The Earth’s crust circulates in a system of destruction and creation called plate tectonics. Earlier in geologic time, California was an active subduction zone, a place where a tectonic plate beneath the Pacific Ocean was moving toward the North America plate and plunging under it. This all changed starting 28 million years ago, and today the Pacific and North America plates rub each other sideways along the San Andreas fault zone. When that change happened, the earlier subduction zone was broken in pieces. Those pieces were then shuffled into the Coast Range as the earth’s crust rumpled up, and one of them is preserved here in Serpentine Prairie, part of Reinhardt Redwood Regional Park. It displays a part of the subduction cycle that is rarely seen—but one that’s unusually common in California.
Serpentine is typically a lustrous, green or blue-green rock with a glassy or greasy looking surface. Often it also has scattered black bits (magnetite or chromite) in it. Serpentine has been California’s state rock since 1965. Geologists make a point of calling the rock serpentinite (accent on the “pent”), saving the word “serpentine” for the family of green minerals that make up serpentinite.
Here’s how serpentinite forms and why it’s rare. When an ocean-floor plate is first made at a mid-ocean spreading ridge, the deepest part consists of a dense, green-black rock called peridotite (accent on the “rid”). As the plate ages and cools, it cracks and allows seawater to enter the peridotite layer, where the most common minerals in the rock (olivine and pyroxene) react with the superheated water and turn into serpentine minerals. These minerals take up more space than the original minerals, about 10 percent more. As you can imagine, that is very disruptive, and more cracks quickly open to let in more seawater and cause more alteration. As long as the conditions are right, most of the peridotite beneath the ocean floor is quickly converted to serpentinite. Here's a page with more details about that process.
Some of the large boulders at the entrance to the prairie are made of peridotite. Take a close look at their color and texture. Parts of them preserve evidence of that alteration process, as seawater invaded cracks in the rock and changed it into new minerals.
Serpentinite is fairly rare because ocean-floor plates almost always get carried down into subduction zones and are recycled in the earth’s mantle. But because in California the subduction zone was interrupted and broken apart, pieces of this weird-looking rock are preserved on land in many places.
The ground here at Serpentine Prairie has no trees because serpentinite has very few nutrients and often has naturally high concentrations of toxic elements, like nickel and chromium. Instead, there is a whole assortment of small plants that specialize in serpentine soils. The exhibits here have more information about these plants.
As you walk along the uphill side of the preserve, observe the rocks underfoot. Mixed with the serpentinite is blueschist, a kind of altered (metamorphic) rock that forms under the high pressures found deep in subduction zones. Depending on the original rock that was put under high pressure, blueschist is not always blue, but at Serpentine Prairie the blueschist is a metamorphosed version of basalt lava, rich in the blue mineral glaucophane. “Schist” refers to any rock made of thin layers of shiny mineral grains or crystals. If you look at it under a hand lens, you may see the short, needlelike glaucophane crystals that account for its distinctive color.
On the west side of Skyline Boulevard, Crestmont Drive goes through extensive exposures of these rocks.
To win the find in this cache, please fulfill the following tasks:
1. Describe the color of the blueschist. What kind of blue is it?*
2. Describe the color of the serpentine. What kind of green is it?*
3. Describe (or submit a picture of) a feature in a peridotite boulder that shows where it was being converted to serpentinite.
*If judging the color is difficult (due to colorblindness), describe the appearance of these rocks.