California State Rock – Serpentine

California State Stuff IV


In recent columns I began reviewing the California State “things.”
We covered the state flower (California poppy), the state bird
(California quail), the state trees (coast redwood and giant
sequoia), the state animal (grizzly bear) and the state marine
mammal (California gray whale).

So what is the state rock of California? Hard rock, punk rock,
hillbilly rock, or, heaven forbid, easy rock ? You have three
seconds to think about it. Sorry, time’s up. It is serpentine rock.
This unique and beautiful rock is rare in most of the world but
here in California we have largest exposures of serpentine in North
America. It’s almost a perfect choice for our state rock except
that it is not found in Southern California.

Because serpentine makes poor soil, few plants can grow on it;
therefore there is little to obscure this showy rock. It’s nearly
always a lovely chrome green but occasionally it can be nearly
black. Often it has a reddish veneer because the iron on its
surface rusts. The word serpentine refers to the mottled, snake-
like pattern periodically seen on the rock as different minerals
run through it in a criss-cross pattern. The Greek physician
Dioscorides suggested ground serpentine as a prevention for snake
bite. Not a healthly idea as you’ll soon learn.

Early geologists in California recognized the economic resources in
serpentine. Mercury, nickel, chromium, magnesite and asbestos were
often found near its outcroppings. Mercury, also known as
quicksilver, was first extracted because of its role in recovering
gold from gravels in sluice beds. Southern Marin gets their water
from the slopes of Mt. Tam where there are large areas of
serpentine. The drinking water therefore contains asbestos fibers.
But asbestos is only bad for you when you breathe it. So be sure to
tell out-of-town visitors not to snort the water in Mill Valley.

Serpentine is so rare because the conditions necessary to form it
are rare. The source material of serpentine is called peridotite
which is upwelled magma mostly composed of two minerals — a hard, greenish, magnesium silicate olivine and pyroxene. Both of these contain large amounts of iron. Our local serpentine formed along the San Andreas fault. As the Pacific plate dived under the North American plate the peridotite was subjected to intense pressure. Because this occurred near the surface and under the sea, the temperatures stayed low. This is a very unusual condition because
usually high pressures generate high temperatures. The process of
“serpentinization” is the incorporation of water molecules into the
crystalline structure of the rock under high pressure but low heat
so it can’t vaporize. So serpentine is a rock full of magnesium and
iron with water molecules stuck in it.

Few plants can tolerate soils derived from serpentine. The outcrops
are often steep and unstable; the soil is highly acidic with toxic
amounts of nickel, magnesium, and cobalt. There is very little
calcium and potassium, two elements absolutely essential to plant
growth. Since these areas have little plant cover they tend to get
very hot and dry. These poor conditions create “islands” of
serpentine outcroppings where only a few kinds of plants have
evolved mechanisms to survive these harsh conditions.

These plants are often endemic to serpentine, that is, they are
found no where else. In Marin we are blessed with a plethora of
rare serpentine plants. For example on Ring Mountain along the
Tiburon peninsula grows Calochortus tiburonensis, a beautiful
mariposa lily that grows no where else in the world. The slowest
hike I ever took was with the California Native Plant Society one
spring on Carson Ridge on the north side of Mt. Tam. So many
serpentine flowers and plants captured our attention that three
hours after starting the hike we could still see the cars!.



Posted on

August 22, 2009