The Hawaiian chain of volcanoes is many things: spectacular, incandescent, and – volcanologically speaking – unique. As a new Nature study by the Australian National University (ANU) has pointed out, these volcanoes are essentially composed of two tracks growing alongside each other, and they think they finally know why.

At present, the region’s effusive volcanism is centered on the volcano of Kilauea, but in 10,000-100,000 years, this will move onto Lo’ihi, an underwater seamount (volcano) that will one day rise up above the waves and form the newest island in the Hawaiian chain.

The creation of the extinct volcanic isles in the chain, Lo’ihi’s birth, and Kilauea’s eruptive activity is all down to an upwelling superheated mantle plume. This type of “hotspot” volcanism can be found in several other places across the world, which causes a vast degree of melting in the crust and produces a lot of magma that often continuously erupts on the surface.

As the Pacific tectonic plate continues to drift around the world, the hotspot remains stationary, so it melts the crust along a path – consequently, you have a chain of volcanoes.

That’s not all there is to the extremely unusual Hawaiian hotspot. If you look at them on a map, you’ll notice another section, the Emperor Chain, bending off quite sharply in another direction.

This couldn’t be explained by the movement of the Pacific plate, but a team of volcanologists last year worked out that it was because the mantle plume itself was bent around 50-100 million years ago. This meant that it wasn’t melting crust directly above it, but to the side of it, so to speak – hence the bend in the Emperor-Hawaiian Seamount Chain.

Whatever caused this plume to warp – a sudden flow change in the mantle, probably – it also appears to have “split” the melting regions in the crust between one of high-pressure and another of low-pressure. This generated two zones of parallel but distinctive volcanism, named Loa and Kea.

^{Hawaii's two volcanic tracks have been known about for sometime, but their origin has been unclear until now. Tim Jones/ANU}

According to the team’s models, around 3 million years ago, this double-pluming was exacerbated by a sudden change in the movement of the Pacific plate. This helped make the two small hotspots more distinct, and as the plate continued to drift around, two volcanic “tracks” were formed.

This helped push magma up across a wider area, which explains why the Hawaiian volcanoes are so voluminous. Indeed, Mauna Loa was for a long time thought to be the world’s largest volcano – this has since been pushed to second place by Tamu Massif in the Northwest Pacific Ocean, which is the size of New Mexico.

One day, the plume will reset itself and align with the Pacific Plate, thereby merging the two volcanic tracks into one.