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March 20, 2024

While California has plenty of active volcanoes, it also has a rich history of volcanism preserved in its Franciscan Complex rocks.

Looking down from a cliff into the sea, a crag of stacked black pillows of volcanic rock rises from the rough surf. Some of the pillows have a greenish tinge.

These rocks, formed between 80 and 200 million years ago, preserve a long, complicated record of subduction where the overriding North American tectonic plate scraped a mixture of sediment and ocean crust off the subducting tectonic plate. This collection of scraped-up-rocks is called the Franciscan Complex or Franciscan melange (French for mixture). And since the ocean floor is composed of a variety of mafic (magnesium-rich) volcanic and intrusive rocks, those rocks are also caught up in the melange!

Part of the transformation of Franciscan Complex rocks to what we see on land today involves metamorphism, where rocks are exposed to extreme heat, pressure, or both at great depths beneath the Earth's surface. These forces affect the fundamental minerals in the rock. In the case of mafic volcanics, this means changing from things like feldspar, olivine, and amphibole to epidote, chlorite, glaucophane, lawsonite, and serpentine. Depending on the conditions they're metamorphosed under, this means basalt, gabbro, and diabase will become the greenstones/greenschists, blueschists, and serpentinites we can see at the surface today.

But not only have been California's old volcanic rocks been boiled, cooked, and crushed, they've also been carried upward by immense tectonic forces. Otherwise, we'd never see them! Different types of meta-volcanic rocks tell different stories about the depths they were subducted to and whether there were liquids and gases there to help the process along. In some cases, metamorphosis was minor and you can still see features like pillow basalt and volcanic breccia. In others, rocks were buried so deep that they're radically different from what they started as, such as blueschists. Each had a volcanic precursor, but different subduction journeys created the fascinating variety of metavolcanic rocks we find in California's Franciscan rocks today.

To learn more about the metavolcanic rocks in the Franciscan Complex, visit

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