The Kīlauea volcano in Hawaii is said to be the world's most active volcano, and yet we still don't really know how it was born.

New research suggests the original womb of magma lies more than 90 kilometers beneath the hotspot. While previous studies have found two shallow chambers of magma beneath Kīlauea, they weren't big enough to explain all the liquid rock this volcano spews.

A larger chamber, about 11 kilometers deep (that's 6.8 miles), was detected using seismic waves in 2014, and yet now it seems the original magma chamber lies even deeper.

A new analysis of broken fragments of ancient volcanic rock, dredged from the south-eastern flank of the Big Island, suggest Kīlauea was born from a pool of pyroclastic material close to 100 kilometers deep.

Sometime between 210,000 and 280,000 years ago, the Pacific tectonic plate shifted and a plume of magma rushed upwards into the sea. As the piping hot liquid cooled and solidified, it formed a large 'shield' that burst through the waves about 100,000 years ago.

So Kīlauea came to be, but the original rocks ejected from this hotspot are incredibly hard to find, buried as they are beneath numerous layers of newer lava. The igneous rock dredged up in the current study provides an unprecedented glimpse into the volcano's deep and distant past.

Before, it was thought the Kīlauea volcano was created through solid rock partially melting from the heat of the hotspot.

The new research, however, finds no evidence to support that hypothesis. The rocks collected were found to contain a suite of rare earth elements that models suggest could only be formed in one specific way.

Instead of partial melting, it seems the Kīlauea volcano was originally formed through fractional crystallization. This term describes the creation of crystals in deep pools of magma, which do not react with residual melt later on.

"We explored the formation of these samples through experimental work, which involved melting synthetic rocks at high temperatures (> 1,100 ˚C) and pressures (> 3 GPa), and by using a new method for modeling their rare earth element concentrations," explains lead author, geologist Laura Miller from Monash University in Australia.

"We found that the samples could only be formed by the crystallization and removal (fractional crystallization) of garnet."

Garnet is a crystal that can form when magma is subject to high pressures and temperatures more than 90 kilometers beneath Earth's crust. The fact its presence is required to explain the composition of rocks from Kīlauea suggests the original eruption came from similar depths.

Or possibly even deeper. Experiments show garnet can be crystallized at depths of up to 150 kilometers beneath Earth's crust.

The original source of the Hawaiian islands may not be that deep, but the new findings suggest Kīlauea's plumbing is not nearly as superficial as we once thought.

"This challenges the current viewpoint that fractional crystallization is solely a shallow process and suggests that the development of a deep (> 90 km) magma chamber is an important early stage in the birth of a Hawaiian volcano," says Miller.

Other volcanoes elsewhere in the world, like Mount Vesuvius, also show crystal formation times that suggest there are "long-lived deep-seated" reservoirs of magma hiding beneath the surface. Yet Kīlauea's original magma chamber seems to be much deeper than most.

Why that is remains a mystery for now.

The study was published in Nature Communications.