Often this rock is referred to as serpentine. However, it is actually called serpentinite and is made up of serpentine minerals. It is usually this distinctive bluish-green color, but can range from apple green to quite dark in other locations. A light and dark mottling is common. The shiny, waxy look is typical and has a slippery feel reminiscent of soap.
Under a hand lens or microscope, serpentinite looks like it is made up of either very thin plates or long fibers depending upon which of the three most common serpentine minerals make up the rock. These minerals are lizardite, chrysotile, and antigorite. Lizardite and antigorite form platy (flat flakes) crystals while chrysotile forms light colored fibrous (long strands) crystals. Fibrous chrysotile is the most common form of asbestos.
Chrysotile tends to be golden or white and should be avoided. If you handle or touch rocks containing Chrysotile, you should wash your hands before eating or touching your mouth.
Serpentinite is a metamorphic rock. But unlike the majority of metamorphic rocks that form from increased heat and pressure, serpentinite forms most commonly when a mantle rock known as peridotite comes in contact with water, rises and cools, usually in a subduction zone or near a ocean spreading ridge. Peridotite is an igneous rock that is relatively low in silica and high in magnesium and iron (otherwise known as mafic).
Mafic rocks typically form in the earth’s mantle below the crust. This deep in the earth, the temperature is very high and the pressure exerted by the overlying rock is very high. The minerals that form during these conditions are stable only under these high temperature and pressure conditions. Mantle rocks also have generally different chemical composition than rocks on the crust. This makes the soils that form from these rocks difficult for most plants to tolerate (see Unusual Plants & Rocks of the Presidio)
As the peridotite is pushed up into the crust at a mid-ocean spreading center and moves away, the temperature and pressure on the minerals is reduced. Fractures from plate movement and cooling rock create conduits for sea water to come into contact with the peridotite. Another major location where fractures are created is at subduction zones where the oceanic plate full of peridotite bends, forming additional cracks, and descends back into the mantle.
Now that the minerals that make up the peridotite are no longer in the high pressure and temperature conditions under which they are stable, they begin to metamorphose. Water is incorporated into the minerals and they become serpentine minerals turning the peridotite into serpentinite. This methamorphic process is called serpentization.
Serpentization increases the volume of the crystals by about 30%, thus decreasing the density of the rock. This further enlarges any cracks in the rock allowing more water to come into contact with unmetamorphosed peridotite, continuing the process deeper into the rock. This less dense rock begins to work its way up through more dense material above it, partially explaining how serpentintie is exposed at the earth’s surface.
It is at subduction zones where serpentinite gets sheared off the subducting plate and attached to the continental plate. A subduction zone was present off the coast of California relatively recently. That is why serpentinite is common in California.
Serpentization also creates heat in exothermic chemical reactions. The chemical reactions are capable increasing the temperature of the surrounding water by about 260 deg C (550 deg F). This creates deep undersea thermal vents as hot water is forced out of sea floor cracks pulling in cooler water. Entire undersea ecosystems have developed around these vents that are completely independent of the sun as the heat and food source (methane) is all derived from the chemical reactions of serpentization. It is thought that such ecosystems may have been important in the formation of early life.
Logging questions:
- The text "GC3BPP5 Golden Gate NRA Serpentinite" on the first line
- Where is the chrysotile found in the outcrop?
- Estimate the relative percentage of Lizardite and antigorite to chrysotile.
The above information was compiled from the following sources:
- http://geology.about.com/od/minerals/ig/silicates/minpicchrysotile.htm
- http://geology.about.com/od/rocks/ig/metrockindex/rocpicserpentinite.htm
- http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Notes/serpentine.html
- California Geological Survey - CGS Note 41-Guidelines for Reviewing Geologic Reports; Serpentine: California State Rock; http://www.conservation.ca.gov/cgs/information/publications/cgs_notes/note_14/Pages/Index.aspx
- Serpentine, Geohiking; http://www.geohiking.com/serpentine.html
- Ask GeoMan...; http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry45.html
- The Deep Meaning of Lost City; A new type of vent sheds light on a major Earth process; By Andrew Alden, About.com ; http://geology.about.com/cs/escibasics/a/aa081003a.htm
- Reaction Sea-water – Mantle: production of serpentine; http://www.ifremer.fr/serpentine/english/scientific-sheet-5.htm
- SERPENTINE and SERPENTINITE, USGS, http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Notes/serpentine.html http://www.nps.gov/goga/forteachers/serpentinite-faq.htm