A Notable and Mysterious Rock

© 2008 Jack Mueller, from Whacking the Punchline.  Used with permission.

The rock below is the most notable in the kitchen, the only one that consistently draws attention, maybe because of its size.  Friends routinely pick it up, shift it in their hands, ask “why is it so heavy?”  [It weighs 12 lb (5.4 kg).]

The shape is interesting, “it’s like a brain!”  [It’s a bit bigger -- 20 cm long, and 12-15 cm wide and high -- and a lot heavier than a human brain, which weighs 3 lb (1.4 kg).]

It arrived in the kitchen several years ago, having been transported by car from the Santa Maria River on the west coast of North America to the interior.  But what is it?

I’m just an amateur geologist, with almost no training in rock id, but I do know it isn’t right to ask for identification of a rock out of context, ex situ.  Unfortunately, this rock was already ex situ when I found it ... a traveler.  It was sitting amidst coarse deposits in a broad sandy river bottom.  It appears to have traveled quite a ways, as it’s well-rounded, smoothed by bumping and grinding.  In my many walks down the Santa Maria River, I've seen only a few green rocks, and none so nicely shaped.

Here's another pretty green rock in the sandy river bed; width of view is roughly 25 cm.

The Santa Maria River is dry most of the year, but can rage during winter storms.  The bed is wide, sometimes almost a quarter mile across, with willow stands and braided deposits of sand and coarser debris.  By the end of spring, it often is covered with a healthy crop of annuals and biennials.

Santa Maria River, from Santa Maria to the sea; click photo to view details.

Such a curious rock -- large, round, an unusual and appealing color.  I decided to take it home.  A few minutes later I heard yelling in the vicinity of a willow thicket.  Sparky (dog) had met up with a man pushing a bicycle across the river.  “¡Hola, hola, buenos días!”  “Buenos días” I replied, “piedra verde grande” (large green rock).  “¡Sí, sí!” he agreed.

Maybe there are clues upstream.  The Santa Maria River is formed by the confluence of the Cuyama and Sisquoc Rivers, which drain the Coast and Transverse Ranges to the east and southeast.  The geology of these mountains is complex and hard to figure.  The area is famous today for earthquakes along the San Andreas Fault, where crustal plates jerkily slide past each other.  But before this lateral (transform) motion started ca 30 million years ago, this was a subduction zone where the North American plate overrode the Pacific plate.  The result is a “grand mess” that keeps geologists thinking and theorizing.  Today’s exposed deformed rocks include spectacular banded deep sea sediments (cherts), sediments from shallower offshore waters, volcanic rocks (metamorphosed and not), pieces of oceanic crust, and even mantle rock that was pushed up during all the crazy activity.  These are lumped together and called the Franciscan Assemblage or mélange, a very puzzling collection of rocks.

“If all rocks resembled the Franciscan Complex, no science of bedrock geology could have emerged.” (Alt and Hyndman 2000).

Look at what was going on along the coast of California 100 million years ago!
Courtesy National Park Service, Presidio of San Francisco (geology).

Among the fascinating and even awe-inspiring features of the Franciscan mélange are rocks not usually found at the surface.  I was suspicious, in fact hopeful, that this rock had come from deep in the earth.  Might it be the state rock of California, serpentine, which is part of the mélange?  (Technically, serpentine is a mineral; the rock is serpentinite.)  The serpentinite masses exposed along the Central Coast are thought to have been squeezed up from the mantle during subduction.  I had explored outcrops in nearby San Luis Obispo just a few days before.  Those exposures are greenish to bluish, and quite smooth, not really like my kitchen rock.  And is serpentinite tough enough to survive a trip down a river?

Poppies and serpentine -- California State Flower and State Rock.

Another possibility is eclogite, an uncommon rock sometimes found in association with serpentinite outcrops in the Coast Ranges.  It forms through metamorphosis of igneous rocks carried down to regions of high temperature and pressure during subduction.  One of my California geology guides (Alt and Hyndman 2000) describes eclogite as “notably dense in the hand ... surprisingly hard to break with a hammer, the quality geologists call toughness”, i.e. tough enough to survive a trip down a river.  This seems a more likely candidate.  A google search for images of eclogite didn’t rule it out, but didn’t produce anything convincing either.  Generally, the rocks in the images were green but more sparkly than mine, with visible crystals of red garnet.

My final candidate is another tough rock, greenstone, suggested by this photo from Shell Beach in northern California.  These rocks are intriguingly similar, though too far away to say for sure.

Greenstone from Shell Beach, northern California.

The chunk of greenstone below, from San Benito County in the Coast Ranges to the north, has white patches and veins like the rock in my kitchen (source; size not given).

So the rock remains a mystery for now ...  or do you know what it is?

Update:  More information and ideas about this rock can be found in this follow-up post.

Sources (in addition to links in post)

Alt, D. & Hyndman, D.W.  2000.  Roadside geology of northern and central California. Missoula MT:  Mountain Press.

Mueller, Jack.  2008.  Whacking the Punchline; Compression Sketches.  The Lithic Press,

1391 19 Road, Fruita, Colorado 81521.