A serpentine soil is derived from ultramafic rocks, in particular serpentinite, an ultramafic rock formed by the hydration and metamorphic transformation of ultramafic rock from the Earth's mantle. The soils derived from ultramafic bedrock give rise to unusual and sparse associations of edaphic plants that are tolerant of extreme soil conditions such as a low calcium-magnesium ratio, lack of essential nutrients such as nitrogen, potassium, and phosphorus, and high concentrations of the heavy metals that are more common in ultramafic rocks. These plants are commonly called serpentine endemics, if they grow only on these soils. (Serpentinite is composed of the mineral serpentine, but the two terms are often both used to mean the rock, not its mineral composition.)

Excellent examples of serpentine soils and the distinctive ecologic communities associated with them have been described in western North America. For instance, in areas where these ultramafic rocks are patchy, such as the Klamath Basin region of northern California, the areas of serpentine soil can be clearly seen as sparsely covered areas bounded by forest on the normal soils.

Areas of serpentine soil are also home to diverse wildflowers, many of which are rare or endangered species such as Acanthomintha duttonii, Pentachaeta bellidiflora, and Phlox hirsuta. In California, shrubs such as leather oak (Quercus durata) and coast whiteleaf manzanita (Arctostaphylos viscida ssp. pulchella) are typical of serpentine soils.

Appearance

Serpentine-rich rock has a mottled, greenish-gray color with a waxy feel to it. These rocks form by the reaction of olivine-rich rock, peridotite, with water. Serpentine-rich rock forms during this reaction under a variety of conditions. For instance, it forms near the surface of the earth, particularly where water circulates in cooling rock near mid-ocean ridges: masses of the resulting serpentine-rich rock are found in ophiolites incorporated in continental crust near present and past plate techtonic boundaries.

It also forms within the upper part of the Earth's mantle during subduction, when crust containing wet oceanic sediments is submerged under a land mass, and water is expelled upwards into mantle peridotite. The resulting serpentine-rich rock is less dense, softer, and more slippery, giving it the ability to work its way upward along fault lines. It rises through rock layers while under pressure and eventually may be exposed at the surface.

Distribution

Serpentine soils are widely distributed on Earth, in part mirroring the distribution of ophiolites. Although it covers only about 1 percent of the state's surface, the state rock of California is serpentine. One such area in California is the Edgewood Park and Natural Preserve. Serpentine soils also are present in small but widely distributed areas within the Appalachian mountains of eastern North America.

Serpentine endemism and bioremediation

The unique plants that survive in serpentine soils have been used in the process of phytoremediation. As those plants developed specialized processes to exist in a metal-saturated environment, the process (also known as bioremediation) utilizes them to treat environmental problems by consuming those minerals from polluted soil.

See also

• Serpentine
• Ultramafic rock
• Pygmy forest

References

1. Alexander, E. B., Coleman, R. G., Keeler-Wolf, T., and Harrison, S., Serpentine Geoecology of Western North America: Geology, Soils, and Vegetation. Oxford University Press, 2007.
2. San Mateo County - Parks Department - - Edgewood Park & Natural Preserve
3. Dann, Kevin T., Traces on the Appalachians: A Natural History of Serpentine in Eastern North America. Rutgers University Press, 1988.
4. NATO Advanced Study Institute on Phytoremediation of Metal-Contaminated Soils, Morel, J.-L., Echevarria, G., & Goncharova, N. (2006). Phytoremediation of metal-contaminated soils. NATO science series, v. 68. Dordrecht: Springer.