New Idria Serpentinite: A Land Management Dilemma

Abstract

The New Idria asbestos-bearing serpentinite represents a unique tectonic situation. Change in plate motion produced transverse compression across the San Andreas transform fault system in the vicinity of New Idria which produced shortening of the former continental margin resulting in sub-parallel thrusting, folding, and diapirism. This tectonic activity has exposed a huge body of serpentinized mantle peridotite (45 sq mi) that may have been part of the Pacific Ocean crust basement. Continued wedging of these peridotites into the water-rich continental crust has caused a chain-reaction serpentinization and expansion. Thousands of tectonic (milling) events acting on the serpentinite near New Idria created a huge deposit of short-fiber asbestos. The main serpentine mineral is chrysotile that forms very short fibers (~5 microns) having a specific surface area 3 to 4 times greater than other commercial asbestos fibers. Its natural chemical solubility reduces its biopersistence resulting in much lower toxicity to humans. During the last 15 million years the asbestos-bearing serpentinite of the New Idria body has been eroded and contributes fibers to alluvial deposits along the west side of the San Joaquin Valley and San Benito River Basin with little to no indication of excessive present-day asbestos-related respiratory diseases in this area. A study by the Bureau of Land Management concerning exposure of asbestos fibers to weekend off-road vehicle riders in the New Idria serpentinite area indicates that their risk of developing lung cancer is low. Knowledge of the geologic facts concerning the natural origin and distribution of asbestos-bearing serpentinite illustrates the dilemma of the U. S. Government attempting to remediate the small EPA superfund mining sites within this vast region of serpentinite.