Monday, February 11, 2013

Mysterious Green Rock, continued

Last week, I posted about a curious rock from the Santa Maria River (California) that I suspected was one of the unusual rock types on the Central Coast, products of subduction back when the North American plate was overriding the Pacific plate.  Readers’ comments and further investigation have made the whole business even more interesting.

My first thought was serpentinite -- mantle material squeezed up during subduction and metamorphosed in the process.  Serpentinite starts out as peridotite, a common igneous rock in the upper mantle, composed mainly of olivine and pyroxene.  These minerals are unstable in the face of weathering, and by the time they reach the upper crust and surface, they usually have been altered to serpentine, producing serpentinite (serpentinite is a rock, serpentine is a mineral; even so, the State Rock of California is called serpentine).  Several readers agreed serpentinite is a strong candidate.

Others suggested harzburgite (also here), a variety of peridotite and in fact the most common one in these types of subduction zones.  Part of the logic for this choice is that serpentinite may not be tough enough to survive a rough trip down the Santa Maria River from its headwaters to where I found the rock.  But harzburgite is a peridotite, with those unstable minerals often altered to serpentine -- is it possible to find chunks of harzburgite at the surface?

How about partially-metamorphosed harzburgite/serpentinite?  It seems there could be various degrees of metamorphism, with intermediates.  So I was really excited to find this thin-section of “serpentinized harzburgite” at Evelyn Mervine’s website, Georneys ...
Serpentinized harzburgite in thin section, plane polarized light.  Used with permission.
... and this photo of “Serpentinite; Harzburgite with Asbestos” (source; no size given)
It looks like the mystery rock also has asbestos veins and veinlets (below, click to view).
Serpentinite sometimes contains the mineral chrysotile, which in fibrous form is a type of asbestos.  In fact, serpentinite was declared the State Rock of California in 1965 in part to promote the asbestos industry.  But times changed.  In 2010, a bill was introduced to strip serpentine of its title because “California should not designate a rock known to be toxic to the health of its residents.”  Californians once again were confusing a mineral with the rock that contains it (sometimes).  Fortunately the bill failed.  See Geotripper’s posts here and here for an interesting and inspiring account of the bill and its defeat.
The South Hills Natural Reserve in San Luis Obispo, California, features hiking trails through beautiful outcrops of serpentin(it)e, still the State Rock.
The California coast ca 100 million years ago, with subducting oceanic crust and serpentinite (lavender).  Courtesy National Park Service, Presidio of San Francisco (geology).
One term led to another, and in the process of reading about various mantle-derived rocks, a clearer picture emerged of how all these things fit together on the Central Coast.  The massive moving plates of the earth’s surface consist of crust and the upper part of the mantle -- mainly peridotite.  Thus oceanic plates are peridotite topped by rocks from marine environments.  During subduction, pieces of oceanic plates sometimes are plastered onto the continental margin rather than subducting.  These are ophiolites, which typically consist of “peridotite plus associated rocks such as gabbro, pillow basalt, diabase sill-and-dike complexes, and red chert”.  As it turns out, California’s Central Coast is well-known for ophiolites, and in fact the Coast Range ophiolite is recognized as an example rich in harzburgite ... another piece of evidence for the puzzle.

Now when friends ask about the striking green rock on the kitchen counter, maybe I will tell them it’s serpentinized harzburgite (wow!).  But without some kind of destructive analysis, I probably can't be sure.  More likely I'll just say it’s mantle rock ... a mysterious traveler from the depths of the Earth.
© 2008 Jack Mueller, from Whacking the Punchline.  Used with permission.

2 comments:

  1. Sorry I didn't catch your response in the previous post- I could've been clearer. When I said "tectonized" I meant to imply that it had been metamorphosed both cataclastically, and almost always hydrated to some extent. So a good deal of the original ultramafic minerals would have (chemically) altered to serpentine minerals at the same time the rock was being (physically) crushed. In other words, two different processes taking place at the same time.

    Dana and I are hoping to get down to the Josephine Ophiolite some time this year, though we haven't scheduled that trip yet. You can rest assured I'll get plenty of photos of very similar rocks in the wild.

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    1. It will be great to see those photos!

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