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Serpentinite

A rock composed almost entirely of serpentine minerals. Also known as serpentine rock. The glossy, glazed luster and green-black color are characteristic of serpentinite.

A common rock composed of serpentine minerals; usually formed through the hydration of ultramafic rocks, dunites, and peridotites in a process known as serpentinization. The result is the formation of hydrated magnesium-rich minerals, such as antigorite, chrysotile, or lizardite, commonly with magnetite or, less frequently, brucite.

A nondescript rock with a dark green to yellow-green color, often streaked or shot through with veins of other colors. Rock tends to be soft (H 3-5), and have a greasy or silky feel, especially on smooth flat surfaces; cut surfaces have a highly polished- waxed slickness to them. Luster tends to be dull or waxy.

Serpentinite is a rare rock type that is developed only in subduction zones, where one tectonic plate is subducted under another. It is a rock comprised of one or more serpentine minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle. The alteration is particularly important at the sea floor at tectonic plate boundaries. It is the state rock of California, USA.

Because serpentinite outcrops form small isolated areas of unusual soil chemistry, endemic plants adapted to these chemical conditions, and the microclimates of a particular outcrop, often evolve there.

The most likely parent rock for serpentinite is the ultramafic igneous rock peridotite. The mafic igneous rocks basalt and gabbro are also possible precursor rocks for serpentinite. Serpentinite is compose of the mineral serpentine which forms as the igneous minerals olivine and pyroxene are subjected to rising temperature and pressure conditions along a subduction plate boundary. Serpentinite can be cut and polished for use as ornamental stone inside buildings.

The term serpentine is applied to any of a grouping of hydrous magnesium-rich silicate minerals that are typically gray, green, or white in color. A type of rock that is almost completely composed of serpentine is known as serpentinite.

Associated with subduction zones, serpentinite is a metamorphic rock that is formed from the action of high pressure and heat upon hornblende schists or igneous rocks composed chiefly of mafic minerals, such as peridotite, gabbro, or basalt. The substance is most often found in areas where mountain ranges have formed due to the sealing off of an ocean basin. For instance, the Coast Ranges in California, where serpentinite is the official state rock, primarily consist of slices of the ocean’s crust that have been faulted and folded along the coastline. Relatively weak, mountains containing high levels of serpentinite may suffer from recurrent landslides and a brisk erosion process.

Due to its derivation from the Earth’s mantle, protrusions of serpentinite are often quite different from the environment that surrounds them. Indeed, the rock’s low levels of calcium and potassium, as well as its significant amounts of such potentially harmful substances as chromium and nickel, would seem to make it an unlikely place for habitation. However, some plant species have been able to successfully adapt to the unusual characteristics of serpentinite. For example, in Presidio of San Francisco, which is part of the Golden Gate National Recreation Area, several endangered species of plants have become acclimatized to life on serpentinite outcrops.

Scientists do not fully understand how serpentinite makes its way from deep in the Earth, where it often forms, to the surface of the Earth. In the process of formation, serpentinite rock actually becomes less dense (more buoyant) as water is added. Thus it essentially floats upwards, buoyed up by the denser rock around it. Serpentinite is also a very plastic and greasy rock that can be easily squeezed into and even lubricate the many faults at plate boundaries. It then may get dragged or “squirted” to the surface along the faults.