Tongariro volcanic eruption and ash
fall
7 August
2012
An explosive eruption overnight
on Mount Tongariro has spread ash across the Central North
Island and led to travel warnings on major highways in the
region.
The volcano
remains at alert level 2,
indicating the onset of eruptive activity and changes to
indicators of unrest. GNS Science is closely
monitoring the situation.
The Science Media
Centre has rounded up reaction to these developments from
volcano and hazard experts.
Dr Jan Lindsay, Senior Lecturer and vulcanologist at University of Auckland comments:
"It will be interesting
to see how the eruption develops - whether it continues for
months, or is over already. If the former we may see ash
reaching Auckland, depending of course on the wind
direction. We know from looking at sediment cores from
Auckland lakes that ash from Tongariro has reached Auckland
many times over the last 80,000 years. Of course it only
needs to be in the atmosphere to cause a problem: we are
already seeing flights cancelled and the impact on flights
in and out of Auckland airport could be significant."
Dr Thomas Wilson, Lecturer in Hazards and
Disaster Management, University of Canterbury
comments:
Type of eruption
"There
are three main types of eruptions you can get in an
explosive event. There's what we call a phreatic eruption, or steam-driven
eruption. This is where water has been superheated by magma
under the volcano, but it's only the water that erupts in an
explosive manner. Basically the hydrothermal system
underneath the volcano erupts, and this is what seems to
have occurred on Tongariro. In a phreatic eruption, it's
existing rock and material underneath the volcano that is
fragmented and erupted. So in a purely steam-driven
eruption, there's usually no new magma.
"Another type
is a phreatomagmatic eruption, where water has come
into contact with magma and caused the magma to fragment in
an explosive eruption.
"Then there's a magmatic
eruption, where there's no water interacting, and it's just
the magma erupting itself. In a magmatic eruption, the ash
particles are usually a bit bigger, and it's very fresh
material, so there may be more soluble chemicals which can
come off the ash. There are also a lot of magmatic gases
which can condense onto the surface of the ash as it cools
down. These are usually acidic, which can cause some
problems.
"Scientists [from GNS] are out collecting
ash samples as we speak, and there'll be more information in
the near future on those specifics, which are relevant for
agriculture and human health. The key thing is that it's a
very small volume of ash that's been produced so far. It's a
very light dusting."
How quickly could things change?
"Things could change quickly. It's a bit
of an unknown. That's the 'volcano problem', if you like.
It's difficult to know what the volcano is going to
do.
"There are three main scenarios:
• It could
stop completely;
• It could continue with these
same-size eruptions, and we might get some more very light
ash fall across the North Island, dependent on the wind
conditions on the time;
• It could be the beginning of
a bigger eruptive sequence, which would probably mean larger
eruptions, with more ash produced, probably leading to more
widespread deposition of ash, to a thicker
amount.
•
"I can't give you any probabilities on
the likelihood of these scenarios. We do know that there
have been eruptions from this part of the volcano in the
past, and these have typically been pretty small on a global
scale. But we can't rule anything out at this stage
"A
steam-driven eruption like this could be a sign that there's
magma moving into place under the volcano, and we might see
a progression through to a magmatic eruption. Or it could
just be that the volcanic hydrothermal system has been
unsettled by these earthquakes, and we're seeing an eruption
as a result of this."
Is it unusual that there
wasn't more warning before this eruption?
"We saw
the volcanic threat level raised about two weeks ago,
following seismic activity and volcanic gases on the
volcano, and that's the same area where the eruption
occurred.
"This is quite normal behaviour for
volcanoes. They'll show vague signs of unrest, like this,
and then we get an eruption. The challenging thing is
knowing when and where it will occur.
"The key message
is that this is a really good example of why we need to be
prepared for volcanic eruptions in New Zealand. These are
totally normal for these volcanoes, and it's what we should
be expecting. Even if you haven't been affected around the
volcano, this is a good opportunity to think about how to be
prepared and read some of the resources online about ash
fall and other hazards."
Shinmoedake volcano in Japan
"In Jan-Feb 2011, there was an eruption in
the Shinmoedake volcano in southern Japan, which is very
similar to Tongariro, and it erupts a similar composition of
magma. It had a magmatic eruption that was much larger than
what we've seen today. We studied its impacts on critical
infrastructure, on towns and communities and agriculture. It
did cause some disruption. There was centimetres-worth of
ash across vast tracts of agricultural land and some towns.
There were some direct impacts to agriculture, but after six
months, things were mostly back to normal. There were some
big issues with towns and roads, and that created
coordination and management issues. There were some
disruptions to electricity supplies as well, and surface
irrigation water. As far as health concerns, limiting
exposure, wearing a mask and avoiding times when ash was
blowing around outside was sufficient to mitigate those
issues. It's quite a good example of what might happen if
there were a much bigger eruption from Tongariro."