A GEOLOGIC DATASET OF NATURAL OCCURRENCES OF ASBESTOS IN THE CONTERMINOUS UNITED STATES—APPLICATIONS TO RISK MANAGEMENT

In 2004, the U.S. Geological Survey began a study designed to identify and locate the natural occurrences of asbestos in the United States, using descriptions found in the geologic literature. The study identified about 800 natural occurrences of asbestos in the conterminous United States; reported asbestos deposits in Alaska will be catalogued in the next part of the study, and no known asbestos deposits occur in Hawaii. Asbestos occurrences have been reported in 35 of the 50 States; they vary considerably in size and character, from scattered, thin veinlets of asbestos minerals to large ore bodies. Their geographic distribution is directly linked to geology. The geologic settings in which asbestos occur are Mg-rich host rocks altered by relatively moderate-temperature and moderate-pressure metamorphism. Specifically, the rock types known to host asbestos include metamorphosed and metasomatized ultramafic rocks (particularly serpentinite) and some mafic igneous rocks, metadolostones, and metamorphosed iron formations. Asbestiform amphiboles (asbestos) can also occur as accessory minerals in several hydrothermally altered types of alkaline igneous intrusions.

Recognizing that asbestos forms in predictable and identifiable geologic environments is information that allows public health agencies to conduct an informed asbestos screening and management program. The first step involves locating and describing the known (reported) asbestos deposits, at a regional or national scale. Next, the geologic units that host known asbestos are mapped, which delineates the extent of possible additional asbestos mineralization. This geological approach allows agencies to plan for the likelihood of encountering asbestos, while also sparing the unneeded expense of asbestos regulation in regions that are unlikely to contain asbestos deposits. A basic understanding of the geology of asbestos can be applied beforehand at any rock and soil excavation project, regardless of scale. Thus, by mapping the terrains most likely to host asbestos mineral deposits, planners can develop dust-control procedures appropriate for the regions where workers and nearby residents are most likely to be exposed to airborne asbestos.