SERPENTINITES IN THE SAN JOSE AREA, CALIFORNIA

In the San Jose Area serpentinite outcrops are usually exposed on structural highs and form low rounded hills. These tracts are present in residential areas, recreational parks, underlay reservoirs and occupy significant tracts of land under control of local, state, and federal governments. Exposed surfaces of the serpentinites are rocky and usually consist of massive serpentinized peridotite that has been eroded out of a sheared serpentine matrix. Light to darker shades of green is predominant but serpentinite rocks may be black, red or even white with all gradations in between. Where shearing develops during deformation individual lens shaped, fragments are formed that exhibit brilliant, smooth, greenish-black polished surfaces (slickentite). Contacts with the country rock are all tectonic and continued tectonic movement along these contacts has often incorporated exotic rock masses into the sheared serpentine. Where soil has developed on the older upland surfaces, it is a black smectite–rich clay. Vegetation is very sparse with a unique ecosystem consisting mainly of grasslands and shrubs. Aeromagnetic anomalies found over the serpentine bodies at Coyote Ridge, Santa Teresa Hills, Tulare Hill, and in the Morgan Hill area suggest that these bodies are tabular and in thrust contact with the surrounding rocks. These serpentinites contain cross-fiber asbestos in variable abundance, but it is unknown if there is a real health risk from exposure to asbestos fibers at the low levels encountered in these serpentine tracts. Serpentine lands are indeed unique because their chemical makeup is toxic for most plants and asbestos fibers can be released during construction disturbances. These same serpentine tracts support endemic plants species of great rarity. We now recognize the importance of preserving these natural serpentine tracts but at the same time, urban development threatens pollution by spreading dust laden with naturally-occurring asbestos fibers.