P?h?honu in Hawai’i is Earth’s largest volcano

An extinct Hawaiian volcano called P?h?honu is the largest on Earth, with a volume twice that of the next leading contender. In addition, the lava that once erupted from P?h?honu is the hottest recorded in the past 66 million years.

At the surface, P?h?honu doesn’t look like much: just a pair of barren, rocky outcrops hundreds of kilometres north-west of the main Hawaiian islands. Its Hawaiian name means “turtle surfacing for breath”.

Michael Garcia and his colleagues at the University of Hawai’i at M?noa in Honolulu surveyed P?h?honu in 2014 using sonar and gravity sensors, which can measure mass. They found it has a volume of 148,000 cubic kilometres. That dwarfs Mauna Loa, another Hawaiian volcano generally thought to be the largest, which is only 74,000 km3, although it is still taller than P?h?honu. The gravity data also revealed that there is one central mass, confirming that P?h?honu is one volcano rather than the result of lavas from several volcanoes flowing together.

The other serious contender for the title of world’s largest volcano was Tamu Massif, a submarine mountain off the coast of Japan that is the size of the British Isles. In 2013, a team of researchers claimed that it was a single, 4-kilometre tall volcano, but in 2019, the same group announced that this wasn’t the case. Tamu Massif is actually the product of magma rising up from seafloor and spreading; so it isn’t a single volcano at all.

There are also supervolcanoes like Yellowstone in the US and Campi Flegrei in Italy, but Garcia says these aren’t directly comparable. “The eruptions [of supervolcanoes] are bigger in the total volume of material erupted”, he says, and the Yellowstone crater is “enormous”, but researchers aren’t sure of their total volume. “They’re known for their very large eruptions, but not necessarily for their total bulk. It’s kind of comparing apples and oranges.”

Analysis of the of the chemical composition of the rocks shows that P?h?honu also erupted unusually hot lava – hotter than any other known volcano from the past 66 million years. Like the other Hawaiian volcanoes, it is powered by a plume of unusually hot magma in Earth’s mantle. Fortunately, P?h?honu has been extinct for at least 12 million years.

This story is based on an article in New Scientist. The original research was published in the journal Earth and Planetary Science Letters.

Bill Gray