Pecked curvilinear nucleated petroglyphs on a rock on Ring Mountain
Ring Mountain is an elevated landform on the Tiburon Peninsula in Marin County, California. This mountain was named for George E. Ring, who served as a Marin County Supervisor from 1895 to 1903.[3]
Ring Mountain is a unique geological site, where rocks that formed in ancient subduction zones can be observed.[8] Serpentinized peridotite crops out on the two summits of the mountain, and the steep upper slopes are underlain by serpentinite-matrix mélange.[5] 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,[9] 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.[10] The blocks preserve mineral assemblages characteristic of greenschist facies, blueschist facies, amphibolite facies, and eclogite facies metamorphism [11] and is the type location of the mineral Lawsonite.[12] The lower slopes are underlain by greywacke sandstones and shales of prehnite-pumpellyite metamorphic grade,[13] 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.
Blueschist block embedded in serpentinite matrix melange on the west side of Ring Mountain.
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.[14] 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.[15]
Ring Mountain is one of the featured field trips found in the Streetcar 2 Subduction online field trip guide series[16] released in December 2019 by the American Geophysical Union.[17]
Animal life
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.
Plant life
Ring Mountain is the home of the only population of Tiburon mariposa lily in the world. This flower grows near the summit of the mountain in the grassy areas. Ring Mountain is also home to other rare plants such as the Tiburon jewelflower and the Tiburon paintbrush. The Nature Conservancy bought the land around the mountain and has been responsible for preserving the rare native plant species of the area.[18]
Recreation
Ring Mountain is a popular hiking and rock climbing destination and provides spectacular 360 degree views of the northern Bay Area.[19]
^Anczkiewicz, Robert; Platt, John P.; Thirlwall, Matthew F.; Wakabayashi, John (2004-08-30). "Franciscan subduction off to a slow start: evidence from high-precision Lu–Hf garnet ages on high grade-blocks". Earth and Planetary Science Letters. 225 (1): 147–161. Bibcode:2004E&PSL.225..147A. doi:10.1016/j.epsl.2004.06.003. ISSN0012-821X.
^Tsujimori, T.; Matsumoto, K.; Wakabayashi, J.; Liou, J. G. (2006-08-22). "Franciscan eclogite revisited: Reevaluation of the P–T evolution of tectonic blocks from Tiburon Peninsula, California, U.S.A.". Mineralogy and Petrology. 88 (1): 243. Bibcode:2006MinPe..88..243T. doi:10.1007/s00710-006-0157-1. ISSN1438-1168. S2CID55963012.
^Ransome, Frederick L. (1895). "On lawsonite, a new rock forming mineral from the Tiburon Peninsula, Marin County, California". University of California. Bulletin of the Department of Geology. 1 (10): 301–312.
^Wakabayashi, John (2012-09-28). "Subducted sedimentary serpentinite mélanges: Record of multiple burial–exhumation cycles and subduction erosion". Tectonophysics. Chaos and Geodynamics: Melanges, Melange Forming Processes and Their Significance in the Geological Record. 568–569: 230–247. Bibcode:2012Tectp.568..230W. doi:10.1016/j.tecto.2011.11.006. ISSN0040-1951.
