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

Silver Cave

Silver Cave is a near-surface lava tube, which is relatively undamaged by collapse. It contains excellently preserved examples of features common to lava tubes, such as lavacicles, dripstone walls, and pahoehoe floors. More complicated features include several kinds of tube-in-tubes; levee-like lobes of lava spilled from cracks in tube-in-tubes; peeled accretionary walls of lava plaster; lava cascades, benches, and gutters; and pillars, alcoves, and skylights.

The area near the head of Silver Cave (see lower right corner of map 14, pl. 5) is unusual. It originates in a lava cascade that blocks further access upstream but geometrically lies within the Post Office system of superposed lava tubes. Downstream 100 ft from this cascade, Silver Cave divides. The north-trending fork leads to Post Office Cave and furnishes the only route by which three short levels within Post Office Cave can be reached. This overlap between the two cave systems is shown on both maps 14 and 15, plate 5.

The core of Silver Cave is 1,400 ft long. More than 850 ft are added if we include the overlap with Post Office, the division around one pillar, and the length of those tube-in-tubes that can be traversed by crawling. Thus the total accessible passage is 2,250 ft.

Silver Cave was named by J.D. Howard, who explored it in January 1918. Near the entrance, percolating water has deposited white caliche on the walls and ceiling of the cave. On a winter day the caliche is frequently covered with droplets of water, which give off a brilliant silvery reflection when a light is played upon it; Howard named the cave for this effect.

Features at Entrance

The collapse at the entrance formerly was a skylight in the roof when lava flowed in the tube. As the tube filled with lava, the lava must have repeatedly surged upward through the skylight during eruptive peaks because the lip surrounding the skylight is covered with driblets of lava that were flowing back into the tube as they solidified. The shell of lava that partially overhangs the entrance was caused by gas pressure that inflated a lava blister of thin semiplastic crust over the skylight. Subsequent partial collapse of more than half of this blister, plus collapse of many similar blisters and of other debris surging with in the skylight, built a mound on the floor of the tube beneath the skylight.

Features Downstream from Entrance

Downstream from the entrance the floor of the tube is hidden by lava driblets and collapse debris for 30 ft, at which point a bench along each wall appears from beneath the rubble. The projection of these benches 2-5 ft from each wall left a narrow central channel between the benches. The floor of this channel is masked with loose rubble for another 25 ft downstream, where a lava lobe with a spiny pahoehoe surface is visible beneath the collapse rubble. The edges of its rounded upper surface are lower than the benches on either wall, and so the lobe is contained in the channel between them.

The gradient of the tube begins to steepen downstream where the spiny pahoehoe in the channel cascaded and broke into an aa surface. In places the aa in the channel overflowed and covered the smooth benches. The gradient is still steeper 150 ft downstream from the entrance; then the gradient decreases, and the northwest wall of the tube is indented by a gently sloping alcove. At the center of the inner edge of this alcove, a large block that fell from the roof stands out conspicuously. Subsequent to its fall this block was completely immersed in lava, which then drained away and left a blanket of lava plaster and dripstone over the surface of the collapse block and the adjoining alcove (see map 14, pl. 5).

Downstream 60 ft from the alcove the benches on both sides of the tube are completely inundated by spiny pahoehoe that welled up and issued from a break in the surface of the aa lobe of lava between the benches. Lava beneath the aa surface apparently ponded and swelled up when it reached the gentle slope downstream from the area of steep gradient. The hydraulic pressure on the molten material beneath the aa crust became great enough to rupture the aa surface; this rupture allowed the molten lava to spill out and continue downstream as a new flow unit of spiny pahoehoe that spread over both the benches and the downstream surface of the aa. Downstream another 40 ft, this lobe of spiny pahoehoe in turn changes into a collapsed tube-in-tube, and several short sections of crude benches built by its collapse are preserved along the walls.

Collapse blocks litter the floor of the cave near its downstream end whereas the walls and ceiling are lined with thin accretionary layers of lava. The inside of each successive tube lining has lavacicles and dripstone on its surface. Some lavacicles were sheared off and embedded in the outer surface of the next successive tube lining that rose and crowded against them. Forty feet from the end of the tube, one of these linings peeled back from the walls while still in a semiplastic state, but 20 ft from the end this lining is not peeled off; it still sticks tightly against the walls and ceiling of the tube. The downstream end of Silver Cave is closed by lava fill and by many thin accretionary linings peeled from the roof and walls.

Features Upstream from Entrance

The floor of Silver Cave is concealed by collapse debris for 75 ft upstream from the entrance skylight (map 14, pl. 5). Within this area of collapse blocks, the tube widens abruptly from 25 to 50 ft. On the east wall, 80 ft upstream from the entrance, a lava cascade joins the tube. This cascade apparently was fed into the main tube from a surface flow that found access through a breakdown.

A smooth pahoehoe floor emerges from the collapse rubble just upstream from this lava cascade. This is the same pahoehoe floor that forms the smooth benches downstream from the entrance to the cave, but here it has no channel. Two shallow breakdowns (collapse trenches) in this floor, however—one 120 ft and the other 160 ft upstream from the entrance—reveal a shallow tube-in-tube that underlies this floor and extends far upstream. The channel between the benches downstream from the entrance is probably the downstream continuation of this episode of draining.

At the upstream end of the shallow breakdowns, a narrow medial crack developed along the middle of the pahoehoe floor and continues upstream for at least another 80 ft. During volcanism the underlying shallow tube-in-tube over-filled, heaved up, split open the pahoehoe floor above it, and spilled lava out through the medial crack. The overflows formed thin lobes of spiny pahoehoe, which flowed down the shallow gutters along the walls of the tube.

Upstream from these overspill lobes the tube makes a sharp 40° bend to the southwest. The ceiling and floor almost close together in the section where the tube changes direction. This results in a very tight 12- to 18-in-high crawlspace for 15 ft along the tube. This difficult crawl is enough to discourage all but the most enthusiastic spelunkers, but it is the only way to visit the upstream part of Silver Cave and three short levels of Post Office Cave.

At the upstream end of this route the tube widens to 30 ft, and ceiling heights gradually increase to 3 ft. Lava pooled here and formed a wide smooth pahoehoe floor. Upstream 30 ft from the crawlspace, a breakdown in the smooth pahoehoe floor opened a 2- to 3-ft-high tube-in-tube in which one can crawl back downstream for 125 ft, directly beneath the upper tube. This tube-in-tube lies exactly beneath the medial part of the upper main tube; it is the upstream part of the tube-in-tube that overflowed through the medial crack and formed the spiny pahoehoe lobes that spread into the gutters along the walls.

Farther upstream from the breakdown that gave access to this tube-in-tube, a larger shallow breakdown extends upstream along the middle of the tube for 80 ft, paralleled by flat benches that project from the walls on either side. The breakdown floor defines a channel between the benches generally 3-4 ft deep. A small natural bridge (fig. 50) unites the benches between the two breakdowns (see map 14, pl. 5). Large collapse blocks on the pooled surface in the area around this natural bridge were later plastered over with lava. Collapse blocks such as these could easily have broken through the thin crust in the middle part of the pooled surface and probably contributed to the 80-ft-long breakdown in this area.

Figure 50. Balcony forms natural bridge in Silver Cave (see fig. 4 and map 14, pl. 5). Lower tube was created when interior of late lava flow drained away.

The floor of the channel between the smooth benches on either side of the 80-ft-long breakdown contains a 1- to 2-ft-high tube-in-tube, which extends upstream far beyond the breakdown. This small tube-in-tube records a series of instructive transformations as it is followed upstream from the natural bridge for the next 410 ft (see cross sections on map 14, pl. 5). Its top is missing 45 ft upstream from the natural bridge, and the remaining sides of the tube form narrow levee-like ridges that contain a shallow channel between them partly filled with spiny pahoehoe. Whether the roof of this small tube collapsed and was carried away by molten lava, or whether it was never roofed over by the buildup of the steep levees on either side is a moot question because each process probably occurred along different parts of this 410-ft stretch. Where steep levees are present, they are bordered by gutters between them and the wall of the adjoining lava bench. In these places the flat-topped benches projecting from the walls overlook gutters, levees, and a partly filled central channel in the breakdown between them.

Upstream from the natural bridges the pahoehoe benches decrease in width until they are completely absent 90 ft above the natural bridge. They are present 40 ft farther upstream where the tube turns 40° toward the south. Farther upstream the top of the small tube between the benches is open, and the gutters, steep compound levees, and a narrow channel half-filled with a small flow of spiny pahoehoe are all contained between the benches. The levees are well developed here, arching up over the channel in places and forming at one point a complete tube-in-tube that persists for 100 ft. Where a medial crack is present, the tube-in-tube changes in form and opens into a channel that is confined by 6- to 12-in.-high near-vertical levees. Because the confining levees were weak in places, the spiny pahoehoe broke through in several spots and filled sections of the bordering gutter.

The height of the pahoehoe benches decreases from 4 ft beneath the natural bridge to 1 ft above the floor 400 ft upstream from the natural bridge. The benches are continuous, very flat on top, and at the same level on both sides of the tube.

The course of the main large tube along this 400-ft section is reminiscent of river meanders. The tube swings smoothly through several gentle curves and is preserved without collapse. High-lava marks are continuous on the walls of the tube 1 ft above the top of the benches. Well-developed dripstone and lavacicles cover the walls and ceiling above these high-lava marks.

Silver Cave subdivides around a large pillar 320 ft upstream from the natural bridge. On the upstream side of the pillar, a tube at a slightly higher level than the main tube departs to the north, turns east, and reunites with the main tube 80 ft downstream. Because the floor of this side tube is 3-4 ft higher than the floor of the main tube, the lava drained back into the main tube from both ends of the side tube as the final lava flow lowered and left small frozen lava cascades at each junction.

The front of a later lava lobe came to rest on the floor of the main tube just 20 ft upstream from the pillar. This new flow covered the benches, gutters, levees, and channel and prevents further study of them upstream. The flow front is a tongue of lava 3 ft high; from it, a pahoehoe toe protrudes into one of the gutters. The viscous character of this flow is shown by its slab-pahoehoe surface upstream, which consists of a 30-ft-long crust of broken, twisted, and imbricated slabs of pahoehoe. This flow continues to form the floor of the main tube upstream to Silver Cave's connection with Post Office Cave. From this junction the flow can be traced upstream another 60 ft until one crawls into a small chamber 20 ft across blocked upstream by a lava cascade. A lower lava cascade just downstream from the chamber shows steeply plunging ridges of a smooth yellowish-brown lava. These parallel ridges evidently resulted when a smooth-surfaced pool of unusually colored lava cascaded from the lip of the upstream chamber. Colorful splashes of yellow dripstone line the roof on the tube in the same area.

The branch to the north is a small part of Post Office Cave. It contains parts of three levels, each of which is terminated by lava fill upstream. Successive downstream breakdown lips drop the two upper short levels in steps to the lowest level of the three. The floor of this lowest level falls away into the Silver Connector. This connector is the product of a complicated series of breakdowns between at least five superposed tubes, accompanied and interrupted by recurrent refilling and draining of lava. Its development evidently spanned several periods of eruptive activity followed by enlargement during post-lava collapse. Vertical layers of lava plaster adhere to the broken separations between levels in some parts of the connector. (See "Silver Connector Level" section and map 15, pl. 5).