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Hawaii's Oahu Used To Be a Bigger Island
sciencehabit (1205606) writes "The two volcanoes long thought to have formed the Hawaiian island of Oahu had a head start: They grew on top of an older volcano that's now submerged northwest of the island and partially covered by it, new research suggests. Tests indicate that the long-lost peak—now dubbed Kaena volcano—grew from the sea floor and broke through the ocean's surface about 3.5 million years ago, eventually reaching a height of about 1000 meters above sea level before it began sinking back into the sea. At its largest, ancient Oahu would have measured about 1900 square kilometers (about 20% larger than modern-day Oahu) or larger. Over the course of its lifetime, Kaena volcano spilled between 20,000 and 27,000 cubic kilometers of molten rock, the researchers estimate. When Kaena volcano became largely extinct isn't clear."
In the case of the Hawaiian Islands they form a chain of islands over a hot spot in the Earth's mantle. You can trace the path of the sea floor over the hot spot by following it island arc. So the original volcano formed and as the position of the hot spot moved southeast the volcanoes that comprise the present day Oahu formed on its flanks. The same thing is presently happening on the Big Island, Hawaii where the current volcanic activity is mostly on the southeast at Kilauea and 22 miles out to sea at Loihi Seamount while the northwest part of the island is eroding away.
So obviously the hot spot is forming new magma channels as the sea floor moves over it and whether any particular vent on the flanks of a volcano is a breakout from old channels or a new one is a moot point. Any specific channel won't last for long (in geological time) as the hot spot moves on.