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Serpentinite
by Jonathon Schaffner
[Bio 28 - Ecology of GGNRA - Oct. 2010 - Crima Pogge, Instructor]
The Golden Gate National Recreation Area comprises of coastal and inland areas of
and near the San Francisco Bay and hills stretching from San Mateo County north
through much of Marin county. Topographic relief ranges vary from sea level to
2300’ high Mount Tamalpais, with flat marine terraces, alluvial deposits, and
gradients from 3 to 35%, as in the tributaries of Bolinas Ridge.
This area is rich with geological activity, of the distant past as well as present day
shifts in plates and fault lines of the earth’s crust below. The formations of rocks we
see are the results of these movements and chemical reactions of minerals in the
mantle under the sea and North American Plate and Pacific Plate. The Pacific Plate
with its border just off the coast 5 miles, is moving northward past the North
American Plate at a rate of 1” per year, as it has for 28 million years. The San
Andreas Fault line runs from Fort Funston to Bolinas Lagoon and Tamales Bay.
Tectonic plate subduction is when the plate ‘subducts’ or moves under the adjoining
plate. The rocks that have been scraped off and plastered to the edge of North
America between 65 and 175 million years ago are collectively called the Franciscan
Complex. Bruno Mountain, Novato Quarry, Marin Headlands, Alcatraz, Nicasio
Resevoir, Yola Bolly, Permanente and Central are terranes of the bay area. The rocks
of San Francisco drop down between the San Andreas and Hayward faults. The
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Colma Formation is a pile of mostly sand that has collected and covered parts of the
Franciscan in this low-lying area.
Serpentinite forms at seafloor spreading centers, in the center of ocean plates,
where new seafloor forms as pillow basalt erupts through very thin ocean crust.
Because the crust is so thin here, the mantle underneath is close to the surface.
Seawater penetrates cracks near the magma chambers that feed the basalt. Hot
water is forced through fractures in mantle rock dissolving silica. These react with
the mantle rock as in olivine to produce serpentine minerals. The water is converted
into hydroxyl groups. Serpentine is composed of magnesium iron silicate hydroxide.
There are three kinds of serpentine mineral depending on the Mg or Fe content:
lizardite, chrysotile and antigorite.
Serpentinite is a metamorphic rock composed almost entirely of the mineral
serpentine. Antigorite, a key mineral in serpentine, is light to dark green in color,
forms in sheets of silica tetrahedrons, is loosely held together by weak bonds and
gives serpentinite rocks a greasy or silky feel, not unlike that of a snake’s scales,
hence the name.
Plants able to survive in serpentine rich soils find themselves in a situation of
drought, nutrient deprivation, exposure to light and exposure to heavy metals. Since
the overall biomass is low, plants are small and often delicate, not providing much
shade or nitrogen later for future species to survive. There are, however, many
endemic species which do survive in this seemingly harsh environment. One such
flower, steptanthus, has adapted well by forming protrusions on its leaves
resembling butterfly eggs. This deters others from laying eggs competing for
resources later.
Serpentine, derived from mantle material differs from rock in the earth’s crust. It is
low in potassium and calcium, nutrients used by plants. And serpentine often
contains high levels of potentially toxic heavy metals, magnesium, chromium, cobalt
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and nickel. Since serpentine forms in isolated areas presenting unusual soil
chemistry endemic plants and particular outcrops have evolved in these soil and
microclimatic conditions. The Presidio Clarkia and Raven’s Manzanita are examples
species growing in serpentine soil.
As serpentine is California’s state rock, there is controvery in the fact that
serpentine may contain asbestos and be harmful. This chrysotile, among 20
minerals found in serpentinite rocks, is a form of asbestos, a known carcinogen.
Exposure to which increases risk of mesothelioma cancer. Thus campaigns formed
to ‘drop the rock’ as the state rock. If rock is found to be brittle and fibrous, this
asbestos may be present.
References;
nps.gov usgs.gov
msnuleus.org
conservation.ca.gov
chemistrydaily.com
souledout.org
wikipedia.org
encyclopedia.org
bioregion.ucdavis.edu