The soft green rock that may have cradled the first life on Earth
Serpentinite forms when seawater is forced deep into rock beneath the ocean floor. The reaction releases hydrogen, feeds microbes far underground, and runs most of the chemistry needed to build acetyl-CoA, which is why its warm vents are a candidate birthplace for life. Californians made the same rock their state emblem.
The rock takes shape where mafic or ultramafic rock is soaked with seawater low in carbon dioxide, far below the seabed, both along spreading ridges and in the forearc mantle where one plate dives beneath another. What emerges is dominated by the serpentine minerals antigorite, lizardite and chrysotile, along with magnetite. When the iron-rich olivine fayalite reacts with water, it yields magnetite, quartz and molecular hydrogen, a fuel that bacteria in the deep subsurface can live on.
The Lost City Hydrothermal Field, which sits a little way from the spreading centre of the Mid-Atlantic Ridge, could be powered entirely by the heat of serpentinization, though geologists still argue about whether that is enough. Unlike black smokers, its vents release fairly cool, highly alkaline fluids of 40 to 75 degrees Celsius and have built carbonate and brucite chimneys up to 60 meters tall, crowded with microbes. In the Marianas forearc, serpentinite mud erupts from large mud volcanoes whose high-pH fluids also host microbial life.
On land the rock is hard on plants. Soils over serpentinite are thin, short of calcium and other nutrients, and loaded with chromium and nickel. Specialists such as Clarkia franciscana and some manzanitas cope, but because outcrops are scattered, their communities behave like ecological islands and many species are highly endangered. In New Caledonia, by contrast, the plants adapted to serpentine outcrops hold their ground against introduced species.
With a Mohs hardness of only 2.5 to 3.5, serpentinite carves easily. College Hall at the University of Pennsylvania is built from it, Inuit carvers made qulliq oil lamps from it, and its bound water, rich in hydrogen, slows neutrons well enough that RBMK reactors such as Chernobyl used it as top shielding. California named it the state rock, and a 2010 bill to strip that status over asbestos worries met pushback from geologists.
Source: Serpentinite
