Hotspots and Hawaii

The Hawaiian Islands are more than 3,200 km from the nearest plate boundary, yet Kilauea is one of the most active volcanoes on Earth.

Volcanoes in the middle of a plate

Most volcanoes stand on plate boundaries, but some do not. The Hawaiian Islands are entirely volcanic and lie in the middle of the Pacific Plate. In 1963 the Canadian geophysicist J. Tuzo Wilson, the same scientist who proposed transform faults, offered an explanation. He noticed that volcanism at places like Hawaii had lasted for very long times, and reasoned that small, long-lasting and unusually hot regions, hotspots, must sit below the plate and supply heat. Heat from the hotspot partly melts rock, the magma rises through the plate and erupts on the sea floor, and over countless eruptions the seamount grows until it breaks the surface as an island volcano.

The plate keeps moving, though, and the hotspot stays put. Plate motion carries the island away from its magma supply, its volcanoes die, and a new volcano grows over the hotspot. The cycle repeats, leaving a long trail. HVO describes the result: the eight main Hawaiian Islands are made of 15 volcanoes, the youngest part of a chain of more than 129 volcanoes that stretches about 6,100 km across the north Pacific. Most of it is under water, as seamounts and flat-topped guyots.

A clock on the sea floor

If Wilson was right, the islands should get older and more eroded away from the hotspot. They do. USGS gives the oldest rocks on Kauai, at the north-west end of the inhabited islands, an age of about 5.5 million years, and they are deeply eroded. On the Island of Hawaii, at the south-east end, the oldest exposed rocks are less than 0.7 million years old, and new rock is still forming. The oldest dated volcano near the far end of the Emperor Seamounts is 81 million years old. A sharp bend in the chain, dated by HVO at 43 million years (some newer studies put it nearer 47 million), records a change in the direction of the Pacific Plate.

You can turn these numbers into a speed. Kauai and Kilauea are about 528 km apart, and Kauai's oldest rocks are about 5.5 million years old. Dividing gives roughly 9.6 cm per year, a rough estimate of how fast the Pacific Plate has moved over the hotspot. The youngest volcano in the chain is still under water: Kamaʻehuakanaloa, called Lōʻihi until 2021, lies off the south-east coast of the Island of Hawaii. Its most recent eruption is thought to have been in 1996, when HVO recorded 4,377 earthquakes in about a month.

Other hotspots

More than a hundred hotspots have been active in the past 10 million years. Most lie under plate interiors, but some sit near mid-ocean ridges, such as Iceland, the Azores and the Galapagos Islands. Under the North American Plate, the best known is Yellowstone, where three gigantic eruptions in the past two million years made large calderas; the heat still feeds more than 10,000 hot springs, geysers and mud pots, and parts of the area rise and fall by up to 1 cm a year. USGS adds an honest note: since its booklet was written, scientists have debated whether hotspots really come from deep, fixed plumes, so the simple picture is still being tested.