Blueschist is not abundant near the California coast. It is scattered among the much commoner greywacke, shale, and chert in a seemingly irregular fashion. Pockets of limestone are more common in the Coast Range than blueschist. The formations here, in fact the very existence of this mineral, were a puzzle to geologists for many years. Nowadays though, blueschist’s presence in the Franciscan complex, a jumbled mixture of differing rocks of some areas of the California coast, is considered proof of plate tectonics.
Schist is common enough worldwide. Blueschist, colored by the mineral glaucophane, is less common. Wherever it occurs, it is found in zones that have undergone the kinds of transformations experienced by the edge of the North American plate, in the Coast Range of California.
Geologists of the 1970s did experiments to find the conditions of temperature and pressure that could have produced this rare mineral. They heated and cooled rocks using varying parameters of pressure, temperature, and time. It turned out that high pressure but relatively low temperature were necessary to produce blueschist.
The theories of plate tectonics predicted an ideal environment for the production of this mineral: a subduction zone. At a subduction zone, an oceanic plate is being forced under a continental plate. The oceanic rock is forced down, compressing and heating it to the right degree to make blueschist.
Geologists now believe that pieces of the oceanic plate, along with metamorphic rock altered during subduction and fragments from the
earth’s mantle, were scraped off against the margin of the overriding continent. They remain near the coast of California as the Franciscan complex. So the presence of blueschist, along with the rocks that surround it, supports the chief tenet of plate tectonics, that the surface of the earth is composed of a series of plates that move about relative to one another, sometimes scraping along side by side, and sometimes one disappearing under another.
Greywacke sandstone, recognized by its dirty-looking color, and shale, also known as mudstone, are sedimentary rocks that were formed deep underwater. They sit exposed in the Coast range of California. Harder chert, nearly pure silicate, is made mostly of the deposited skeletons of radiolaria microorganisms, piled up in the ooze at the bottom of ancient seas. Greenstone is found in the Franciscan complex too, probably extruded in underwater volcanism and then slowly transferred to the edge of the continental plate. Beautiful jadeite is also part of this complex, sometimes found lying loose on sunny beaches.
Soft green serpentine is part of the complex too. It is formed when periodotite, an igneous (volcanic) mineral, is transformed by subduction. Serpentinite is the state rock of California, and known for the way non-native plants have difficulty growing in the unusual soils formed from its decay. It sometimes contains natural asbestos.
The Franciscan Complex and blueschist are evidence of the way the edge of California has been altered, and is still being altered, by plate tectonics. There are probably remnants of at least nine plates that came up against the edge of the continent and were subducted. Each added its bit of territory to California.