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

SELECTED CAVES AND LAVA-TUBE SYSTEMS IN AND NEAR LAVA BEDS NATIONAL MONUMENT, CALIFORNIA

By AARON C. WATERS, JULIE M. DONNELLY-NOLAN, and BRUCE W. ROGERS

Lava Beds National Monument (fig. 1) lies on the north slope of the huge Medicine Lake shield (fig. 2), a complex volcanic edifice of greater volume than the steep-sided Mount Shasta volcanic cone, which towers as a snowclad landmark 40 mi southwest of the monument (fig. 3).

Figure 1. Index map showing Lava Beds National Monument, Medicine Lake volcano, and distribution of basalt of Mammoth Crater (shaded pattern), which is host to most of the lava-tube caves in the monument. (click on image for an enlargement in a new window)

Figure 2. View of Medicine Lake shield volcano from northern edge of Lava Beds National Monument. Gillem Bluff to right. Field of view is about 15 mi across.

Figure 3. Late afternoon view across Lava Beds National Monument with upper part of Medicine Lake shield volcano on left skyline. Snowcapped Mount Shasta is about 40 mi distant on right skyline. View is southwestward.

Much of the north and south flanks of the Medicine Lake shield were built from molten lava transmitted through lava tubes. These tubes formed beneath the congealing surface of basalt flows in somewhat the same way that a brook may continue to flow beneath a cover of its own winter ice. As molten lava emerges from a vent and flows downslope, congealing lava from the top and sides of the central channel often forms a bridge over the lava stream. The sticking together of bits of lava spatter and fragile lava crusts strengthens the bridge in the manner that thin crusts of floating ice raft together to cover a brook during early stages of a winter freeze. Eruption of basalt lava, however, is a much more violent and spasmodic process than the steady gathering of water that feeds a brook. If liquid lava stops rising from its source deep within the earth, the still-molten lava moving beneath the crusted-over top of a lava flow will continue to drain downhill and may ultimately leave an open lava-tube cave—often large enough for people to walk through. It is rare, however, to find such a simple scenario recorded intact among the hundreds of lava-tube caves in the monument. Even before the top and walls of a lava flow have time to cool during a pause in lava supply, a new and violent eruption of lava may refill the open tube, overflow its upper end, and spread a new lava flow beside or on top of the first flow. Even if the original tube is large enough to contain the renewed supply of lava, this tube must deliver the new lava beyond the end of its original flow and thus the lava field extends farther and farther downslope. If the gradient of flow flattens, the tube may subdivide into a number of smaller distributaries, which spread laterally over the more gently sloping ground.

Within Lava Beds National Monument, most lava tubes are found within the basalt of Mammoth Crater (figs. 1 and 4). Complicated and intertwining lava-tube systems originating from Mammoth Crater and other vents have built a broad fan of complexly interfingering lava flows that form the northeast perimeter of the Medicine Lake shield. Most of this lava was delivered through lava tubes. Some tubes conveyed lava underground 15-20 mi from their sources. Nevertheless, today one cannot walk for a distance of even 4 mi within any one lava tube. Large parts of the roofs of most lava tubes have fallen in, hiding the floor of the tube under huge piles of breakdown or angular broken rock, often stacked so tightly that access to both upstream and downstream portions of the tube is closed. In some places, however, collapse of the tube's roof has provided a large entrance into the lava tube through which one can walk with ease. In some collapse piles where access appears to be lacking, one can search the maze of tumbled blocks and perhaps find a crawlhole into a lava tube. Openings into caves may be detected by noticing the runways of small animals or testing the direction of air flow. On sparklingly clear, very cold winter days, openings into underground caverns will emit a white fog, just as one's exhaled breath does on such a day.

Figure 4. Location map of Lava Beds National Monument showing major lava-tube systems, cave locations, and other selected features. Basalt of Mammoth Crater shown in blue. Other lava flows shown in red. (click on image for an enlargement in a new window)

Holes in the landscape surface formed by failure of part of a lava tube's roof are called collapse pits, breakdowns, or more commonly, collapse trenches (see maps 2, 5, 10, and 20; plates 1, 2, 4, and 6). While walking across the relatively flat surface of the lava flows, you are seldom aware of their presence until a large and deep hole yawns at your feet. Some small breakdowns are dangerous death traps for animals. Unwary humans have met a similar fate (see map 12, pl. 4, and the "Skull Cave" section).

Once underground within a lava tube you may find your way impeded or blocked by a variety of features. Piles of loose rock that have peeled off the ceiling and walls of the tube may clutter the floor of the cave and slow your pace. Where no fallen blocks are present, the smooth to ropy (pahoehoe) surface of the lava on which you walk may change gradually to a very rough surface composed of bubble-filled loose blocks of a spiny (aa) lava. In some cases it may even completely block the cave entrance. The words pahoehoe and aa come from the Hawaiian language. Most lava tubes are found in pahoehoe lava (e.g. Greeley, 1971a; Harter, 1971), but occasionally they occur in aa lava (Guest and others, 1980).

Geologists recognize several varieties of pahoehoe (MacDonald, 1953; Wentworth and MacDonald, 1953). The smooth but thin and partly congealed skin on the surface of the molten lava may become wrinkled and twisted into small ridges that resemble ropes, as the hot and plastic crust is dragged along by molten lava beneath. These ropes in turn may be dragged and stretched out into attenuated lobate forms (fig. 5). Near the end of the period of consolidation some ropy pahoehoe may be cut by closely spaced vertical shears to form laminated or cauliflower pahoehoe.

Figure 5. Ropy pahoehoe. A pasty red-hot rind of partly congealed magma at the surface of a lava flow was folded and twisted into rope-like ridges as it was dragged forward by the molten rock beneath. Chilled by air, the lava surface congealed into lustrous black glass. Near Giant Crater, south flank of Medicine Lake volcano (see fig. 1).

Subtle transitional changes in a pahoehoe surface can be recognized where pahoehoe changes to aa downstream (Peterson and Tilling, 1980). The smooth to ropy forms begin to lose the glassy luster that formed as a thin skin of chilled basalt glass, and the small spherical bubbles confined beneath this glass skin increase in number, grow larger, and become visible on the surface as bumps and broken bulges of the glass crust. As the bubbles grow larger and more irregular in shape, many of them explode outward through the sticky glass crust, and with further movement this prickly surface breaks up into small discrete blocks, completing the transition to aa lava (fig. 6).

Figure 6. Increasing vesiculation and turbulence may cause a lava flow to change from pahoehoe to aa downstream.

In lava tubes the transition from pahoehoe to aa is frequently found downstream from an area where molten lava was violently churned up while tumbling over a lava fall or down a series of lava cascades.

Two additional varieties of pahoehoe are commonly recorded by observers of actively erupting flows in Hawaii but are more difficult to recognize in the congealed flows at Lava Beds National Monument. Pahoehoe toes from 1 to several feet in length may sprout forward all along the front of some advancing lava flows. In places the chief manner of forward movement is by the extending and overriding of successive pahoehoe toes. Shelly pahoehoe congeals where large hollow lava blisters 3 ft or more in diameter have formed beneath a thin crust of erupting volatile-rich lava. These large lava blisters flow out, flatten, and override one another. Fleener Chimneys, in the monument, erupted shelly pahoehoe as the last part of the eruption that produced the Devils Homestead flow. Much of this flow is aa (fig. 7), particularly farther from the vents at Fleener Chimneys.

Figure 7. Example of aa lava, broken surface of Devils Homestead lava flow (see fig. 4).

The bubbles and blisters that form in molten lava are produced by release of water and other gases from the molten rock. When pressure is lowered by rise of molten liquid, called magma, to the surface, or by the turbulence of tumbling over a cascade, the lava may froth just as the dissolved carbon dioxide in beer will froth and form bubbles as you open the can and tumble the beer into a glass.

Not all collapses of lava-tube roofs took place after volcanism ceased. Many lava tubes contain easily decipherable records of breakdown that occurred when molten lava was flowing through a tube. Careful examination of the congealed surface of the last flow of lava down a tube is likely to reveal both small and large blocks of rock that tumbled from the roof of the cave and were then rafted downstream on the molten flood until it, in turn, congealed into rock. Large rafted blocks are shown on the maps of this report.

If a large segment of a tube roof collapses while the tube is still filled or half-filled with flowing lava, a number of events can occur that leave their record in the rocks to be examined long after volcanism ceased. If the tube is only half full of flowing lava, and the thickness of collapse debris is nearly equal to the flow, then the lava may pool behind the obstruction, flow over the tumbled blocks, and cascade off the downstream side. Alternately the molten lava may penetrate between the fallen blocks and buoy them up enough that with the additional hydraulic energy of lava ponding behind the obstruction, the flow is able to entrain and bulldoze enough of the obstruction for the lava river to restore its former gradient. Much of this buoyed material is deposited downstream in alcoves, where the tube widens, or on the inside of curves, where the stream velocity slackens. Examples of these features are well preserved in the central part of Valentine Cave (map 8, pl. 3), in parts of Tickner and Berthas Cupboard Caves (map 9, pl. 3), and in many other lava-tube caves.

When a roof collapse is so large that it effectively plugs a tube filled with flowing lava, the molten lava in the tube downstream from the obstruction flows on, leaving an open lava tube; however, minor leaks through or around the plug may continue to feed a small flow into the eviscerated tube below. Upstream from the plug the molten lava backs up and fills the tube to its roof. This process gradually increases the hydraulic pressure on all parts of the tube until a weak spot is opened, generally in the cave's roof. The lava then pours out of this hole and forms a new surface lava flow, which spreads downstream from the point of egress. As this flow advances downslope one or more lava tubes may develop within it. With further spreading and subdividing, one lobe may find a breakdown leading to an open tube below. Thus a part or all of the flow may be diverted, tumbling as a lava fall through this breakdown—perhaps into the same tube that was plugged by a breakdown upstream.

Studies of the many lava-tube caves in the monument also provide alternate interpretations of what has happened in places under essentially these same conditions. If the obstruction cannot be bulldozed away by the lava, the pressure of backed-up lava may also be relieved by the formation of a bypass around the obstruction. Such a bypass is very possible if the flowing lava remained hot beneath its already firmly congealed crust. The hotter liquid magma within the tube simply pushes the cooler, plastic material aside, and a bypass is formed around one side of the obstruction. In some tubes two bypasses may form, one on each side. Such a double bypass is present near the downstream end of Tickner Cave (map 9, pl. 3).

Relief of the pressure in a backed-up lava tube can also come from collapse of the floor of the filled lava tube downward into an underlying lava tube. In each of the three major lava-tube systems in the monument there are numerous examples where this has happened. If the lava in both tubes then drains out, the connector, as the underground collapse conduit is called, remains open and can provide access to a cave passage that might never have been discovered otherwise, connectors, once formed, tend to persist. The Silver Connector, shown on map and section of Post Office Cave (map 15, pl. 5), passes through lava tubes at five different levels, but it is entirely underground—not a surface collapse. Flowage within connectors was not always down. Some of them transmitted lava from a lower ponded level to an upper open level, but the evidence for this is not likely to be discovered unless the plugged lower level also obtained release at some lower point to allow both it and the connector to drain. Otherwise the connector remains filled with congealed lava and so would remain unidentified or possibly be mistaken for the vent of a new volcano.

Indeed, open lava tubes, and open connectors of any kind between lava tubes, are unusual features. From the very nature of the way they develop, lava tubes cannot remain open unless the lava field forms over a topographic slope that affords sufficient gradient for lava to drain out of the tube after eruption ceases. Tubes cannot develop within lava that remains ponded until solidification. Furthermore, flowing lava, like water, spills into any opening available. So it is quite normal that a walk downstream within a lava-tube system will reveal that each lava tube and its distributaries are likely ponded to the roof with the final flow of lava that entered. You will first notice that lava on the floor of the cave begins to rise against the walls of the cave, and it acquires a smooth ponded surface with few of the usual pahoehoe ropes. The surface of the pond appears to rise downstream until it intersects the roof of the cave; actually, it remains level, whereas the ceiling and floor of the tube slope downstream. The lava that rose in the tube was pooled by an obstruction to this level, and congealed because it was unable to drain out.

From studying partly eroded shield volcanoes, geologists find that lava tubes containing a filling of congealed lava are much more abundant than open lava tubes. Open lava tubes will be more common among the youngest lava flows in a volcanic pile, for most older open lava tubes may have been filled with the lava from later eruptions. It has been estimated that only 10 to 20 percent of the lava tubes of a flow drain and remain accessible to an explorer. Nevertheless, because of the complexities of intermittent pauses and recurring floods of magma, combined with the interruption of flow in tubes by roof collapse, it is likely that a few lava tubes will remain open on the steeper flanks of a shield volcano, even if they are buried under hundreds of feet of new flows.

Some lava tubes receive fillings of material other than lava. Sand, gravel, or volcanic ash washed in by surface water may fill them. A rise in the water table after volcanism ceases may drown the underground passages. In the monument, large tubes that are 100 ft or more below the ground surface may be filled completely with ice, or else have their walls decorated by a frieze of large frost crystals interspersed with draperies of long icicles. Crystal Cave (map 18, pl. 6) is an outstanding example.

Intact parts of the ceiling in most caves show fine displays of lavacicles. As the name implies, they are like icicles but were formed as molten lava dripped from the roof of the cave. Undamaged parts of most tube walls show linings of dripstone (figs. 8 and 9). These capture the flow forms taken by congealing liquid lava as it splashed against or dripped off the walls of a tube when the lava surface quickly lowered in the tube. Lavacicles can weld together into a dripstone drapery where lava drips slowly from an overhanging ledge.

Figure 8. Lava dripstone trails down wall of Post Office Cave (see fig. 4 and map 15, pl. 5). Reddish color was produced by oxidation of hot lava surface.

Figure 9. Lava driblet on wall of Fern Cave (see fig. 4 and map 17, pl. 5). Pencil for scale.

Because of similarity with features found in limestone caves many authors use the name lava stalactite instead of lavacicle. The process of formation, however, is utterly different. Limestone cave stalactites are formed from material precipitated as a water solution degases and evaporates. Icicles and lavacicles are caused by the freezing of a liquid. Many stalactites in limestone caves have a companion stalagmite that grows up to meet them when water droplets falling from the tip of the stalactite degas and evaporate on the cave floor, leaving a deposit.

On the surface of some large rafted blocks, however, splatters of lava and pieces of plastically deformed lavacicles that tumbled onto the block as it traveled down the lava tube are likely to be present. Companion lava stalagmites (fig. 10) are sparse in lava tubes because drip from the tip of a lavacicle in most cases fell into the molten flood below. Where the floor had already solidified, stalagmites consisting of droplets of lava welded together are often present. Occasionally these display frozen rivulets of lava, which ran down their sides and partially smoothed their surfaces.

Figure 10. Lava stalagmite formed by dripping of still-hot lava from ceiling of Post Office Cave (see fig. 4 and map 15, pl. 5) onto still-moving flow. The 2-ft-high stalagmite was apparently rafted downstream from the ceiling drip that formed it. Hammer for scale.

High-lava marks on the walls of a tube, like the high-water marks of a river in flood, record the position of lava at some former high stage in its flow. If lava remains constant for considerable time at one level high within a tube, the congealing of the lava surface inward from the walls may build a lava balcony; if ponding occurs lower on the walls (less than 3 ft), a lava bench may form. Most maps in this report show where balconies and benches are present. For excellent examples, see the maps of Silver (map 14, pl. 5), Tickner (map 9, pl. 3), Balcony (map 13, pl. 4), and Valentine (map 8, pl. 3) Caves.

In places, a flow that was building paired benches—one from each wall—may form a crust of congealed lava extending completely across the tube. If the still-molten lava flowing beneath this crust drains out later, a two-storied tube remains—an upper older story—beneath which a newer lower tube remains active. If the magma in the lower story then drains out, a tube-in-tube is formed. Another type of tube-in-tube forms when a small lobe of new lava invades an older and larger open lava tube and then drains out soon after a thin exterior crust has solidified. Even more interesting examples of stacked tube-in-tubes occur in places where small tubes, 3 to 7 ft in diameter, have been occupied by brief periodic surges of lava—a crust forms that encircles each new surge of lava, but if the flow is too small to fill the tube, this new crust develops some distance from the roof and upper walls, while firmly attached to the floor. Thus a few flow surges of diminishing size will produce tube-in-tubes stacked within one another that resemble nested concrete culverts of varying size. Examples may be seen in Tickner Cave, and at the downstream terminations of Arch and Silver Caves.

The formation of thin accretionary crusts of basalt magma at places where it comes in contact with air or with cold rock is responsible for many interesting minor features, both on the surface and within lava tubes. At first these crusts are plastic and mobile, and with added cooling they may be folded into many small lobes whose surfaces resemble sections of coiled ropes congealed into stone. Such accretionary lava crusts are visible in many lava tubes. Coatings of lavacicles on the roof of a cave may have peeled off and exposed another thin layer underneath, which also has lavacicles. Observe the dripstone on the wall of a cave over an area of several square meters, and you are almost sure to find "pull outs" where the dripping plaster of this final coat sagged down or peeled away from the wall (fig. 11). Behind the pull out another layer of dripstone is exposed on the wall. Examine the cross section edge of a large lava tube sliced by a major breakdown, and you will probably see layer after layer of accretionary lava plaster called linings welded together in the cross section of the tube. Every accretionary layer represents a separate volcanic surge followed by a period of quiescence. Most accretionary layers, as can be seen by their tight welding, resulted from small-scale fluctuations in the amount of magma coursing through the tube.

Figure 11. Lava dripstone and "pull out" in wall of Mushpot Cave (see fig. 14 and map 1, pl. 1). Dark area in center is a pull out where pasty red-hot dripstone sloughed off the wall and oozed downward. Pencil for scale.

Instructive examples of the transitory skins that form on moving basalt lava are present in many of the 3-ft-high benches that border the walls of large-diameter (30-60 ft) cave passages such as in Craig and Valentine. In places where a falling roof block has sliced such a bench, vertical inward-sagging thin layers of basalt can be seen beneath the final coating of lava plaster covering the bench. Some of these layers exhibit torn, crumpled, and pulled-out edges, all of which indicate that their extensions were sheared off and distorted by the pull of the lava flowing beside them (fig. 12). A thin plastic layer of congealing basalt cannot remain arched over a cave of large dimensions, but it can be preserved in small tube-in-tubes, such as those in the Garden Bridges area. Generally, such skins of congealing lava are continuously rafted forward and simultaneously sag, shear, and pull loose along the walls of the cave. The result of these processes is precisely what can be seen within the broken benches of Valentine and Crystal Caves.

Figure 12. Broken, partly collapsed lava bench on wall of Crystal Cave (see fig. 14 and map 18, pl. 6).