Still trying to think of a better title for the blog... and ruminating about ophiolites thanks to this terrific page. The author, Mike Strickler, is a high school and community college prof, which gives him just the right balance of technical and accessible writing. He provides a detailed tour of the Josephine Ophiolite, which sits across the northwest California - southern Oregon border. It's a group representing the ophiolite sequence, which is what happens when the very heavy ultramafic rocks of the ocean crust get shoved up on land by colliding plates. The ultramafics turn into peridotite and serpentine when they reach the surface, because it's a chemically instable environment for them. They "like" the high-temp high-pressure, semi-plastic environment of the upper mantle, which is where they originate. Atop the heavy ultramafics are lighter mafics - gabbro, basalt, and a system of water-modified rocks between them - and then a veneer of ocean sedimentary rocks (slate, chert, conglomerates).
What's interesting here is Strickler's argument that the earth sorts itself by density. Heavy elements (iron, magnesium) seek the center, while lighter stuff (aluminum, silicon) float at the top. This means the (ultra)mafics, high in the heavy elements, tend to be ocean crust and upper mantle. The granites, higher in light elements like silicon, float high and make continental rock. An ophiolite is the rare chance to see deep crustal rock exposed.
The Bay Area has some ophiolite exposure, too: near the Golden Gate, and in Napa, between Clearlake and Lake Berryessa, what's called the Putah-Cache Ophiolite.
The word "ophiolite" comes from the Greek ophios, "snake", by way of the ophiolite's invariable association with serpentine (Latin "snake"). So the snake-rock is a remnant of the earth before the fall, before differentiation by density separated the layers.
Posted by Chris at January 10, 2004 12:46 PM