25.12.2015 Views

Lava cascade in Thunderbolt Distributary of Labyrinth Cave system

report

report

SHOW MORE
SHOW LESS

Create successful ePaper yourself

Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.

tubes created a cave floor that is hummocky<br />

and difficult to traverse. Many<br />

blocks are exceptionally large-a few<br />

are as long as 40 ft. Very few "<strong>in</strong>-place"<br />

remnants <strong>of</strong> the walls <strong>of</strong> the orig<strong>in</strong>al lava<br />

tubes have survived. The remarkable<br />

feature about this level is that it did not<br />

completely collapse to the surface and<br />

form an open collapse trench like the one<br />

at the downstream entrance. One possible<br />

explanation <strong>of</strong> the strength <strong>of</strong> this part<br />

<strong>of</strong> the ro<strong>of</strong> is that the highest accessible<br />

lava tube at both the upstream and<br />

downstream ends <strong>of</strong> the Post Office <strong>Cave</strong><br />

is filled with congealed lava. This fill<strong>in</strong>g<br />

may have formed a strong massive strut<br />

that has prevented <strong>in</strong>terior collapse from<br />

work<strong>in</strong>g upward to the surface.<br />

Remnants <strong>of</strong> certa<strong>in</strong> passageways<br />

that collapsed together to form the high<br />

central part <strong>of</strong> the Silver Connector level<br />

have been given names to facilitate description<br />

(see map 15, pl. 5). Where a<br />

passageway consists <strong>of</strong> an open<strong>in</strong>g<br />

formed by the <strong>in</strong>tegration <strong>of</strong> two or more<br />

tubes by collapse, we call it a "level. " If<br />

only one ma<strong>in</strong> lava tube is dom<strong>in</strong>ant, we<br />

call that one a "tube." Only the larger<br />

tubes and levels have been named. Small<br />

vertical breakdowns between tubes or<br />

levels are unnamed, but we use "connector"<br />

for the larger semicyl<strong>in</strong>drical<br />

breakdowns that cross vertically through<br />

the position <strong>of</strong> two or more tubes and yet<br />

do not reach the surface.<br />

Two connectors described here<strong>in</strong> at<br />

some length are the Silver Connector<br />

(fig. 51 and map 15, pl. 5) and the<br />

Cataract Connector. The Cataract Connector,<br />

near the east end <strong>of</strong> Post Office<br />

<strong>Cave</strong>, is an excellent example <strong>of</strong> a<br />

connector that formed by collapse between<br />

the lava tubes while one or more<br />

were occupied by molten lava. One <strong>of</strong> its<br />

walls has an 18-ft-high lava cataract that<br />

was frozen <strong>in</strong> place while molten lava<br />

was cascad<strong>in</strong>g <strong>in</strong>to the hole. Other parts<br />

<strong>of</strong> this connector's walls are plastered<br />

with lava dripstone that leaked through<br />

holes <strong>in</strong> the fragile floor <strong>of</strong> a level above<br />

them.<br />

The Silver Connector gives its name<br />

to the Silver Connector level. The Cataract<br />

Connector gives its name to two<br />

lava tubes, the Upper Cataract Tube and<br />

the Lower Cataract Tube. Each tube is<br />

compound and <strong>in</strong> places splits <strong>in</strong>to two or<br />

more parts, but this splitt<strong>in</strong>g is due to the<br />

build<strong>in</strong>g <strong>of</strong> <strong>in</strong>ternal balconies and not to<br />

breakdown between separate tubes.<br />

The Upper Cataract Tube lies just<br />

beneath the eastern part <strong>of</strong> the Silver<br />

Connector level and at its west end<br />

merges with that level through a ro<strong>of</strong><br />

collapse. In the much deeper parts <strong>of</strong> the<br />

Silver Connector level farther west, the<br />

Upper Cataract Tube is undoubtedly one<br />

<strong>of</strong> the components <strong>of</strong> the enlarged Silver<br />

Connector level (see longitud<strong>in</strong>al section<br />

on map 15, pl. 5).<br />

At the upstream end <strong>of</strong> the Post<br />

Office <strong>Cave</strong> the relations <strong>of</strong> passageways<br />

are similar. Above the Cocoa Pipel<strong>in</strong>e<br />

the four largest tubes or levels have been<br />

given names. Above the Cocoa Pipel<strong>in</strong>e<br />

tube is the Cocoa entrance level, which<br />

<strong>in</strong>volved two tubes, one above the other.<br />

Still higher is the upper entrance level; it<br />

subdivides eastward <strong>in</strong>to upper and lower<br />

tubes. And still higher are the Silver<br />

<strong>Cave</strong> levels (maps 14 and 15, pl. 5).<br />

The Silver Connector extends vertically<br />

through at least four and perhaps<br />

seven <strong>of</strong> these levels; it stops below them<br />

but above the Cocoa Pipel<strong>in</strong>e. With<strong>in</strong><br />

200 ft downstream from the Silver Connector,<br />

most <strong>of</strong> these levels have merged<br />

through collapse to form the highceil<strong>in</strong>ged<br />

part <strong>of</strong> the Silver Connector<br />

level. The Cocoa Connector, and the<br />

Central Connector downstream, are<br />

breakdown holes that reach the Cocoa<br />

Pipel<strong>in</strong>e.<br />

Because <strong>of</strong> the difficulty <strong>of</strong> show<strong>in</strong>g<br />

superposed levels on a map, some levels<br />

are <strong>of</strong>fset for the sake <strong>of</strong> clarity and l<strong>in</strong>e<br />

variations are used to help dist<strong>in</strong>guish<br />

certa<strong>in</strong> levels. Collapse rubble completely<br />

covers the floors <strong>of</strong> the Silver Connector<br />

level and Cocoa entrance levels<br />

but is almost absent <strong>in</strong> the ro<strong>of</strong>ed-over<br />

parts <strong>of</strong> the Cocoa Pipel<strong>in</strong>e and a few <strong>of</strong><br />

the other tubes.<br />

With this prelim<strong>in</strong>ary explanation <strong>of</strong><br />

the features, an upstream traverse<br />

through Post Office <strong>Cave</strong> is described. It<br />

starts at the collapse trench that gives<br />

access to the ma<strong>in</strong> entrance and ends at<br />

the upstream end <strong>of</strong> the Cocoa Pipel<strong>in</strong>e<br />

with a climb through overly<strong>in</strong>g tubes and<br />

levels to the upstream exit.<br />

Collapse Trench at<br />

Downstream Entrance<br />

The collapse trench that leads to the<br />

downstream entrance <strong>of</strong> Post Office<br />

<strong>Cave</strong> (map 15, pl. 5) is 220 ft long, 80<br />

ft across at its widest po<strong>in</strong>t, and 50 ft<br />

deep. Its walls <strong>in</strong> most places are nearly<br />

vertical, and large underground caverns<br />

open at either end <strong>of</strong> the trench. The<br />

downstream cavern leads beneath Post<br />

Office Natural Bridge to a downstream<br />

breakdown trench; the upstream cavern<br />

conta<strong>in</strong>s the entrance to Post Office<br />

<strong>Cave</strong>.<br />

In January <strong>of</strong> 1918, J.D. Howard<br />

clambered down the upstream wall <strong>of</strong> the<br />

trench, explored the cavern at its head,<br />

and from the side <strong>of</strong> this cavern crawled<br />

upstream along the downstream entrance<br />

tube (see map 15, pl. 5) to the po<strong>in</strong>t<br />

where his way was blocked by lava that<br />

filled the tube to its ro<strong>of</strong>. Evidently he<br />

did not f<strong>in</strong>d the small and <strong>in</strong>conspicuous<br />

crawlway through collapse debris <strong>in</strong> the<br />

middle <strong>of</strong> the entrance cavern floor that<br />

gives access to the ma<strong>in</strong> parts <strong>of</strong> Post<br />

Office <strong>Cave</strong>, for he disappo<strong>in</strong>tedly<br />

wrote, "it isn't much <strong>of</strong> a cave, as it is<br />

only an open<strong>in</strong>g with a crawler at the<br />

back end." Howard called the cave Post<br />

Office because <strong>of</strong> the many "pigeon<br />

holes" <strong>in</strong> the cliff above the entrance<br />

cavern. These rem<strong>in</strong>ded him <strong>of</strong> post<br />

<strong>of</strong>fice boxes. This clutter <strong>of</strong> small open<strong>in</strong>gs<br />

is one <strong>of</strong> the particularly <strong>in</strong>terest<strong>in</strong>g<br />

features <strong>of</strong> the collapse trench. They<br />

apparently formed <strong>in</strong> either a skylight on<br />

the top <strong>of</strong> a large tube or <strong>in</strong> a surface<br />

channel through which molten lava was<br />

actively mov<strong>in</strong>g downstream. In either<br />

case the surface <strong>of</strong> the molten flood (as<br />

observed <strong>in</strong> Hawaii and at other active<br />

volcanoes by Peterson and Swanson,<br />

197 4) congeals from the walls <strong>of</strong> the<br />

open<strong>in</strong>g, but before it can crust over<br />

completely, m<strong>in</strong>or fluctuations <strong>in</strong> the<br />

height <strong>of</strong> the lava river cause recurrent<br />

partial crusts to form and attach to the<br />

walls <strong>of</strong> the open<strong>in</strong>g at slightly different<br />

72 Selected <strong>Cave</strong>s and lava-Tube Systems, lava Beds National Monument, California

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!