Ring Mountain (California)

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
Highest point
Elevation603 ft (184 m)  NAVD 88[1]
Prominence402 ft (123 m) [2]
Coordinates37°54′35″N 122°29′09″W [1]
Geography
LocationMarin County, California, U.S.
Topo mapUSGS San Quentin
Pecked curvilinear nucleated petroglyphs on a rock on Ring Mountain

A number of rare and endangered flora inhabit Ring Mountain.[4] The mountain's twin summits consist of serpentinite,[5] 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.[6]

Native American pecked curvilinear nucleated petroglyphs created by the Coast Miwok people are also found here.[7]

Geology

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] The lower slopes are underlain by greywacke sandstones and shales of prehnite-pumpellyite metamorphic grade,[12] 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.[13] 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.[14]

Ring Mountain is one of the featured field trips found in the Streetcar 2 Subduction online field trip guide series[15] released in December 2019 by the American Geophysical Union.[16]

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.

Recreation

Ring Mountain is a popular hiking area and provides spectacular 360 degree views of the northern Bay Area. [17]

See also

References
  1. "Ring". NGS data sheet. U.S. National Geodetic Survey. Retrieved 2014-02-12.
  2. "Ring Mountain, California". Peakbagger.com. Retrieved 2014-02-13.
  3. "Ring Mountain, Open Space Preserve". County of Marin. Archived from the original on 2008-06-21. Retrieved 2009-12-07.
  4. Earth Metrics Inc. (1989) Marinero Estates Environmental Impact Report, Tiburon, California, prepared for the city of Tiburon, Ca.
  5. Bero, David A. (2014). "Geology of Ring Mountain and Tiburon Peninsula, Marin County, California". National Geologic Map Database. Retrieved 2019-09-28.
  6. 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. doi:10.1016/j.epsl.2004.06.003. ISSN 0012-821X.
  7. Hogan, C. Michael (2008). Andy Burnham (ed.). "Ring Mountain, The Megalithic Portal".
  8. Tsujimori; Matsumoto; Wakabayashi; Liou (2006). "Franciscan eclogite revisited: Reevaluation of the P-T evolution of tectonic blocks from Tiburon Peninsula, California, USA". Mineralogy and Petrology. 88 (1–2): 243. doi:10.1007/s00710-006-0157-1.
  9. Raymond, Loren (1 August 2019). "Origin of Mélanges of the Franciscan Complex, Diablo Range and Northern California: An Analysis and Review". Geosciences. 9 (8): 338. doi:10.3390/geosciences9080338.
  10. Mulcahy, Sean R.; Starnes, Jesslyn K.; Day, Howard W.; Coble, Matthew A.; Vervoort, Jeffrey D. (2018). "Early Onset of Franciscan Subduction". Tectonics. 37 (5): 1194–1209. doi:10.1029/2017TC004753. ISSN 1944-9194.
  11. 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. doi:10.1007/s00710-006-0157-1. ISSN 1438-1168.
  12. "Tectonostratigraphic Terranes of the San Francisco Bay Region". 1984. Cite journal requires |journal= (help)
  13. 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. doi:10.1016/j.tecto.2011.11.006. ISSN 0040-1951.
  14. Ernst, W. G. (2016-04-03). "Franciscan mélanges: coherent blocks in a low-density, ductile matrix". International Geology Review. 58 (5): 626–642. doi:10.1080/00206814.2015.1108879. ISSN 0020-6814.
  15. "Google Earth". earth.google.com. Retrieved 2019-12-22.
  16. "Streetcar2Subduction". www.agu.org. Retrieved 2019-12-22.
  17. http://www.marincounty.org/recreation Retrieved 2017-3-21
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