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Serpentine rock is
apple-green to black and is often
mottled with light and dark colored
areas. Its surfaces often have a shiny
or wax-like appearance and a slightly
soapy feel. Serpentine is usually
fine-grained and compact but may be
granular, platy, or fibrous in
appearance. The term "serpentine" is
commonly used by the general public to
refer to the rock type that geologists
call "serpentinite." Serpentine occurs
in central and northern California -- in
the Coast Ranges, the Klamath Mountains,
and in the Sierra Nevada foothills.
Serpentine rock is
primarily composed of one or more of the
three magnesium silicate minerals, "lizardite,"
"chrysotile," and "antigorite."
Chrysotile often occurs as fibrous
veinlets in serpentine. Chrysotile in
fibrous form is the most common type of
asbestos. Asbestos is a term applied to
a group of silicate minerals that
readily separates into thin, strong, and
flexible fibers that are heat resistant.
Lizardite and antigorite do not form
asbestos fibers and instead are
plate-like in form. Because serpentine
often contains some asbestos, and
exposure to asbestos fibers have
potential human-health consequences, the
Air Resources Board adopted regulations
in 1990 restricting the use of this rock
type as an unpaved road surfacing
material. Further information on
restrictions for serpentine use in
California can be obtained by contacting
Air Resources Board at (916) 322-8285,
or the local Air Pollution Control
District Offices. See our asbestos page
for information about asbestos in El
Dorado County, a brochure that describes
the issue, and related web links.
Serpentine is
considered by geoscientists to be the
metamorphosed remains of magnesium-rich
igneous rocks, most commonly the rock
peridotite, from the earth’s mantle. The
mantle is a thick layer of rock just
below the earth’s crust. One theory for
serpentine formation and occurrence
currently in favor with many
geoscientists is that peridotite
underlying oceanic crustal rocks have
been metamorphosed to serpentine in
subduction zones that existed at various
times in California’s past. A subduction
zone is an area where ocean crust rocks
run into and slide underneath the edge
of a continent. Because serpentine has a
much lower density than peridotite, it
rose toward the surface along major
regional thrust faults associated with
the subduction zones.
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