Joaquin Miller Park contains excellent examples of serpentinite. This large boulder, placed by the park entrance, is a textbook example of how this rock type forms. (Click the photo for a 1000-pixel version.)
Serpentine rock starts out as peridotite (“per-RID-a-tight”), a very important rock type that is rarely seen because it composes most of the Earth’s mantle, beneath the crust. Bear with me as I take you through the outskirts of plate-tectonic petrology.
The Earth’s mantle is hot and under pressure — so hot and under so much pressure that if you ease up on the pressure, even a little bit, it starts to melt. That’s what happens in places where the crust is spreading apart. Just a fraction of the mantle rock melts, only a few percent, and the melt — magma — leaks upward, seeking to erupt as lava. Magma isn’t the same composition as the rock it leaves behind. It’s enriched in elements like silicon and aluminum, and depleted in others like magnesium and iron. Silicon is the biggie that governs all of magma chemistry, and geologists track it (like other elements) in terms of its oxide, SiO2 or silica.
The mantle is real low in silica, around 40%. The first melt that comes out of the mantle is about 50% silica and hardens into the rock called basalt. The crust of the ocean floor is almost entirely basalt. Beneath it is peridotite, the badass dregs that the magma left behind.
As plate tectonics keeps sweeping the dense ocean crust back down into the mantle, remelting keeps concentrating silica in the magmas, which become less and less dense until they end up in the continents. Granite, the continents’ workhorse rock, is over 70% silica. (Granite is made of the minerals quartz, which is 100% pure silica, and feldspar, which is about 50% silica.)
The upshot of all this is that peridotite, the dense left-behind dregs rich in magnesium and iron, rides along on the bottom of the ocean crust, and almost all of it stays deep in the Earth. Occasionally chunks of ocean crust end up on land, where they’re called ophiolites (“OH-fee-alights”). Oakland contains bits of the well-known Coast Range ophiolite.
Peridotite, badass as it is, is helpless against superheated water, which reacts with its minerals (olivine and pyroxene) to form a hydrated mineral, serpentine. Let me show you what happens in these photos from the Klamath Mountains, America’s largest exposure of peridotite. This is a split-open boulder sitting in a roadside turnout up there. You can see similar examples up at the Serpentine Prairie preserve, off Skyline Boulevard.
When all of this rock was still a few miles underground, superheated water entered the gray-green peridotite along cracks, and the alteration spread outward from the cracks. Serpentine takes up more space than the unaltered minerals, so the outcome is just like driving wedges into the rock. Serpentine is also softer. That’s how this peridotite outcrop ended up looking like it does — alteration, then erosion.
This stage of alteration is preserved because conditions cooled off before the serpentinization process could finish. Usually, peridotite is completely altered. After that, serpentinite tends to slip and slide and flow, erasing any hints of its original structure.
Look again at the big boulder at Joaquin Miller Park. It shows those same spikes on its upper rim, plus a spiderweb of alteration cracks in the center. I find it mesmerizing.