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A seamount is a volcano that forms below the sea's surface, and may one day rise above it to form an island. There are at least 63 seamounts off the coast of California, spanning the entire length of the coastline. Most are located more than 100 miles offshore and a mile or more beneath the ocean's surface. None are active today, but they were between 10-25 million years ago. Seamounts are home to unique ecosystems of organisms that evolved to take advantage of hydrothermal activity, and are oases of life on the bare deep sea floor. 

Davidson Seamount is the most well-studied of these volcanoes, and was the first underwater peak to be named a seamount. Located about 80 km (50 miles) off the coast of Big Sur, it's shaped like an elongated arrowhead made up of parallel ridges of steep volcanic cones. Most of these erupted about 10-15 million years ago, and are made up 320 cubic km of hawaiite, mugearite, and alkalic basalt - types of basalt commonly found on ocean island volcanoes and spreading ridges. Photo: The robotic manipulator arm on the ROV Tiburon prepares to place another geological sample in the sample drawer. Image courtesy of Davidson Seamount Exploration 2002, NOAA/OER. 

To read more about this unique volcanic environment in California, check out https://tos.org/oceanography/assets/docs/23-1_clague.pdf and https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2009GC002665