Pillow basalt is a type of volcanic rock that forms when hot lava erupts underwater, typically on the ocean floor. As the lava rapidly cools upon contact with cold water, it solidifies into rounded, pillow-shaped formations—hence the name “pillow basalt.” These structures often look like a pile of overlapping, smooth, bulbous stones. Because the lava cools so quickly, the outer shell hardens first while the inside remains molten for a short time, allowing the lava to continue oozing out and forming new “pillows.”
This unique formation process makes pillow basalt an important clue for geologists studying underwater volcanic activity and how new oceanic crust forms. You can find pillow basalt not only in the deep sea but also on land, in places where ancient ocean floors have been uplifted and exposed over millions of years. Their distinctive appearance and formation story make them a fascinating natural record of Earth’s volcanic and oceanic history.
A notable example of this is in the Franciscan Complex, a geologic formation along the coast of California that includes rocks formed in an ancient subduction zone. Pillow basalts found here are remnants of oceanic crust that were thrust onto the continent as tectonic plates collided and one slid beneath the other. These rocks, now visible in places like the Marin Headlands, provide clear evidence that the region was once part of the deep ocean floor. Their presence helps scientists piece together California’s dynamic geological past, shaped by plate tectonics and seafloor spreading.
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- Graywacke
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- Radiolarian chert
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