Ring Mountain's Serpentine

Looking east from one of the summits of Ring Mountain across the prominent saddle where melange is exposed, one can see the sheet-like, bouldery outcroppings of serpentinite, a rock whose chemical composition (primarily iron and magnesium with silica and oxygen) suggests that it had its origins from somewhere in the earth's mantle, beneath the surficial crust.

In some places in the Coast Ranges, the serpentinite appears to have been melted and has intruded into cracks or faults between rocks much as molten lava often does. But in other Coast Range locations, the serpentinite lacks the appearance of thermal alteration; it does not appear to have been melted: the arrangement of mineral grains is not typical of an igneous rock, nor have surrounding rocks been metamorphosed by heat, i.e., they do not show evidence of having been "cooked."

These "cold intrusions" of serpentinite usually occur in anticlinal arches through which they have been forced as massive elongate plugs, Mt. Diablo which can be seen in the distance in the above photo is a prime example. It is thought that stray masses of serpentinite may have squeezed through the compacting mass of Franciscan sediment penetrating wherever they found a way. Some of the the ocean floor materials of the Franciscan may have been swept deep beneath into the interior before they escaped back to the surface today forming the masses that now contain the unique blueschist minerals.

Through this action of the subduction zone active earlier in the history of Marin County's rocks, perhaps these diapiric (cold) intrusions occured in anticlinal arches where the serpentinite forced its way upward and with continued earth movements and erosion of the encasing, easily eroded melange are all possible factors in contemplating the current placement of these two units.

Unlike rocks of the earth's crust, serpentinite contains almost no aluminum. Lacking aluminum, its weathered sediments are unable to form clay, an essential ingredient of fertile soils. The weathering rocks do not form insoluble residues that accumulate and transform into soils, but slowly dissolve and run off in both surface and subsurface waters. Not surprisingly, the serpentinite soils of the upper slopes of Ring Mountain (locally called the "Henneke" soils) are bare and thin between rocky outcroppings of rock.

As a final challenge to plants trying to live here, serpentinite is almost devoid of potassium, sodium, calcium and phosphorus, all important fertilizers, AND it is unusually rich in magnesium, chromium and cobalt, heavy metals which are toxic to most plants.

It is rather incredible, then, that three plant species have evolved to thrive on this soil and are found today only on the Henneke soils of the Tiburon Peninsula. (Castilleja neglecta, Streptanthus niger, and Calochortus tiburonensis). It has been reported that the Calochortus, a tall Mariposa lily, occurs only within the 80 acres of the Ring Mountain Preserve, a small piece of heaven for this hearty plant.


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