Dr C.P. Rajendran
The writer is an adjunct professor at the National Institute of Advanced Studies, Bengaluru, and author of a forthcoming book, Earthquakes of the Indian Subcontinent.
Geographically relegated to a distant corner of the southern Pacific Ocean, the Polynesian island nation of Tonga – an archipelago of more than a hundred tiny islands – was thrust suddenly onto the international news circuit by a rare geological event: A colossal volcanic eruption! The explosive activity on the early morning of January 15 was the most violent in the past 30 years of the global record of volcanism. The eruption generated a 20-km-high mushroom cloud of smoke and ash in the near-field and sent shockwaves halfway around the planet, triggering an oil leak near Peru and raising concerns of a tsunami across the Pacific. The low-lying unpopulated islands nearby were indeed inundated by 4-5-metre-high waves in the aftermath of the initial event.
Tongatapu, the main island of the Kingdom of Tonga and the host of its capital Nukuʻalofa, where most Tongans live, was struck by a 1.5-m-high tsunami and was also covered by a patina of volcanic dust and ash transported by winds. This pollution – of the air, the water and the islands’ foods – is likely to persist for months. The volcano itself consists of two small uninhabited islands, Hunga-Ha’apai and Hunga-Tonga, and raises its head 100 m above sea level 65 km north of Tonga’s capital. But hiding below the waves is the rest of the colossus, around 1,800 m high and 20 km wide. It is one of a series of such features along the Tonga-Kermadec volcanic arc – formed as part of the ‘Ring of Fire’, or the circum-Pacific belt.
This global tectonic junction runs along the rim of the Pacific Ocean and is known for its deep trenches. These are the sites where tectonic plates grind against each other, creating a continuous belt of volcanoes and earthquakes all along. They were responsible, for example, for the 1960 Chile and 2011 Tohoku (associated with the infamous Fukushima incident) events. As an extension of this zone of tectonic turbulence, there is an active subduction trench along the Tonga archipelago, where the Pacific plate is pushed beneath the Indo-Australian Plate. In fact, the Tonga group of islands owes its origin to volcanic activities that have been going on for millions of years in the region.
The Hunga-Tonga-Hunga-Ha’apai volcano has erupted intermittently, in bursts, in the past few decades. In 2009 and 2014, for example, jets of magma and steam burst through the waters. These initial rumblings were first sensed by volcanologist Shane Cronin of Auckland University, New Zealand, and his group during one their surveys in the region. Cronin has been conducting pioneering work in that part of the Pacific Ocean for years, including monitoring volcanic activity in the area around the Tonga group of islands. According to Cronin, in late December 2014, an undersea volcano erupted between two small islands in the Tonga volcanic arc, northeast of New Zealand, sending steam and dense plumes of ash high into the air. The eruption lasted for about five weeks and the ash finally settled and interacted with sea water, chemically solidifying it. What sprang out of this claggy setting was a new island that became a bridge of sorts between the two original adjacent islands – the Hunga-Tonga-Hunga-Ha’apai.
After 10 months, when Cronin visited this ‘baby’ island, named Hunga, he noticed that it had already become home to flowers and birds. Such islands don’t last long thanks to sea erosion and future volcanism, but they are a boon for scientists studying young landforms and their effects on life-sustaining processes. The series of smaller to moderate eruptions that continued also indicated that increasing gas pressure was increasing in the upper part of the volcano, and that its magma chamber and plumbing system were getting ready for a bigger event. A seafloor survey revealed a large, shallow underwater depression – or a caldera – to the south of the volcano, about 4 km by 2 km. This feature was consistent with some earlier observations as well. The survey also found a broad, shallow area associated with the 2009 eruptions near an island formed in 2015 and a chain of cones created in 1988.
The surveyors thought that the caldera likely represented an older Hunga edifice that had collapsed violently into the sea – a major triggering factor in generating tsunamis. This collapse may have been the source of a mysterious South Pacific eruption about 1,000 years ago – a predecessor event whose signs geologists have found in deposits. (One question that has been raised about the explosiveness of the January 15 event off Tonga is why the sea-water didn’t cool the magma from the eruption. The answer is the Leidenfrost effect: if magma slowly rises into sea-water, even at a temperature of about 1,200º C, a thin film of steam forms between the magma and water. This layer insulates the outer surface of the magma. But when magma enters the water rapidly, the steam layer is quickly destroyed and the hot magma comes in direct contact with cold water, at a temperature far greater than the water’s boiling point. Volcano researchers call this a fuel-coolant interaction: it produces volcanic particles and blasts moving at supersonic speeds.)
The predictive power of volcanology has been constantly increasing. For example, the March 1980, eruption of Mount St Helens was preceded by a month of earthquakes and lots of steam being vented at the eruption site, creating a large bulge on the volcano’s northern slope. These were indications of an imminent eruption, and the US Geological Survey issued daily bulletins to help locals and other government officials prepare. Advances in ground-measurement systems and satellite observation have further revolutionised our ability to detect and identify changes around the surface of a volcano as it prepares to erupt. History is replete with legendary volcanic eruptions – for example, of Mount Vesuvius in 79 CE, which destroyed the ancient cities of Pompeii and Herculaneum. Krakatoa in Indonesia in 1883 and Pinatubo in the Philippines in 1991 both reminded us that these hazards are never someone else’s problem. The Barren Island volcano in the Andaman Sea is a live volcano located much closer home.
Volcanic eruptions have been identified as the most dramatic agents that may impact the near- and far-field regions. The droplets of sulphuric acid emitted to the atmosphere during eruption can temporarily alter the climate, sometimes for years. The Tonga eruption is a stark reminder of such hazards – as well as the importance of efforts to study them.