Pillow lava in the road

Several large pillows of green, black, and reddish basalt are visible in the road. Pillow lavas are formed under water and are the most abundant volcanic rocks on earth. A pillow starts as a crack in an undersea lava flow, protrudes, and then extends the main lava flow. A pillow may also implode or explode from gases within the flow. The outside of a pillow (the chill margin) is originally black obsidian, which has long since worn off the pillows seen here. These pillows were presumably formed at the East Pacific rise zone of sea floor spreading separating the Pacific plate from the Farallon plate and Nazca plate of South America. Basalt that has been altered by warm water forms greenstone. White streaks in the rocks here are probably calcite or quartz introduced long after their volcanic origin.