FRIENDS OF EDGEWOOD NATURAL PRESERVE
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FRIENDS OF EDGEWOOD NATURAL PRESERVE
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By Carolyn Curtis Serpentine is not a rock, but a group of minerals, which can include chrysotile asbestos. Serpentinite, the term that probably makes most sense to use when referring to the rocks and soils at Edgewood, is a type of metamorphosed sedimentary rock. The type of metamorphism known as serpentinization can happen in many ways. Many minerals can be transformed into serpentinite, as, for example, when olivine and orthopyroxene combine, which happens mainly on land. For other serpentinites, the parent mineral, such as peridotite, a tan, blocky mineral, can crystallize when it is brought up from the sea floor through the crust of an opposing tectonic plate. Chemically, serpentinites are at the heavy end of the metal spectrum, containing relatively large amounts of magnesium and/or iron. Because of these elements, serpentinite is referred to as ultramafic (Mg + Fe). Serpentinite and peridotite can have a rusty-looking crust because of the oxidation of the iron (magnetite) they contain; California has many a "Red Hill" and "Red Mountain" as a result. Origin and Distribution The parent rock to serpentinite started out on the sea floor, in the lateral grooves between ridges in the deep trench off the margin of continents. As the sea floor moves up and encounters another plate, the leading edge goes down under the other plate, which is known as subduction. The lighter components of the sea floor are scraped up on top of the other plate; on the West Coast, the result became the Franciscan formation. When the subduction ceases, sedimentary rock forms in the normal way, and lots of ultramafic rock comes to the surface. Most serpentinite in California’s inner coastal ranges, such as the serpentinite in the Oakland-Berkeley hills, results from subduction. The layers of younger sedimentary rock that formerly covered this serpentinite have weathered away, exposing the serpentinite. The coast ranges are a big compression zone; the Sierra is moving west, pressing the Great Valley rocks. In this situation of moving layers, the slippery and easily mobilized serpentinite pops up along fault zones. In addition, there is tremendous hydrostatic pressure, so the serpentinite also rises along thrust lines as well. Serpentinite and Vegetation These serpentinite chemical characteristics inhibit plant growth:
The crystal structure of serpentine won't allow calcium, so this element comes out in the ground water. Serpentine seeps are calcareous water, resulting in travertine aprons where native orchids sometimes grow. The cobbles in stream beds near serpentinite areas can be cemented with this calcium. Plants have adapted to this lack of calcium in serpentinite soil in various ways: some require little calcium, others accumulate it. Calcium is highly important for plant growth; plants growing in serpentinite areas tend not to be tall. Likewise, some plants adapt to the concentrations of iron and magnesium by accumulating it. The magnesium in the serpentinite is not buffered because of the lack of calcium there, so it is toxic to most plants. Studies have shown that toxic metals aren't the problem, but rather the low nutrient levels and dryness of serpentinite areas. Plant adaptations to low nutrient levels and to aridity look the same. Despite the superficial aridity of serpentinite areas, water comes out all over serpentinite areas, drained from adjacent places; however, because serpentinite is so fractured and sheared, the water’s availability is problematic. Most serpentinite areas can have persistent streams and springs, unlike the surrounding sandstone, which dries up. |
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