Echo crater of most attractive sight in  Waimangu Volcanic Valley near Rotorua.

The devastating impact of volcanic eruptions

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28 February 2022

Reading Time: 3 minutes

When volcanoes erupt, they rock human lives – and ocean-dwellers suffer, too.

If you live in a volcanically active area, you know all too well the familiar rumbles and motion as the earth’s crust moves under your feet. Activity along the chain of volcanoes dotted from the central North Island, Bay of Plenty and along the Kermadec Ridge remind us we live in a geologically lively country. Occasionally it wakes us in the night and out childhood earthquake drills kick into gear.

Aotearoa New Zealand has had its share of dramatic volcanic events. Our youngest land mass, Rangitoto Island, emerged from the sea only 600 years ago. The well-established communities around the Hauraki Gulf witnessed the creation of Rangitoto and the devastating clouds of steam, ash and rubble.

In 1886, the Pink and White Terraces in Lake Rotomahana, once an international tourism destination, were devastated in a serious of large eruptions that blew the lake apart and changed the landscape forever. Treasured by the Te Arawa for centuries, the terraces were suddenly gone. Many people lost their lives and were displaced as the land was uninhabitable for many years.

Thankfully the world rarely experiences such catastrophic volcanic eruptions, but on January 15, 2022, the active volcanic region of Tonga came alive. The two islands of Hunga Tonga and Hunga Ha’apai were joined together in 2015 after many years of volcanic activity. They are now torn apart by one of the strongest volcanic blasts measured on Earth in recent times.

It was the mixture of concentrated magma from the Earth’s core contacting a vast amount of seawater that caused such an enormous explosion. The eruption caused devastation with an umbrella cloud of debris from the volcano reaching stratospheric heights, rising more than 30km high. The cloud was 500km wide, and debris feel across the Pacific region, covering the buildings, forest, crops and people in a grey blanket of ash.

The Pacific is the world’s largest ocean region with very little land other than around the edges – look at a global map centred on the Pacific and you will see what I mean. When considering this world view, the Hunga Tonga-Hunga Ha’apai volcanic event impacted the ocean more than the land, so what are the consequences of an event like this for ocean-dwellers? The undersea noise would have been immense, causing physical or hearing damage to animals that could not get away. The shaking and eventually fracturing of the undersea landscape will have impacted the deep ocean trenches and near shore reefs.

We know that the 7.8 magnitude Kaikōura earthquake in 2016 triggered massive underwater slips and destruction of life on the seabed, much as a landslide destroys the forest around it. With the seabed in ruins, there was widespread disruption to food availability from the smallest invertebrates to the largest animals, the parāoa (sperm whales). Researchers found the whales had to work harder to find their preferred prey – deep-sea squid and fish – and moved further offshore to areas less impacted by the slips. Due to the remoteness of Hunga Tonga-Hunga Ha‘apai, we can only use these other examples to suggest what’s happened to ocean life.

Immediately adjacent to Hunga Tonga-Hunga Ha’apai the coast has changed completely. The coral reefs, rich in biodiversity, the reason many of us visit the Pacific Islands, will be largely wiped out. There is new habitat formed, new rock upon which seeds will grow, seabirds will eventually nest there again and around the edges, coral reefs will form again. It is remarkable how resilient ocean life is and as I write, larval plankton and algae will be colonising the new undersea habitat in the near shore waters. Not all of the reef has been destroyed so there will be a source of life close by; spawning corals, fish eggs and anemones to get the reef ecosystem up and running again.

Because it is so remote, there are few direct pressures from humans that threaten other reefs. Although the ever-present threat from climate change makes coral reef formation more challenging as the ocean warms and acidifies.

With the recent event in Tonga, the tsunami added another form of destruction. Tsunamis occur when a large mass of ocean is moved suddenly, generating slow-moving but large waves 10 to 60 minutes apart. They have immense power and with estimated speeds of more than 800 km/hr, the Hunga Tonga-Hunga Ha’apai tsunami washed over low-lying islands and atolls with little to stop it. The whole Pacific was on tsunami alert. Any animal not swimming fast enough would have been pushed onto the land and then sucked back into the ocean.

Unfortunately, the ocean also drags in buildings, vehicles and anything else in its path. The coral reef communities around the islands will have taken a massive hit. There will be much-needed clean-ups, something that will come in time once the people get back on their feet. Among this devastation, new ocean communities will spring up, life will continue, much as it will for the Tongan people.

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