Lava cascade in Thunderbolt Distributary of Labyrinth Cave system
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<strong>in</strong>to the distributary <strong>in</strong> the general area<br />
where the Mushpot tube expands to 40 ft<br />
<strong>in</strong> width. The low ro<strong>of</strong> over the west side<br />
<strong>of</strong> the Mushpot tube is, therefore, the<br />
cont<strong>in</strong>uation <strong>of</strong> the eastern distributary<br />
ro<strong>of</strong>, upstream from the po<strong>in</strong>t where the<br />
two tubes merge. <strong>Lava</strong> from Mushpot<br />
tube filled the lower distributary tube to<br />
its ro<strong>of</strong>, except for the low-ceil<strong>in</strong>g area<br />
on the west side <strong>of</strong> the enlarged Mush pot<br />
tube and the 50 ft that the distributary<br />
tube extends to the east. Detailed evidence<br />
<strong>of</strong> exactly how the two tubes<br />
jo<strong>in</strong>ed is obliterated by the deep lava fill<br />
<strong>in</strong> the lower tube, but enough <strong>of</strong> the ro<strong>of</strong><br />
rema<strong>in</strong>s to establish the overall relation<br />
<strong>of</strong> one tube to the other.<br />
Western Tributary<br />
The western tributary is a small tube,<br />
less than 6 ft wide and 3 ft high. It is<br />
floored by a small stream <strong>of</strong> sp<strong>in</strong>y pahoehoe,<br />
<strong>in</strong> places only a few <strong>in</strong>ches<br />
thick. In the upper part <strong>of</strong> the tube the<br />
lava flowed along an <strong>in</strong>cl<strong>in</strong>e <strong>of</strong> 11 o for 54<br />
ft. A traverse up this tube is strictly a<br />
crawl; <strong>in</strong> only one spot, adjacent to the<br />
small pillar halfway along this passage,<br />
is it possible to stand upright. Access to<br />
<strong>Lava</strong> Brook <strong>Cave</strong> is blocked upstream by<br />
a passage less than 6 <strong>in</strong>. high, but this<br />
same po<strong>in</strong>t can be approached <strong>in</strong> the<br />
tube's cont<strong>in</strong>uation for 20ft as a distributary<br />
from <strong>Lava</strong> Brook <strong>Cave</strong>.<br />
Balconies and Skylights<br />
Downstream along the Mushpot tube<br />
trail, beyond the complex area <strong>of</strong> tube<br />
junctions, are three features that <strong>in</strong>terrupt<br />
the smooth contours <strong>of</strong> the cave's arched<br />
ro<strong>of</strong> and walls.<br />
The largest is a balcony located 130<br />
ft downstream from the cave entrance. It<br />
hangs as a graceful semicircle 9-12 ft<br />
above the floor <strong>of</strong> the tube. The smooth<br />
upper surface <strong>of</strong> the balcony is plastered<br />
aga<strong>in</strong>st the upstream half <strong>of</strong> a dome <strong>in</strong> the<br />
ro<strong>of</strong> <strong>of</strong> the tube. It probably formed when<br />
a lava pool filled most <strong>of</strong> the tube<br />
completely but rose only part way to the<br />
top <strong>of</strong> the dome. Cool<strong>in</strong>g lava at the top<br />
<strong>of</strong> the pool evidently attached to the tube<br />
walls and formed a narrow platform.<br />
Before thicken<strong>in</strong>g, the lava beneath the<br />
platform dra<strong>in</strong>ed out <strong>of</strong> the tube and left<br />
the platform hang<strong>in</strong>g at its present level.<br />
This crescentic balcony overlooks the<br />
underground lecture hall on the floor <strong>of</strong><br />
the cave.<br />
A second feature <strong>in</strong> the cave's ro<strong>of</strong> is<br />
located another 160 ft farther downstream.<br />
It resembles part <strong>of</strong> a culvert<br />
from which the bottom half has been<br />
removed. This rounded ceil<strong>in</strong>g channel<br />
can be followed for 35 ft along the top <strong>of</strong><br />
the larger tube, until it turns <strong>in</strong>to the<br />
south wall and is lost with<strong>in</strong> a few feet<br />
under a plaster <strong>of</strong> dripstone. It def<strong>in</strong>itely<br />
records the presence <strong>of</strong> a small open lava<br />
tube, only about 8 ft <strong>in</strong> diameter, whose<br />
floor collapsed <strong>in</strong>to the large Mushpot<br />
tube below.<br />
The third and highest feature is a<br />
niche <strong>in</strong> the south wall 17 ft above the<br />
tube floor. This niche is part <strong>of</strong> a high and<br />
narrow cupola <strong>in</strong> the tube's ro<strong>of</strong>. It is<br />
located at the sharp bend 150 ft upstream<br />
from where the ma<strong>in</strong> tube is blocked by<br />
pond<strong>in</strong>g <strong>of</strong> lava. The cupola and niche<br />
probably are a ro<strong>of</strong>ed-over skylight <strong>in</strong> the<br />
tube's ceil<strong>in</strong>g. Its orig<strong>in</strong>al walls are<br />
obscured by a plaster <strong>of</strong> dripstone. Many<br />
<strong>of</strong> the th<strong>in</strong> plates <strong>of</strong> lava that formed by<br />
the bridg<strong>in</strong>g <strong>of</strong> congeal<strong>in</strong>g lava <strong>in</strong><br />
skylights-as observed on active lava<br />
tubes <strong>in</strong> Hawaii (Greeley, 1971b, 1972;<br />
Peterson and Swanson, 1974)-are<br />
clearly visible as horizontal ridges partly<br />
smoothed over by the dripstone plaster.<br />
Repeated freez<strong>in</strong>g-over <strong>of</strong> the lava surg<strong>in</strong>g<br />
with<strong>in</strong> this narrow skylight built a<br />
cover across a hole, which formerly<br />
opened to the surface. When the lava <strong>in</strong><br />
Mushpot <strong>Cave</strong> withdrew to its present<br />
level, only a cupola and niche <strong>in</strong> the wall<br />
rema<strong>in</strong>ed to <strong>in</strong>dicate the position <strong>of</strong> the<br />
ro<strong>of</strong>ed-over skylight. Today an electric<br />
light illum<strong>in</strong>ates the niche. Downstream<br />
from this po<strong>in</strong>t the trail ends. Though the<br />
ceil<strong>in</strong>g is low, you can still crawl for<br />
another 100 ft by avoid<strong>in</strong>g two areas <strong>of</strong><br />
collapse blocks. The tube is closed by a<br />
lava fill<strong>in</strong>g to its ro<strong>of</strong>.<br />
Dripstone and <strong>Lava</strong>cicles<br />
Both Mushpot and <strong>Lava</strong> Brook<br />
<strong>Cave</strong>s provide such excellent examples<br />
<strong>of</strong> dripstone and lavacicles (such as those<br />
shown <strong>in</strong> figs. 8 and 9) that further<br />
descriptions and locations <strong>of</strong> these features<br />
are useful. F<strong>in</strong>e examples <strong>of</strong> lavacicles<br />
cover the large domes <strong>in</strong> the area<br />
where Mushpot tube widens at its junction<br />
with the eastern distributary. Excellent<br />
places to study them more closely at<br />
eye level can be found along the ma<strong>in</strong><br />
tube ro<strong>of</strong> throughout its length, except <strong>in</strong><br />
areas <strong>of</strong> ro<strong>of</strong> collapse. In these areas,<br />
many stages and changes <strong>in</strong> the development<br />
<strong>of</strong> the lavacicle and dripstone<br />
plaster are recorded. Many <strong>in</strong>terest<strong>in</strong>g<br />
patterns result from the response <strong>of</strong><br />
molten lavas <strong>of</strong> vary<strong>in</strong>g viscosities to the<br />
pull <strong>of</strong> gravity. On steep and smooth<br />
walls, pull-out patches <strong>in</strong> the dripstone<br />
are common (fig. 11). F<strong>in</strong>e examples <strong>of</strong><br />
such slough<strong>in</strong>g are present on the southeast<br />
Mushpot wall between the large<br />
balcony and the ro<strong>of</strong>ed-over skylight.<br />
Good examples <strong>of</strong> pull-out patches from<br />
a lavacicle-covered ro<strong>of</strong> can be seen on<br />
the high domes <strong>in</strong> the tube junction area.<br />
Other <strong>in</strong>terest<strong>in</strong>g forms, better seen <strong>in</strong><br />
Valent<strong>in</strong>e and Post Office <strong>Cave</strong>s, are<br />
near those high-lava marks where the<br />
mov<strong>in</strong>g lava dragged and distorted hot<br />
and sticky dripstone. In places the dripstone<br />
is sheared <strong>in</strong>to narrow benches and<br />
strips or broken <strong>in</strong>to foliated masses.<br />
Caliche, False Gold, and<br />
Phosphorescent Deposits<br />
Changes after solidification also produce<br />
<strong>in</strong>terest<strong>in</strong>g patterns superimposed<br />
upon the lavacicles and dripstone. Water<br />
from ra<strong>in</strong> and melt<strong>in</strong>g snow seeps down<br />
<strong>in</strong>to the caves and then may spread out,<br />
wett<strong>in</strong>g the surface <strong>of</strong> the lavacicles and<br />
dripstone. Some <strong>of</strong> this water evaporates<br />
and leaves a th<strong>in</strong> coat <strong>of</strong> caliche (calcium<br />
carbonate plus m<strong>in</strong>or amounts <strong>of</strong> other<br />
soluble salts and clay) as a white filigreelike<br />
tracery upon the dark lava surface<br />
beneath (fig. 17). Excellent examples <strong>of</strong><br />
this lace-like natural ornamentation can<br />
be seen on both walls <strong>of</strong> Mushpot <strong>Cave</strong><br />
between the large balcony and the<br />
ro<strong>of</strong>ed-over skylight. J.D. Howard gave<br />
the name "White Lace" to a cave with<br />
walls <strong>of</strong> this k<strong>in</strong>d <strong>of</strong> caliche. If cave<br />
walls are completely covered with white<br />
14 Selected <strong>Cave</strong>s and <strong>Lava</strong>-Tube Systems, <strong>Lava</strong> Beds National Monument, California