How Volcanic Islands Form.

The Volcanism page explains hotspots and why Hawaii forms as a chain — but not how a volcano actually becomes an island, or what happens to it afterward. That's its own real story, and Hawaii happens to lay out the entire life cycle in one place, start to finish.

It starts thousands of feet underwater. A hotspot volcano begins erupting on the ocean floor, sometimes in water over 3,000 feet deep, with no sign of it at the surface at all. Each eruption piles more lava onto the last, slowly building a submarine mountain called a seamount. This can take a very long time — most seamounts never make it to the surface at all.

If it erupts long enough, it breaks the surface and becomes an island. The newly emerged land is fragile at first — loose ash and rubble, easily worn away — but continued eruptions armor it with solid lava flows that can resist wave erosion. As long as the volcano sits over the hotspot, it keeps growing, eventually building the broad, gently-sloped shield shape described on the Volcanism page. This is the island at its biggest and most active — Hawaii's Big Island, still sitting directly over the hotspot today, is at exactly this stage.

Then the plate keeps moving, and the volcano's fuel gets cut off. Because the hotspot itself stays put while the Pacific Plate drifts slowly northwest over it, the volcano eventually moves off the plume that's been feeding it. Eruptions taper off and stop. From this point on, the island only gets smaller: wave erosion wears down the exposed rock, and the island also physically sinks — partly because the crust cools and contracts as it moves away from the hot mantle plume beneath it, and partly because the sheer weight of the volcanic mass presses down on the plate it's sitting on (the same kind of load-bearing sinking, at a much larger scale, as a heavy object slowly pressing into soft ground).

This is where coral takes over the story. While the island is still above water, coral reefs begin growing in the shallows around its shore — a fringing reef, hugging the coastline. As the island keeps subsiding, the reef keeps growing upward to stay in sunlight, gradually becoming a barrier reef, separated from the shrinking island by a widening lagoon. Charles Darwin worked out this sequence in the 1830s, well before anyone could see the seafloor to confirm it directly, purely by reasoning through the geology.

Eventually, the volcano disappears beneath the waves entirely. If the coral has kept pace with the sinking the whole way, what's left is an atoll — a ring of coral reef surrounding a lagoon, with no island left in the middle at all. If the water is too cold for coral to keep growing (coral needs warmth and sunlight, and the Pacific Plate carries these islands slowly toward cooler, higher latitudes), the reef can't keep up, and the drowned volcano becomes a guyot instead — a flat-topped seamount, its peak planed flat by wave erosion before it sank too deep to be touched by waves at all.

Hawaii shows every stage of this at once, laid out across the map. The Big Island is still actively erupting, directly over the hotspot. Move northwest and each island is progressively older and further along the same decline: Maui, dormant and eroding; Oahu, more heavily eroded, its shallows lined with fringing reef; Midway, the volcanic peak gone entirely, now just an atoll. Keep going and the chain bends northward and continues as the Emperor Seamounts — fully submerged guyots, stretching all the way to Kamchatka, Russia, tracking roughly 80 million years of Pacific Plate motion. Kure Atoll, the northernmost point in the chain where coral can still barely keep up with subsidence, is even known to scientists as the "Darwin Point" — named for exactly this process.