Volcano
VulcanSpirit
Richard & Alison Brunstrom
Mon 26 Aug 2013 13:38
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Vanuatu is an extremely volcanic place. There are
at least six active volcanos on land and two underwater and several dormant and
extinct (?!) ones. Some of them are classified as highly dangerous -
particularly those where there is a high risk of seawater contacting the magma
which can lead to catastrophic explosions.
We have visited the island of Ambrym which has two
active craters at present, Marum & Benbow. They are particularly famous
because they hold three lava lakes - a lake of boiling rock.
The whole island of Ambrym is a huge basiltic
volcano rising to about 1100m above sea level at present. In the cantre of
the island is a caldera measuring 12km by 8km resulting from a titanic Plinian
eruption in 50AD (a Plinian eruption is one of a type similar to that famously
observed in detail by Pliny the Younger in AD79 when Vesuvius buried Pompeii -
typified by a column of gas and ash rising high into the stratosphere and the
ejection of a large amount of pumice). The amount of magma erupted here was so
great that the top of the then existing mountain collapsed forming this very
large caldera. Subsequently the caldera has been filled almost to the
brim with lava and ash forming a 100 square km ash plain. Ambrym still
erupts every year, but the last really serious event was in 1913. The volcano is
regarded as highly dangerous with almost the entire island at risk from ash,
lava and pyroclastic flows.
We walked up it with a guide (entirely necessary
for finding the path across the ash plain). It's a very good walk, and the three
of us were the only people on the mountain. This is quite a long way off the
beaten tourist track.
Here is the ash plain at about 1000m with our guide
Robert:
 In the background is an extinct cone; the Marum
cone and crater are in the right background obscured by smoke and cloud. The
plain is really very flat indeed, and fills the caldera almost to the brim - the
flowing ash must have been extremely fluid. I think the layer we were walking on
was from 1913, but I'm not sure. It's extremely difficult to find the path
without a guide as the plain is randomly vegetated and the landmark cones are
usually hidden by cloud. A fascinating place.
Here is the plain and caldera seen from the
summit with more recent bare ash in the foreground:
 And this is the Marum crater with the lava lake at
the bottom:
 When you get to the crater rim you are standing on
a narrow frangible ridge of ash, looking 500m straight down into the lava.
Safe it is not - but no fence. As you approach the lip you can hear
the magma bubbling away furiously just like a pan on the stove, and as you
look over the edge you can feel the heat even from half a kilometre
away!
The lava is molten rock at a temperature in a range
from 700 to 1700 degrees Celsius, and it is boiling furiously throwing great
gobbets up into the air as you can see in this telephoto shot:
 This place is truly awesome.
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