NZ Hot Spot: Volcanoes

Just northeast from Tauranga, 50 km off the coast from Whakatane, is White Island, New Zealand’s most active cone volcano. Captain Cook, not realizing it was a volcano, gave it that name because it was always shrouded in white mist. White Island has been built up by continuous volcanic eruptions over the past 150,000 years with only 30% of it being visible above the surface of the ocean. On a clear day, you can see it from here far offshore with a puff of smoke above it.

White Island (1)

White Island (1)

You can take tours to the island, which is currently uninhabited. The volcanic cone continuously belches acidic gas. The tour company gives you hard hats to wear and gas masks to protect you from the harmful vapors. They also advise you and take no responsibility for the inherent danger of walking around on an active volcano. A significant eruption can occur at any time.

If you continue southwest from White Island in a broad swathe approximately 350 km long by 50 km wide roughly halfway across the North Island, you encompass the rest of what is called the Taupo Volcanic Zone, one of the most active volcanic zones in the world and part of the Pacific Ring of Fire.

The Taupo Volcanic Zone includes a huge number of geothermally active sites. Driving through parts of this area, you can literally see countless areas of steam rising from thermal vents amidst the rolling green hills. The Taupo Volcanic Zone includes Rotorua, a popular tourist destination known for its geysers, mud pots and hot pools. Continuing farther southwest are two frequently active volcanoes, Ruapehu and Tongariro.

As an aside, a friend of ours (our one friend) here is going skiing on Ruapehu this week. Here is a photo captured by a skier in 1996 when Ruapehu suddenly decided to erupt.

Eruption of Ruapehu, 1996.

Eruption of Ruapehu, 1996.

Finally as part of the Taupo Volcanic Zone is Taupo itself, a “super-volcano” responsible for the world’s largest known volcanic eruption in the past 70,000 years.

Just as earthquake intensity is measured with the Richter scale, volcano explosivity has its own scale termed the Volcanic Explosivity Index (VEI) which has a scale from 1 to 8 based on the amount of material ejected, to what height and the length of the eruption. Similar to the Richter scale, the VEI is a logarithmic scale; each increase in one on the scale represents a ten-fold increase in the relative power of the eruption.

What is termed the Oruanui eruption of Taupo 26,000 years ago was VEI 8, termed “Mega-collosal” and again the highest on the scale, and covered potions of the central North Island with pyroclastic flow material up to 200 m deep. Pyroclastic flow refers to the deadly flow of rock, ash and gases that can sweep down the sides of a volcano during an eruption—that big dust cloud you see billowing down from the volcano in some photos. Pyroclastic flows can travel at speeds up to 100 km/hr near the ground and send clouds of volcanic ash thousand of meters into the air.

Lake Taupo, a giant lake with a surface area of 616 square kilometers (238 square miles) is readily visible on maps of the North Island. This mammoth peaceful lake known for its exceptional trout fishing is part of Taupo’s caldera (collapsed volcanic cone) generated from this past enormous eruption.

That giant lake in the center of New Zealand, Lake Taupo, is the part of the caldera from a volcano.

That giant lake in the center of New Zealand, Lake Taupo, is the part of the caldera from a volcano.

A lesser eruption of Taupo, VEI 7 and termed “Super-collosal”, a mere 1,800 years ago is the most violent volcano eruption in the world in the last 5,000 years.

By comparison the famous eruption of Vesuvius in Italy in 79 AD was 5 on the VEI, as was the 1980 eruption of Mount St. Helens in Washington State. You may have seen videos of the amount of ash and pyroclastic material produced by Mount St. Helens. By comparison, the Oruanui eruption of Taupo emitted 1000 times that amount.

Volcanic and thermal features of New Zealand are part of the Pacific Ring of Fire. This horse-shoe shaped region encompassing most of the lands bordering the Pacific Ocean is where a series of tectonic plates meet and slide into each other, away from each other and sideways to each other causing all sorts of geothermal mischief.

Ring of Fire

Ring of Fire

Where tectonic plates meet, one plate can be forced underneath the other creating chemical reactions which reduces the melting point of rocks and consequently magma to form. The magma rises toward the surface forming large magma chambers which can expand and rise up through crustal fracture lines producing a volcano. Most magma never reaches the earth surface but rather just cools where it is.

Volcanic activity can also occur at locations where the plates are being pulled apart from one another causing a relative weakening of the crust. This often occurs at the ocean bottom; indeed a significant amount of volcanic activity is submarine in nature.

Volcanoes are essentially ruptures in the earth’s crust allowing magma (molten rock and other materials) and gases to escape. Lava is the term used for expelled magma.

Volcanoes are popularly classified as active, dormant or extinct but this is a relatively arbitrary classification having to do with whether they have been active in historical times. But there can be gaps of up to a million years between the eruption of a given volcano. Scientists tend to term a volcano active if it has erupted in the last 10,000 years. By this measure there are 1500 active volcanoes in the world, of which some 50 erupt each year.

There are lots of ways a volcano can kill you.

Despite what you may think, being overrun by lava is not one of the most common ways.

Volcanic ash can block out the sunlight and alter the climate causing life forms to freeze to death, or over time to starve to death. Enormous amounts of dust and sulfur dioxide are released into the atmosphere by a volcano. These particles absorb sunlight reducing the amount of sunlight that reaches the earth. Relatively recent volcanoes in the historical past are estimated to have lowered the average temperature of earth by 10 degrees Celsius (50 degrees Fahrenheit) for several years duration. Volcanoes have been responsible for the extinction of species. At the end of the Triassic period, massive volcanic eruptions caused the extinction of over half the species living on land and the oceans. It could happen again.

Ash, which can spread over vast distances, can also damage infrastructure with the electrical power grid being particularly vulnerable.

Volcanic eruptions at sea can produce tsunamis. One quarter of deaths attributable to volcanoes in the last 250 years have been due to tsunamis.

The pyroclastic flow from a volcano, that roiling mass of gas and debris discussed above can engulf you and smother you or the poisonous gases can kill you.

A lesser known phenomenon but one that is responsible for thousands of deaths when volcanoes erupt is termed a lahar. This is a mudflow of pyroclastic material, debris and water. It often has the consistency of wet concrete and flows like a river engulfing and destroying anything in its path.

Pyroclastic flow

Pyroclastic flow

A significant volcanic eruption is a distinct possibility here in New Zealand. A Volcanic Alert System is in effect throughout the country with all known volcanic areas being constantly monitored.

Auckland, the country’s largest population center, although outside the Taupo Volcanic Zone, is also at risk since the hills surrounding Auckland are all a series of extinct or dormant volcanoes. A government brochure lists what one should do to protect oneself during a volcanic eruption. The exhibit at the Auckland museum also provides this bit of advice—

“Handy hint: if your car is covered in volcanic ash, wash it, don’t wipe it: wiping it is like rubbing it with sandpaper.”

What, really? Okay, I’ll remember that. But I imagine if my pumice-covered bum (as they say here) itself survives, the last thing I’ll be worried about is scratches on my car . . .

(1) White Island Tours, Whakatane






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