Ring Mountain (California)
|Elevation||603 ft (184 m) NAVD 88|
|Prominence||402 ft (123 m) |
|Location||Marin County, California, U.S.|
|Topo map||USGS San Quentin|
A number of rare and endangered flora inhabit Ring Mountain. The mountain's twin summits consist of serpentinite, a rock which is very high in magnesium, producing soils of unusual chemistry (serpentine soil). The landscape is strewn with many sizable boulders which exhibit a variety of lithologies including high-pressure metamorphic rocks of amphibolite, blueschist, greenschist, and eclogite grade.
Ring Mountain is a unique geological site, where rocks that formed in ancient subduction zones can be observed. Serpentinized peridotite crops out on the two summits of the mountain, and the steep upper slopes are underlain by serpentinite-matrix mélange. The melange contains blocks of high-pressure, low-temperature metamorphic rocks associated with subduction zone metamorphism. Melanges of this general style are known from the Franciscan Complex, but this melange is particularly notable for the size and variety of the metamorphic blocks. Dating of metamorphic minerals in the blocks indicates that they were produced over a protracted history of subduction which began ~175 million years ago. The blocks preserve mineral assemblages characteristic of greenschist facies, blueschist facies, amphibolite facies, and eclogite facies metamorphism. The lower slopes are underlain by greywacke sandstones and shales of prehnite-pumpellyite metamorphic grade, but the contact between the sandstones and the serpentinite-matrix melange is not exposed. Landslides and their deposits are abundant on Ring Mountain, for example at Triangle Marsh, and they carry serpentinite and metamorphic blocks far downslope from their in situ positions.
The origins of the serpentinite-matrix melange, and the mechanism of mixing the metamorphic blocks of different ages and apparent thermal-burial histories, has been a matter of debate. Some authors argue that the metamorphic rocks were exposed at the surface, eroded and re-deposited into a subduction trench to form the melange as an olistostrome. Others interpret the melange as having formed in a subduction plate boundary where blocks of meta-basalt from the downgoing plate were mixed with serpentine from the upper plate mantle.
Though Ring Mountain is an island ecosystem surrounded by Highway 101 and suburbs, it hosts a variety of wildlife, including coyotes, deer, skunks, and many birds and reptiles.
Ring Mountain is a popular hiking area and provides spectacular 360 degree views of the northern Bay Area. 
- Calochortus tiburonensis
- Castilleja neglecta
- Streptanthus niger
- Triangle Marsh
- Category: Endemic flora of California
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- Ring Mountain Preserve
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