Download Volcanoes - Sanctuary Mountain Maungatautari

Essay Volcano
Our outlook!
What a view we have from the Visitor Centre! The Rangitoto Ranges, Kakepuku, Ruapehu on a good
day, and over to the left, Lake Arapuni, and even the outline of Mt Tarawera. There lies the history
of New Zealand, under a mantle of bush and pasture. And we are standing on our ‘own’ volcano.
This view always excites me as I try to visualise the events leading to its formation over 100 million
years ago. Maungatautari Volcano is a youngster on this time scale, being ‘only’ about 1½ million
years old. In its rampaging youth, it blew out its western side, and repeatedly spewed out quantities
of andesitic debris. These layers are beautifully exposed in the cutting at the edge of the carpark.
Here layers are visible, distinguished by slight differences in colour and often by a line between
layers.
But we are getting ahead of ourselves. Underlying the entire landscape is the root rock of New
Zealand, greywacke. This is where we go back more than 100 million years, when the continent of
what we now call ‘Zealandia’ was forming. It is a fairly new concept to think of ourselves as just a
minor part of a large mostly-submerged continent stretching from New Caledonia in the north to
Campbell Island in the south, and east to the Chatham Islands. Just because it is submerged does not
disqualify it from being continental, both because it is all less than 2000 m. below sea level, and
because its rocks are typical of continental crust. It covers an area of almost 5 million sq. kms even
though only 6% is above sea level. That 6% is the New Zealand we know (those interested in this
should read ‘Zealandia’, by Mortimer and Campbell. Publishers Penguin and GNS).
Our greywacke was the continental sediment washed off the east coast of the ancient land of
Gondwana, which later split up into Antarctica and Australia. The adjacent continent of Zealandia
shifted eastwards to its current situation from about 80 million years ago. The sediment that made
this new land varied from mud, to silt, to sand, and over the ages this all turned to hard rock. Where
it became heated, it metamorphosed into the schist that forms much of the spectacular landscape of
Otago. The greywacke is the underlying structure of the landscape we see from the Visitor Centre,
and forms the Rangitoto Range to the south, and the Kaimai and Mamaku Ranges to the east. And
look at the sealed road you drove in on – the ‘blue metal’ chips that form the road’s surface are
crushed greywacke, probably sourced from one of the many quarries in the region.
Now we come to the volcanos. In our region, they began off the west coast, hence all that black
sand. Volcanic activity moved eastwards (it still moves that way) and created Karioi, Pirongia and
Kakepuku around 4½ million years ago. Then it moved on to form Maungatautari about 1½ million
years ago.
Looking east again we look out over the Taupo Volcanic Zone. We think of Taupo as being ‘the big
one’ but the Taupo eruptions were a mere puff compared with the truly ‘ginormous’ Mangakino
explosion around half a million years ago. Invisible unless one knows what to look for, it shows as a
vague depression, the caldera, south of Whakamaru. It threw out some 2000 cubic kilometres of
material, the largest eruption in the Southern Hemisphere in the last million years. To get some idea
of the quantity, imagine a line from Auckland to Ruapehu: all the area between that line and the
west coast would be covered by volcanic debris more than half a kilometre thick. This pyroclastic
flow actually covered a vastly greater area of central North Island, including huge sheets between
Mangakino and the West Coast. Most has been weathered away, but there are numerous cliffs and
boulders still visible as you drive around.
There are two main types of volcano in NZ. One type is those that have the classic conical shape
such as Taranaki, Ngaurahoe, and Ruapehu (although this has been eroded away from being conical)
and our own Maungatautari. The other type is those that just explode, like Taupo, Rotorua, and
Mangakino. Taupo really began 23,000 years ago, and has continued to explode every few thousand
years until the last one some two thousand years ago. From the Visitor Centre, its most visible
effects are the way the debris from the explosions re-routed the Waikato River from time to time.
The present-day valley of the river is now filled with Lake Arapuni, but at least four times in the past
this was filled to overflowing with pumice, which blocked the flow of the river and was then
gradually washed away again. The most visible recent explosive eruption was Mt. Tarawera, in 1886,
seen in the distance away over to the left (east).
Having set the bones in place (greywacke) and the flesh (volcanic soils), we now come to the clothing
of the landscape. It must have been incredibly desolate after the eruptions. But nature is nothing if
not persistent. In not many centuries the bush would have re-appeared. Then we humans came
along, and in the flick of an eye we had the current patchwork of pasture, houses, bush, exotic trees,
roads, power lines, dams ….. and pest-proof fences!
Enjoy!