One of Earth's most EXPLOSIVE volcanoes is refilling with magma

The reservoir of one of Earth's most explosive volcanoes is quietly refilling with magma, scientists have revealed.

The underwater Kikai volcano, off the coast of Japan, last erupted 7,300 years ago in what was the largest explosion in modern times.

It ejected more magma than could cover the whole of Central Park 7.4 miles (12 kilometres) deep, leaving a large and shallow crater called a 'caldera'.

Now, researchers have discovered a region beneath this caldera has been slowly refilling with fresh magma over the last 3,900 years.

And it sparks concerns that another giant eruption could be on the cards.

In recent years onlookers have noticed steam being released from the crater and the area has experienced more than a dozen small earthquakes.

'Due to its extent and location it is clear that this is in fact the same magma reservoir as in the previous eruption,' geophysicist Professor Seama Nobukazu, from Kobe University, said.

'We must understand how such large quantities of magma can accumulate to understand how giant caldera eruptions occur.'

During the previous Kikai eruption, around 36 cubic miles (160 cubic kilometres) of dense rock was spewed across 1,700 square miles (4,500 square kilometres).

The violent blast sent pyroclastic flows up to 93 miles (150km) from its epicentre – roughly the same distance from Sheffield to London.

That supereruption is thought to have wiped out the prehistoric Jomon civilisation in southern Japan.

While the volcano hasn't done anything dramatic since, it is still active. In the centre of the caldera is a lava dome that has been gradually forming for nearly 4,000 years.

The researchers used airgun arrays to cause artificial seismic pulses and measured how these waves moved through the Earth's crust.

Analysis confirmed the hidden reservoir's size and shape – and that it's filling up with magma.

Chemical analysis showed that the material produced by this and other recent volcanic activity is of a different composition than what was ejected in the last giant eruption.

'This means that the magma that is now present in the reservoir under the lava dome is likely newly injected magma', Professor Nobukazu said.

Alongside Kikai, other famous calderas include North America's Yellowstone and Indonesia's Toba.

These powerful volcanoes are known to go through long-term cycles of being 'asleep' before reawakening and erupting in dramatic style.

But the mechanics of this rhythm are still not fully understood.

'This magma re-injection model is consistent with the existence of large shallow magma reservoirs beneath other giant calderas like Yellowstone and Toba,' Professor Nobukazu said.

'We want to refine the methods that have proved to be so useful in this study to more deeply understand the re-injection processes.

'Our ultimate goal is to become better able to monitor the crucial indicators of future giant eruptions.'

The study, published in the journal Communications Earth & Environment, reveals that the estimate for the current magma reservoir is a depth of 1.5-3 miles (2.5-6km).

This matches the estimated magma depths of 1.8-4.3 miles (3-7km) for the previous Kikai eruption.

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The study reads: 'These melt re-injection processes in a magma reservoir at a shallow depth just beneath the caldera could be a step towards the next giant caldera eruption.'

A separate study previously predicted that if the volcano were to erupt again, it could eject huge amounts of debris into the atmosphere, potentially blocking out the sun in some areas and triggering a 'volcanic winter'.

It could also cause tsunami that would hit southern Japan and the coasts of Taiwan and China, before striking the coasts of North and South America.

Professor Yoshiyuki Tastsumi, magma specialist and first author of the earlier study, told The Mainichi newspaper: 'Although the probability of a gigantic caldera eruption hitting the Japanese archipelago is 1 percent in the next 100 years, it is estimated that the death toll could rise to approximately 100 million in the worst case scenario'.

WHAT IS EARTH'S RING OF FIRE?

The Ring of Fire is a horseshoe-shaped geological disaster zone and hotbed of tectonic and volcanic activity.

Roughly 90 per cent of the world's earthquakes occur in the belt, which is also home to more than 450 volcanoes. 

The seismic region stretches along the Pacific Ocean coastlines, where the Pacific Plate grinds against other plates that form the Earth's crust.

It loops from New Zealand to Chile, passing through the coasts of Asia and the Americas on the way. 

In total, the loop makes up a 25,000-mile-long (40,000km) zone prone to frequent earthquakes and eruptions.

The region is susceptible to disasters because it is home to a vast number of 'subduction zones', areas where tectonic plates overlap.

Earthquakes are triggered when these plates scrape or slide underneath one another. When that happens at sea it can spawn tsunamis.