Wednesday, October 28, 2009

Cave Bacon and Stalactites on Drugs: The Mother Lode Underground

During our field trip outdoors last weekend, we spent a lot of time underground. One of our stops included a tour of Black Chasm Cavern near the town of Volcano in the Mother Lode of the Sierra Nevada. It is a strikingly beautiful cave in a region that is not really known for its cave systems, but they are there and they are a truly unique place to learn about geological processes in the Sierra Nevada.

Most people, if they ever think of it all, will associate the Sierra Nevada with granite, an igneous rock that forms from the slow cooling of magma miles beneath the surface. As a point of fact, only about three-quarters of the Sierra is composed of granitic rocks, and only a small percentage of that is technically granite (mostly it is granodiorite). The remainder of the range is covered by volcanic rocks, or is composed of older metamorphic rocks, including a significant amount of marble. Marble is derived from the heating and deformation of limestone, which formed on the floors of Mesozoic and Paleozoic tropical seas, possibly as coral reefs or carbonate shelf deposits. The rocks were lifted from the ocean floor and added to the edge of the North American continent when the terranes collided with the Cordilleran subduction zone in Mesozoic time.

Marble, like the limestone from which it was derived, is composed of the mineral calcite. Weak acids in the soil and groundwater react with the calcite, carrying it away in solution, forming the openings that eventually become the caverns. As the Sierra Nevada rose and deep canyons were eroded across the range, the groundwater table dropped, exposing the caverns to the atmosphere. Water dripping or seeping into the cavern evaporates, leaving behind the mineral deposits that hang from the ceiling of the cave or rise from the floor (dripstone deposits), or which cover the walls of the cave (flowstone deposits). Collectively these cave decorations are called speleothems.

More than 1,000 caverns grace the Sierra Nevada foothills. Some of the caves are world-class in their intricacy and decoration. One cave in Kings Canyon National Park has more than 2o miles of passageways. Many of the caves are also extremely dangerous: most are developed in vertical passageways with deep unexpected dropoffs, and these serve as sinks for "bad" air (carbon dioxide is a heavier gas). On the other hand, some of the very nice caves are open for guided tours: Crystal Cave in Sequoia National Park, Boyden Cave near Kings Canyon, Moaning Cave north of Columbia, Mercer Cavern at Murphys, California Cavern up the hill from San Andreas, and Black Chasm, site of our exploration last week. A trail near New Melones Reservoir leads to the Natural Bridges, where Coyote Creek flows through a cavern.

Black Chasm is named for the deep cleft not far from the cavern entrance that drops 90 feet or so into a small lake. The lake extends at least another 60 feet downwards into the mountain. The chasm prevented the miners and other early explorers from exploring (and vandalizing) the more remote passages of the cave. As a consequence, the owners were able to develop the cave with a mind towards preserving the most pristine parts of the cave. They constructed stairwells and pathways across the chasm, with handrails preventing visitors from accessing and breaking the most fragile speleothems. The cave was opened for tours about 2001, and it quickly became my favorite choice for our geology field trips.

Besides the well-placed lighting system that highlights some beautiful draperies (cave bacon) and stalactites (see the second picture), the cavern offers some of the best developed helictites to be found anywhere (top picture). Helictites are essentially stalactites that have forgotten to follow the law of gravity. The precise details of the their origin are enigmatic, but water pressure clearly plays a greater role than gravity does. They are exceedingly fragile and are usually the first things to be vandalized in unprotected caves. Black Chasm has a spectacular wall covered with them (my photo is of a small corner of the entire panel).

The operators of Black Chasm offer a group discount, and the guides enjoy speaking to geology groups when they get the opportunity. It is well worth a visit!

If you are interested in exploring wild caves, preservation and protection is the highest priority. As such, you should get in touch with the local grotto of the National Speleological Society if you want to be involved in protecting this special resource. If you have followed my blog for any period of time, you will know that I think that cave vandals are one of the lowest and most moronic forms of humanity in existence.

1 comment:

Gaelyn said...

Wow, that's a lot of helictites. They are so amazing, as are all the cave formations. I've wanted to see some of these marble caves ever since I worked at Oregon Caves, also marble. I've seen some awesome photos of the wave-like striations in Crystal Cave.
I highly recommend Karchner Caverns in AZ, if you haven't already been there.
Excellent post. Again I missed a great field trip.