Why so wonky, Hawaii? University of Sydney

There’s something strange about Hawaii. Part of the Hawaiian-Emperor seamount chain, it’s well acknowledged that the region’s effusive volcanism – primarily centered on the volcano of Kilauea – is down to an upwelling superheated mantle plume. This type of “hotspot” volcanism can be found in several other places across the world, but it appears to have made this particular chain of islands somewhat wonky.

The chain of islands are observed to suddenly take a sharp bend of about 60°, and no other seamount chain does this. For decades, scientists assumed that, as the Pacific tectonic plate continued to drift, the hotspot beneath it remained stationary. This meant that new seamounts, which are mounds of volcanic material, would keep popping up in the Pacific Ocean, with each new active volcano leaving a trail of extinct ones in its wake.

However, a new Nature study has suggested that something far more peculiar is happening beneath Hawaii. Guided by surface observations, the team from the University of Sydney used a supercomputer to simulate the mantle flow patterns underneath the Pacific Ocean and found that the superheated plume decided to suddenly change course 50-100 million years ago.

As it turns out, the mantle plume itself isn’t always perfectly vertical, and its chaotic motion is causing new volcanoes to rise through the sea somewhat irregularly. Combined with the motion of the Pacific plate, this ultimately led to the generation of the spectacular wonkiness of the seamount chain.

“These findings suggest the shape of volcanic seamount chains record motion in the deepest mantle,” Rakib Hassan, a Ph.D. student at the University of Sydney and lead author of the study, said in a statement.  “The more coherent and rapid the motion deep in the mantle, the more acute its effects are on the shape of seamount chains above.”

The supercomputer simulation of the Hawaiian mantle plume. University of Sydney via YouTube

Simulating flow within the partly solid, partly molten mantle is notoriously difficult. It’s huge, after all – it makes up around 84 percent of the volume of the Earth – and studies continuously reveal that its gigantic convection currents are incredibly complex, to say the least.

Mantle plumes are particularly mysterious; they originate from the boundary between the mantle and the liquid outer core, and their formation mechanism is highly debatable. Researchers have long assumed that they were always vertical, but this team of scientists weren’t buying this.

Using the Southern Hemisphere’s most highly integrated supercomputer, Raijin, they plugged in the most cutting-edge observational data to create one of the most accurate 3D simulations of the mantle beneath the Pacific Ocean to date. This gave them the opportunity to look through 200 million years of the Earth’s geological history, and around 50-100 million years ago, they noticed that the plume beneath Hawaii started to misbehave.

Some classic Hawaiian volcanic activity. Alex GK Lee/Shutterstock

Although remaining vertical for most of its 140-million-year-long lifetime, the plume warped and bent during this particular period of time, causing an abrupt change in the direction of volcanic islands appearing at the surface. The simulations suggest that vast slabs of cold tectonic plates continually sank into the mantle beneath the north Pacific, which disrupted the plume’s upward flow and caused it to tilt.

“It is now clear that we first need to understand the dynamics of the deepest 'Underworld', right above the core, to unravel the history of volcanism at Earth's surface,” concluded Dietmar Müller, a professor of geophysics at the University of Sydney and one of the authors of the paper.

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