Blueschist

Blueschist is a rare type of metamorphic rock that forms under high-pressure, low-temperature conditions, typically deep within subduction zones—areas where one tectonic plate is forced beneath another. Its striking blue color comes from the presence of a mineral called glaucophane, a bluish amphibole that gives the rock its distinctive hue. Blueschist often forms from basalt, a volcanic rock, and its transformation requires intense geological processes that occur deep beneath Earth’s surface.

Because of its unusual formation conditions, blueschist is an important clue for geologists studying plate tectonics and the history of Earth’s crust. It provides evidence of past subduction and helps scientists understand how continents have shifted and collided over millions of years. Although rare at the surface, blueschist can be found in certain mountain belts where ancient subduction zones have been uplifted.

In the Franciscan Complex of coastal California, blueschist plays a key role in unraveling the region’s geological past. This vast and complex geologic formation was created by the subduction of the oceanic Farallon Plate beneath the North American Plate during the Mesozoic Era. Blueschists in the Franciscan Complex are particularly valuable because they reveal the extreme pressure conditions these rocks endured, even as the surrounding temperatures remained relatively low. Their presence helps confirm the long and dynamic history of subduction that shaped much of California’s rugged landscape.

Resources

• Click here for a 3D image you can rotate.
• More on Geologyscience.com
• Video: George the Geologist – Blueschist.

Glossary links