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One of California's coastal volcanic fields, the Pinnacles volcanic field (or Pinnacles National Park) is an eroded collection of mostly rhyolite lava flows, intrusions, and pyroclastic breccias erupted around 23.5 million years ago.

A broad orange and grey pyramid-shaped rock face contrasts with a clear blue sky, and sits among smaller spires and pillars of the same rocks. All are cut by deep cracks and depressions, and subtle angled layers are visible in the largest rock face. Their bases are surrounded by sagebrush and scrubby trees, and in the background is a low boulder-strewn hill of the same rocks.
Weathered rhyolite breccias form the spires, towers, and caves that characterize Pinnacles National Park. These relatively soft volcanic deposits are perched on harder, more resistant lavas and intrusions that make up the base of the volcanic field. USGS photo by Jessica L. Ball

These fields likely formed from the subduction of a spreading ridge on the Farallon tectonic plate, allowing hot material to pop up through the coastal mountains and erupting in fits and starts. Many cycles of explosive and effusive rhyolitic eruptions built up the Pinnacles in their original location, just to the north of Santa Clarita, CA. 

But the Pinnacles aren't anywhere near Southern California now - so what happened? 

That's the next part of the story. As the Farallon tectonic plate was completely subducted beneath the North American plate, tectonic movement switched from plates colliding to plates sliding past each other - a transform boundary. This was the famous San Andreas fault zone, and as it split the coastal mountains, it also dragged most of the Pinnacles volcanic rocks northward on its west side. The remaining bits are still located in Southern California and are known as the Neenach Volcanic Formation, but the rest moved 195 miles (315 km) northward to their current location 30 miles (48 km) southeast of Salinas.

Along the way, thousands of feet of rubble covered and then were eroded off the volcanic rocks, and even the intrusive rhyolites were exposed to weathering. Over time, the softer breccias were worn away into tall spires and hollowed out into caves, still perched atop the more resistant lavas and intrusions. Nowadays, these fantastic formations make up one of the newest National Parks, and are also home to a thriving population of California Condors!

Read more about the geology of Pinnacles in