Wednesday, Jan 08 2014 06:00 AM

Pen in Hand: When it comes to interesting Tehachapi boulders, basalt rocks!

Related Photos

A basalt boulder is host to a thriving lichen colony. Photo by Jon Hammond

Orange lichen coats a basalt rock. Photo by Jon Hammond

An unusual textured surface in a small basalt boulder. Photo by Jon Hammond

Basalt cinders or scoria are porous as a result of escaping gas bubbles that left behind tiny cavities in the cooling rock. Photo by Jon Hammond

An ancient weathered outcropping of basalt rises like an island among yellow Hilltop Daisies and purple Gilia west of Highway 58. Cracks and fissures in the rock are home to deep orange lichen colonies. Photo by Jon Hammond.

The Tehachapi Mountains are known for having limestone deposits close to the surface, which provide the main material used by the Lehigh Southwest and Cal Portland cement plants. There are also granite and tonalite outcroppings, marble deposits in a few places, quartz, as well as significant sandstone throughout Sand Canyon. One of the less common rocks found in our area is basalt.

Basalt is a volcanic rock formed by the rapid cooling of lava on the surface or close to it. Basalt is frequently porous-looking, which geologists describe as being "vesicular," because it has many tiny holes or gas pockets formed when the magma approached the surface. At that point gases were able to escape the molten lava, and these gases left behind small cavities called vesicles as the magma rapidly cooled.

The older name for these lightweight, spongy-looking rocks was cinders and they have frequently been bagged up and sold for use in landscaping as "lava rock," and have also been used in gas barbecues. Cinders are also ground up and used on roadways in the winter in the place of salt to melt ice. Inyo County has huge deposits of lava rock, and CalTrans and other agencies use these reddish cinders on Highway 58 and other local roadways to speed ice melt and provide better traction. In recent times the more accepted geological term for cinders is scoria.

Basalt typically starts out black or gray, but frequently weathers to brown or reddish as its iron-rich minerals oxidize and turn to rust. Basalt is one of the most common rock types in the world, but is not highly represented in the Tehachapi Mountains -- it is far more common in some parts of the Mojave Desert, for example, and as I mentioned earlier, Inyo County has many areas of substantial ancient basalt flows in the Owens Valley, as does Mono County farther to the north.

With its often porous surface that weathers relatively quickly, basalt frequently provides an excellent surface for lichen to become established, and some of the basalt formations in the Tehachapi area are host to beautiful lichen colonies. There are some basalt outcroppings just west of Highway 58 near the Tucker Road offramp, and these almost seem painted with orange and pale yellowish green as a result of thriving lichen colonies. Local Indian people once heated basalt rocks in fires for use in sweat lodges, and made bedrock mortars in the surface of some basalt boulders.

Both the Tehachapi Mountains and the Sierra Nevada Mountains are considered volcanic and granitic in origin, and interestingly both of these major rock types have the same parent material: magma. In the case of basalt, the magma reached the Earth's surface or at least came close enough to cool very quickly and didn't have time to form large crystals. Granite, on the other hand, cooled deep below ground level and cooled very slowly, forming much larger crystals, and was later pushed upward and above ground by earthquake faulting action.

Basalt in the Tehachapi Mountains is all the more interesting because of its relative scarcity -- encountering a basalt outcropping is like finding a stand of Sugar Pines or nesting American Robins: they are here, but certainly not in abundance. . .

Have a good week.

Jon Hammond has written for the Tehachapi News for over 30 years. Send email to:

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