c. 28 Million BCE

Hawaiian Islands

John Tuzo Wilson (1908–1993)

The Earth’s mantle releases internal heat in giant cycles of convection—rising of hot deformable or molten rock to the surface, where it cools, becomes less buoyant, and sinks back down. Geologists have a special name for places where the hot, rising plume of magma reaches close to—or breaches—the surface: hotspots. Hotspots are places where volcanoes erupt seemingly “in the middle of nowhere,” compared to other more common volcanic eruptions along the highly active boundaries of converging or diverging plates. Examples of famous past and present hotspot volcanoes include the Deccan Traps, the Yellowstone Caldera, Iceland, and the Hawaiian Islands.

The Big Island of Hawaii and its neighboring islands and undersea mountains (“seamounts”) are aligned in two long segments, first a 1,500-mile-long (2,400-kilometer) chain of large islands and seamounts running from the Big Island to the northwest, and second a 1,000-mile-long (1,600 kilometer) archipelago of seamounts running much more north–south all the way to the Aleutian Islands of Alaska. The linear nature of the alignment of the islands, including the sharp bend in the alignment toward the north, provided a clue to geologists: perhaps there was a hotspot in the middle of the Pacific Ocean that the Pacific plate has slowly been moving on top of over time.

The second important clue to this hotspot hypothesis for the formation of the Hawaiian chain comes from the ages of the volcanic rocks on these islands. On the Big Island, fresh volcanic rock from the Kilauea and Mauna Loa volcanoes is erupting onto the surface and growing that island today. But the rocks on the next island west, Maui, are about a million years old. Continuing west from there, the rocks on Molokai, Oahu, and Kauai are about 2, 3, and 5 million years old, respectively. The pattern was clear to Canadian geologist John Tuzo Wilson, who first figured it out in 1963: older island and seamount rocks heading northwest mean that the Pacific plate is slowly moving to the northwest over the relatively stationary Hawaiian hotspot. The oldest exposed island, tiny Kure atoll, is around 28 million years old, marking the approximate time the Hawaiian Islands first appeared above sea level.

NASA satellite view of the Hawaiian Island chain, from the Big Island of Hawaii in the southeast to Kauai and tiny Ni’ihau in the northwest. The full chain of smaller islands and atolls above sea level extends more than another 620 miles (1000 km) to the northwest.