A "hydrothermal" eruption is
one in which no molten rock is
directly involved. It happens in
a hydrothermal system when
conditions arise which lead to
the explosion of water residing
there at temperatures elevated
above ground surface boiling
temperature. The latter is
controlled by atmospheric
pressure and at Waimangu this
is just under 100 degrees
Celsius. Temperatures at
relatively shallow depths in
intense hydrothermal systems
commonly exceed 200 degrees
Celsius.
Because of the youth of the
surface activities at Waimangu,
and because the subterranean hydrothermal system had to
adjust to the existence of new fluid outlets through the
craters formed in the 1886 eruption, it is perhaps not
surprising that several hydrothermal eruptions have reshaped
the valley since the 1886 formative event. (And Waimangu
Geyser - see Eruption Chronicle Issue 3 - itself can be
regarded as having been a “periodic hydrothermal eruption".)
During the ten to twenty years after its formation on 10 June
1886 Echo Crater had been partly filled by alluvium
comprising a mixture of sandy volcanic deposits that were
progressively washed into it at the southeast and northwest
ends from the surrounding country
by small streams. With steam and
hot water escaping from numerous
small vents in this flat floor, its
appearance and the sounds
accompanying the discharges
earned it the name Frying Pan Flat.
After the extinction of Waimangu
Geyser (1900-1904) hydrothermal
activity in the western part of Echo
Crater seems to have increased. In
1915, several small hydrothermal
eruptions occurred at different
places on Frying Pan Flat. But
none of these (and other events)
gave any intimation of the
magnitude of what was to follow.
This was to be the largest
hydrothermal eruption in the
history of Waimangu
Shortly after six o'clock on the morning of April 1 1917 almost
the whole area comprising Frying Pan Flat suddenly exploded.
The initial blast consisted of steam loaded with the sandy
material that had formed the Flat and with rock from beneath
the old crater floor. It was funneled mostly in a southwesterly
direction 800m away, and directly in its path, stood the
Waimangu Accommodation House . (The site of this building
lies just beyond the present tearooms at the vista point at the
head of the valley.) The force of the blast was so intense it
lifted the roof of the Accommodation House and scattered
The force of the blast was so intense it lifted the roof off the Waimangu accommodation house
resulting in the deaths of a woman and her child.
debris to a distance of 70 metres further to the southwest. It
also plastered the front of the ruined building with an
adhesive coating of sand.
Waimangu House was occupied at this time by resident
guide Ernest McCormack and his wife and little boy. The
inmates were woken by the sound of stones falling on the
roof. McCormack later reported that when the blast struck
it "snatched off the roof as if it was paper" engulfing them
all in the scalding steam and sand.
The outburst had been witnessed also by farmer Gordon
Woods and his wife from their house some three kilometers
away. They at once set out to render what assistance they
could to the injured family and were responsible for
summoning medical assistance. While McCormack
eventually recovered, his wife and child unfortunately
succumbed to their burns.
The Woods and other early visitors noted that the eruption
explosions threw material up to 300m high with ejections
commonly reaching 180-240m and with rocks "falling like
meteors". During the next few days the eruptions gradually
subsided, but still occasionally threw boulders hundreds of
metres into the air.
The vegetation around Echo Crater, particularly to the north
and to the southwest, was "leveled to the earth ... as though
by some mighty cyclone". Plant life that during the thirty
years since the 1886 eruption had managed to regain a foot
hold at Waimangu was blasted by this next eruption and was
By this next eruption and was wiped out for a second time in
the affected area.
The 1917 event blew away almost the whole of Frying Pan
Flat and indeed it re-excavated and enlarged the western part
of Echo Crater to the north and west. Water soon began to
collect in the new crater, and there were occasional geyser
eruptions from several vents within it. By 26 June 1918
"Frying Pan Lake" almost filled the subcraters formed in
1917 and on that date a controlled outflow was dug to release
the accumulated volume and prevent the possibility of its
spilling and catching any unwary visitor in the valley below.
Although activity generally subsided in the years after 1917,
the form of Frying Pan Lake was slightly reshaped by a small
hydrothermal eruption in 1973.
Frying Pan Lake is one of the largest active geothermal
features in the Taupo Volcanic Zone and is the largest hot
spring in the world (38500m2).It has an average depth of
only 6m but is fed by numerous deep vents, the deepest being
21m. The highest measured lake surface temperature is 67
deg.C and hovers around 55 deg. C in the discharge stream.
The water in the lake is classed as an "acid sulphate" but it is
derived from the "alkaline chloride" water that enters through
the deep vents when atmospheric oxidation of sulphur gases
gradually occurs during their residence time in solution in
the lake.
The Frying Pan Flat eruption was one of relatively few major
hydrothermal eruptions witnessed in historic times.
Author: Professor RF Keam Physics Department University of Auckland
Waimangu Volcanic Valley, P O Box 6141, Rotorua, New Zealand
Phone 64 7 3666 137, Fax 64 7 3666 607, Email: [email protected]
www.waimangu.com