We have been warned repeatedly that rising sea levels will wipe small Pacific Island nations from the map of the world thanks to climate change; that they are particularly vulnerable to human-caused emissions of carbon dioxide. Not so the island with the crazy-sounding name Hunga Tonga-Hunga Ha’apai. It wasn’t overtaken by rising sea levels. Rather it blasted itself into the sky! Obliterated in an instant. The massive volcanic eruption caused sonic booms, tsunami waves, spectacular lightning bolts, and a giant umbrella cloud that rose to a height of 30 kilometres and reached 500 kilometres in diameter in less than two hours.
I was reminded of that exhaustingly long novel Hawaii by James Michener. It begins with a comment about the relentless surge of the universe, the violence of birth, the cold tearing away of death, and yet how promising was the interplay of forces as an island struggled to be born, vanishing in agony.
This work of fiction gets closer to the truth than many of the leaders of Pacific Island nations who lament carbon dioxide emissions from human sources but are apparently blind to the reality that they are standing atop extinct volcanoes that once belched carbon dioxide. And never mind that a recent University of Auckland study found the purportedly ‘at risk’ islands of Kiribati, as well as the Maldives archipelago in the Indian Ocean, have grown in land area by up to eight percent over the last six decades, despite rising sea levels – because of the accumulation of coral sediment.
To be clear, most volcanic islands that have persisted, do eventually, by their very nature, gradually sink back into the ocean because they are effectively floating on top of hot ductile rocks that slowly give way. Observed subsidence of many hundreds of metres in the case of flat-topped, wave-eroded seamounts cannot be explained by sea level rise alone. The islands that continue to exist in tropical waters have coral growing on top of their subsiding summits, with some of the dead coral accumulating as sediment, thrown, or blown above sea level during storms and cyclones. Kiribati, Tuvalu and the Marshall Islands in the Pacific, and the Maldives and Seychelles in the Indian Ocean, are all atolls built up from dead coral that grows on top of long extinct subsiding volcanoes.
The spectacular emergence of Hunga Tonga-Hunga Ha’apai as a small island just a few years ago is evidence that the long drawn-out geological processes that can create new Pacific islands continue to the present day. The island’s even more spectacular obliteration through an explosion on 15 January, may not be its end. It has erupted on a similar scale at least twice before in the last few thousand years, and this latest eruption is perhaps just a short setback in a million-year-long story.
We marvel when volcanoes explode, and mostly ignore them the rest of the time. Hawaii’s Mauna Loa is the world’s largest active volcano, but it does not erupt explosively.
It emerged above sea level about 400,000 years ago and has been steadily dribbling out low-viscosity basaltic lava, along with lots of steam and some carbon dioxide, for at least 700,000 years. Mauna Loa is the quintessential shield volcano with broad, rounded slopes – growing slowly, and for so long. It is now one of the largest single mountain masses in the world, rising more than 4 kilometres above sea level with a total height of more than 9 kilometres from base to summit. It is an example of ‘hot spot’ ocean island volcanism, characterised by the formation of individual seamounts. A lot of the world’s coral reefs are hosted by these volcanoes – many of them now extinct and slowly sinking under their own weight into the sea floor.
Most volcanism commences underwater. Much more extensive than ‘hot spots’, the Earth’s mid-ocean ridges are a continuous network of volcanically active undersea ridgelines crisscrossing Earth’s seafloor like stitching on a baseball – the Earth being the baseball. Mid-ocean ridge volcanism is relatively slow and steady, and almost entirely non-explosive. The material that erupts through fissures at what are called ‘spreading centres’ along the ridge lines is primarily basalt, the most common volcanic rock on Earth. This is where the thin and young oceanic crust of the Earth is being created, at what are called constructive divergent plate boundaries.
Volcanologist Arthur Day explained to me that Hunga Tonga-Hunga Ha’apai is not part of the mid-ocean ridge network, nor a ‘hot spot’ volcano like Mauna Loa. The tectonic setting, magma composition, and eruptive style of this volcano are completely different. Hunga Tonga-Hunga Ha’apai sits on the shallow Tonga-Kermadec Ridge and is a volcanic arc volcano. Volcanic arcs occur at convergent zones where the Earth’s crust is being destroyed.
Hunga Tonga-Hunga Ha’apai first appeared above sea level in 2015 as a large crater volcano with a cauldron-like pit in an area of subsidence. Its magma has more silica, making it more viscous and therefore more explosive. In addition, the explosion of Hunga Tonga-Hunga Ha’apai was made all the more violent due to two more effects: the sudden depressurisation of dissolved gases as the magma escaped from its submarine magma chamber, plus the vaporisation of vast amounts of sea water as the then-unsupported chamber collapsed inward. This submarine collapse may have displaced enough sea water to also trigger the associated tsunami.
Those concerned about human-caused climate change have been quick to explain that the amount of carbon dioxide expelled from the depth of the Earth by this Tongan volcano on January 15, was insignificant relative to the relentless activities of humankind. I agree. But the blast did penetrate the stratosphere and could cause global cooling. The eruption of Mount Pinatubo in the Philippines in 1991 caused a drop in global temperatures of 0.6 degrees Celsius for the next 15 months that is visible as a blip in the monthly global satellite temperature record. If Hunga Tonga-Hunga Ha’apai were to trigger catastrophic global cooling, would the rest of the world be entitled to sue the people of Tonga?
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