New Zealand is host to more than a dozen active volcanoes. The vast majority of these are located
on the north island. Some of these volcanoes have produced spectacularly large eruptions,
such as Lake Taupo in 233 AD, or Mount Tarawera in 1886. However, none of the volcanoes located
on the north island are as active as Mount Ruapehu. Although this mountain's eruptions are
comparatively small, it produced the deadliest eruption in New Zealand's history in 1953.
This video will discuss this active volcano, and mention what its future hazards are.
Mount Ruapehu can be found in the south central portion of the north island of New Zealand.
Specifically, it is 15 kilometers south of another volcano, Mount Ngauruhoe.
Ruapehu began forming 250,000 years ago, when a series of basaltic andesite lavas erupted
onto the surface. These early eruptions occurred 6 kilometers north of the modern
summit. Over tens of thousands of years, these eruptions formed a tall volcanic cone.
Then, 130,000 years ago, the lava became slightly more viscous.
As a result, eruptions were more explosive. Some of these eruptions started to build the
modern summit of Ruapehu, which now represented the main vent.
Today, an outcrop of lava from this period can be found 5 kilometers east-southeast of its summit.
Between 64,000 and 35,000 years ago, spectacularly large eruptions infrequently
occurred once every few thousand years. These eruptions produced both andesite and
highly explosive dacite lavas, primarily on the west and south slopes of Mount Ruapehu.
Since then, eruptions have slowly become less explosive over time as the erupted
lavas contained less silica. One eruption that interrupted this trend occurred 10,000 years ago.
This eruption was at least 10x larger than any witnessed historical eruption, and ejected ash
high into the atmosphere. In addition to generating a lahar, several pyroclastic
flows raced down the mountain and traveled at least 8 kilometers from Ruapehu's summit.
For the next 8,000 years, eruptions were very small and infrequent. Then, 1800 years ago,
eruptions became more common, largely matching the frequency we see today.
The first historically documented eruption at Ruapehu occurred in 1861 and was an average sized
eruption. Over the next 70 years, 19 eruptions occurred. The main hazard posed by these eruptions
was not from ash, lava, or pyroclastic flows. Rather, the primary hazard was something known
as a lahar. Lahars are a type of debris flow which have a viscosity of wet concrete, and can reach
more than 50 meters in height. During an eruption, these can form if a retaining wall collapses
or if lava melts snow or ice on the summit. One of these lahars from a relatively small
eruption in 1953 caused the worst lost of life in the country from a volcano. A lahar destroyed a
railroad bridge only minutes before a train passed over it, causing a derailment with 151 fatalities.
The aftermath of this disaster eventually resulted in the creation of a lahar
warning system and stronger bridges, which prevented a similar disaster 54 years later.
In 1995, the largest eruption of the last 100 years took place. As hundreds of skiers were
on the mountain, the volcano violently erupted. This sent boulders 2 kilometers from the crater,
and generated a plume of ash which reached 12 kilometers into the atmosphere.
Miraculously, this eruption resulted in no fatalities or serious injuries.
One year later, a similarly large eruption began. Both of these eruptions were rated as
a 3 on the volcanic explosivity index. Since then, three other eruptions have
occurred. The most recent of these ended on September 25th, 2007.
In early 2021, the alert status of Ruapehu was raised once again. Scientists who had
been monitoring the crater lake noticed a rise in temperature as the high altitude lake was measured
at 43 degrees Celsius. Luckily, the temperature decreased and the alert level was lowered. If
this crater lake temperature was to reach at least 50 degrees Celsius, it would almost certainly mean
than another eruption would soon occur. During periods of raised alert status, an exclusion
zone of several kilometers would be needed on the mountain. Pyroclastic flows at the mountain
do occur but are rather rare, and typically do not travel very far. If you live nearby and witness an
eruption, get out of any streams or riverbeds. Do not drive over any bridges or roads which go
across rivers, due to the threat of lahars. I hope that you enjoyed this video. If
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