, the still molten lava beneath the crust would have drained away and left the crust as a septum dividing a former large tube into upper and lower parts. Where the crust was thin, parts of it might have tumbled into and been carried away by the lava as it withdrew. Only the margins were left hanging as balconies along the wall.

This hypothesis does not stand up, however, even though there is much evidence of local balcony building at various times during the formation of the middle and lower levels. The detailed history of lava occupancy and other events, which shaped these two levels, is far too complex to unravel completely from the limited exposures provided by Crystal Cave—only a small sample of a lava-tube system many miles long. The collapse trench and other associated features show that the tube system extended far upstream and downstream from the places where its continuations are blocked by collapse debris or by ice in Crystal Cave. Nevertheless, even a leisurely 2-hour traverse through the middle and upper levels of Crystal Cave discloses a long and complicated history of recurrent filling and draining of molten lava, not a single episode of lava ponding, balcony building, and drainage. Significant parts of this record are summarized below.

1. If we could remove the septum between the middle and lower levels—which is exactly what happened in the upstream 250 ft of the lower level—we would see that the cave's cross section does not have the oval shape indicative of hydraulic equilibrium from a single lava tube encased in a basalt flow. Instead, this cave is elongated vertically; ceiling heights are 35-45 ft and widths are less than 20 ft except where enlarged by collapse of the walls. This shape implies the merging by collapse of at least two superposed tubes.

2. In the upstream 250 ft of the lower level, more than one basalt flow can be seen in collapsed parts of the walls. The downstream half of the lower and middle levels cuts into red tuffs and breccias, which underlie the basalt flows constituting the wall rock of the upstream part of the cave. Good contact relations between a basalt flow and the red breccia immediately beneath it can be seen in the middle level just below the wall of the access breakdown between the upper and middle levels, and on the north wall of the middle level 120 ft farther upstream. In the downstream part of the lower level, red breccia and tuff is present nearly everywhere slides or cracks in the wall lining of the tube expose the wall rock. Continuation of the lower level downstream is blocked at the Red Ice Room by red breccia rock slides cemented with ice. Other cracks and slides in the coatings of lava plaster and dripstone that line the lower level show that this part of the lava tube is completely surrounded by red breccia. For 120 ft upstream, most slides and cracks in the walls also reveal red pyroclastic material as the dominant type of wall rock. (For hypothesis on the origin of the red color in these breccias and the probable mechanics of emplacement of lava tubes within red pyroclastics, see "Red Tuff and Volcanic Breccia" section and map 12, pl. 4.)

3. The septum between the middle and the lower levels is more variable in thickness than would be expected from simple balcony building. This is best seen by comparing the thickness in the walls of the six breakdown holes through this septum. Also, note the measured thicknesses shown in the longitudinal section and compare with ceiling heights shown on map 18 (pl. 6); as shown on the longitudinal section, remnants of a shallow-ceilinged tube are exposed within those breakdowns where the septum is thickest.

4. The walls of the breakdowns also record episodes of lava refilling after the collapse occurred. In several places remnants of lava plaster or dripstone coat the walls of a breakdown. The first three breakdowns upstream from the one noted by the match line (map 18, pl. 6) show large patches of such plaster within their openings, and through them lava overflowed from the lower level and spread as lobes over the Middle Earth Passage floor (position and direction of flow shown on map). Many more lobes undoubtedly spread from several breakdowns on this level but are buried beneath the thick mantle of collapse rubble covering the floor of the middle level.

5. Many streams of lava that coursed through the lower level during late stages of volcanism were not voluminous enough to inundate the middle level. Numerous narrow benches and stretched or sheared curbs of formerly hot and sticky lava indicate the position of high-lava marks. A particularly informative place to examine how these congealing features were sheared by the moving lava is a 200-ft stretch of the southwest wall in the middle of the lower level (fig. 25 and map 18, pl. 6).

6. One of the final streams of lava, only about 6 ft deep, built a balcony completely across the floor of the lower level for a distance of 100 ft. When the thin crust forming this balcony was less than 2 ft thick, the still-molten lava below it drained and left a shallow flat-topped tube-in-tube only 2-5 ft high. Collapsed areas in its roof give access to this tube-in-tube (see longitudinal section on map 18, pl. 6). Downstream from the lowest breakdown this crust was too thin to survive collapse, and farther downstream continuations of this crust are preserved only in discontinuous remnants of a 3-ft bench along the walls.

Overpass Level

At a point 40 ft downstream from the Blue Glacier, a narrow ramp-like ledge starts at the floor on the west wall of the lower level and climbs at an angle of 20°-30° up the wall until it enters a hole in the roof. Here the ledge widens and is obviously the congealed surface of a lava cataract, which continues upstream another 35 ft, where it opens into a high-ceilinged room in an overlying level. The ramp along the wall is the collapsed lower end of this cataract. The level at the head of the cataract can be traversed for another 200 ft upstream before it is blocked. Near its upstream end, a hole 10 ft long and 2-3 ft wide extends across part of the floor of the level. This hole drops onto a very steep ice cascade, which descends precipitously into a room called "Crystal Grotto" that is evidently an upstream segment of the lower level. Because this level lies above a collapsed part of the lower level, to which it is connected by a lava-cataract ramp on one end and an ice cascade on the other, we named it the "Overpass."

A large roof collapse near the downstream end of the Overpass reveals segments of still another lava tube, the Fantasy Passage, 35 ft above the floor of the Overpass (see longitudinal section on map 18, pl. 6).

Original details of the walls, floor, and roof of the Overpass have been largely obliterated by collapse or obscured by rimes of hoarfrost. The floor of the middle section of this level is considerably lower than either end because it tumbled into and filled the lower level below. An ice pool formed in this low spot, and undoubtedly the ice here is interconnected with the ice that fills the spaces of the collapse breccia in Crystal Grotto upstream from the Blue Glacier Room. Upstream from the ice pool, much of the Overpass level's collapse debris is also cemented by ice. Drip water collected and flowed along the floor adjacent to the east wall, producing an "ice brook" that steepens into an ice cascade as it approaches the pool.

Ice Deposits

There are two basic requirements for permanent ice to develop in caves within a temperate climatic zone: (1) Rainwater or snowmelt must penetrate down to the cave through cracks or other openings, and (2) the air in the cave must remain relatively stagnant throughout the year. Strong underground circulation of air through caves—particularly those that have a surface opening in the low area of a basin or valley and a second on a high ridgecrest—prevents the ice that may form in winter from surviving the following summer.

Crystal Cave is not only an excellent example of conditions that are almost ideal for the growth and stabilization of ice in caves, but it also demonstrates the use of the convection principle in discovering previously unknown caves. Aside from small pores and cracks through its roof rock, Crystal Cave has only one surface opening: the entrance collapse, located at the highest point of this cave's underground passageways. In the winter months the cold dense air above ground descends through this hole and displaces the lighter and warmer air within the cave up to the surface. On a cold, sunny, and windless day in winter, especially when the temperature has dropped substantially overnight to below freezing, the cave passages appear to be "breathing." As the cold surface air descends into the cave, the lighter, warmer, and more moist air is pushed upward onto the surface. Chilled in the freezing temperature above ground, the small amount of water vapor dissolved in the cave air immediately condenses into tiny water droplets, just as your breath does in cold air each time you exhale. Thus a persistent plume of fog rising from a patch of loose boulders indicates that a minor excavation of the boulder pile might reveal an opening into a cave,

In caves like Crystal the cold air that seeps down to fill all the passageways in winter does not warm in summer. Because the summer air outside the cave is less dense than the heavy cold air inside, it cannot descend to displace the cold air. By late summer, however, the temperature of the air in such a cave may rise as high as the average annual temperature of the region. As soon as it warms above 32 °F melting of the ice begins. However, because it requires 80 calories to melt one gram of ice and a further 539 calories to convert a gram of ice water to water vapor, the temperature of the almost stagnant air immediately adjacent to the ice mass soon stabilizes at or near the freezing point of water (32 °F). Ice will either melt or build up by filling the openings until a fairly stable equilibrium is reached in which the amount of ice melted and vaporized in summer fluctuates little from the amount of ice accumulated during the winter months. It is a sensitive equilibrium: the heat supplied and the agitation of the air caused by a large number of visitors passing through a cave in summer increase melting; visitors in winter add to the ice by precipitation of moisture from their breath. This is the reason that access to Crystal Cave is restricted to small groups of visitors on a limited schedule.

Ice in Crystal Cave occurs in four main forms:

(1) Icicles and Ice Stalagmites. Crystal-clear icicles form where water drips from cracks in the roof or dribbles over ledges, and ice stalagmites grow up from the floor where falling water droplets freeze on impact.

A spectacular drapery of coalesced icicles festoons the southeast wall of the lower level from 145 ft to 200 ft downstream from the Blue Glacier Room near the Dolphin, a 4-ft-high ice stalagmite (see map 18, pl. 6). Individual icicles in this drapery are up to 20 ft long (fig. 61) whereas others are coalescing and overlapping shafts 3-15 ft long. All are so transparent that details of the wall rock behind them are clearly visible through the 0.5- to 3-ft-thick covering. Ice stalagmites have grown up from the floor to meet the icicles in parts of this drapery but are not conspicuous. On the other hand, large stalagmites as much as 4 ft high and several feet in diameter are scattered along the floor of the cave in front of the icicle drapery and for another 50 ft downstream. No correlation appears to exist between the size of a stalagmite on the floor and the size of an icicle directly above it; some large icicles have grown completely from roof to floor without a companion stalagmite rising to meet them (fig. 62). Variations in the ratio of water supply to rate of freezing appear to cause these differences in size of icicles versus ice stalagmites. A similar, smaller drapery of icicles occurs at the upstream end of the upper level and throughout Fantasy Passage.

Figure 61. Ice drapery on wall of Crystal Cave (see fig. 14 and map 18, pl. 6).

Figure 62. Ice stalactites from roof joined floor to form this ice fall in Crystal Cave (see fig. 14 and map 18, pl. 6).

One of the most beautiful displays of icicles and other ice forms is found in the Red Ice Room (fig. 63), which lies at the extreme downstream end of the accessible part of the lower level. The 35 ft long and 20 ft wide Red Ice Room is seldom visited because it requires a crawl in ice water beneath a 1-ft-high roof. This crawlway is at the downstream base of the large collapse pile, which descends to the lower level. As previously noted, red tuffs and breccias line the tube in this passage. It is these walls of red pyroclastics enhanced by draperies of icicles and a varnish of clear ice that give color to this room—not red ice, but clear icicles and ice dripstone covering a pink to red background. In addition to "pink" icicles, this room contains a floor of "pink" ice. Slides of coarse red rubble that block the tube downstream are cemented by and coated with transparent ice as well.

Figure 63. Icicles, ice glaze, and ice crystals decorate wall of Red Ice Room in Crystal Cave (see fig. 14 and map 18, pl. 6).

(2) Ice Pools and Ice Mounds. Melt-water pools that have frozen into ice are rare in Crystal Cave, but large parts of the cave floor are covered by ice mounds and hummocky ice slopes. These are built by drops of water and melted bits of icicles and frost that fall from the roof and walls. The ice slope below the entrance of Crystal Cave is a large mound that accumulated from rain and snow blown in through the entrance. The semicircular ice mound along the west wall another 20 ft downstream is an overgrown stalagmite formed by drip from the roof. At the south end of the lower level is the Blue Glacier Room, so named because the large ice mound on its floor is a murky powder blue. It is not known whether admixed dust, algal growth, or a finely cracked texture of the ice is responsible for the scattering of light that causes this color—it may be a combination of all three.

(3) Ice-cemented Breccia. Collapse breccia at the upstream and downstream ends of the upper level are encased in ice, but because of the rock color here, the results are less spectacular than the ice-encased breccias in the Red Ice Room. As noted before, almost the entire floor of the Overpass is a hummocky pile of collapse blocks encased in ice, as are the breccias adjacent to the Blue Glacier, which block upstream continuation of the lower level.

(4) Frost. Rivaling the draperies of icicles in beauty are the rimes of pure white hoarfrost that cover large parts of Crystal Cave's walls and roof (fig. 64). Hoarfrost is extensive near the larger masses of permanent ice, especially in Fantasy Passage, and can also be found spreading outward from the large drapery of icicles on the lower level. It grows most abundantly on the one lava flow in the wall that is more porous than the others. Unlike the transparency of an icicle display, hoarfrost forms pure white rimes that appear to be opaque when viewed at a distance. Closer inspection shows that they are composed of intricately interlocking frost crystals, some in well-developed hexagonal plates or in cleavage rhombs two inches or more across. Others are lacy and interlocking crystals of varying sizes. In fact, each crystal is transparent or translucent, but the lacy intertwining forms and abundant cleavage cracks give frost rimes a whitish luster (fig. 65) quite different from the limpid transparency of ice stalagmites and icicles.

Figure 64. Hexagonal ice crystals as much as 1 in. across present in winter on walls of Fantasy Passage, Crystal Cave (see fig. 14 and map 18, pl. 6).

Figure 65. Ice stalactites in the Red Ice Room in Crystal Cave (see fig. 14 and map 18, pl. 6) composed of 1-in-diameter hexagonal ice crystals stacked one atop another.

Some of the frost crystals show evidence of repeated, probably seasonal, growth. Zonally arranged inclusions of fine dust were deposited upon an irregularly etched crystal, then covered by an overgrowth of clear ice, which is in optical continuity with the core crystal. Several such zonal bands can be counted within some of the larger hoarfrost crystals. Very likely they record seasonal changes: etching and pitting of the crystals during the late spring and summer thaw; dust and some organic debris deposited in the late summer and autumn; and another clear ice rim added during the late winter and early spring.

Hoarfrost in the Red Ice Room varies in appearance from pale pink to white, depending on the thickness of the rime. In places, however, so much red dust has been entrained within the crystals that they retain their pink color even if removed from the wall.