Mount Taranaki stands alone as a mountain. It's a very dramatic peak and it dominates the landscape
in this area. And it's important to respect that in Māori tradition this mountain has a sacred status.
Taranaki is a dormant volcano. It last erupted a few hundred years ago
and could erupt again in the future and no doubt surely will.
We're on the eastern slopes. We've been able to drive up the road through the forest
and the trees have got shorter and shorter, and where we are now is a kind of tussock vegetation.
That's telling us something - it's telling us that the conditions here
for plant life are much more extreme. Very strong wind, it's very cold in the winter, it's
sometimes very dry, sometimes very wet - so these plants are adapted to survive these conditions.
So the extreme conditions that we have on this mountain also impact on the way it erodes.
The frost cracks the rocks, the steep angle means they easily topple down the slopes
and when there's a storm of snow or rain, avalanches and floods gush down into the gullies
like this one and carry these boulders with them. So there are many active processes going on
on a mountain like this and that's why we see all around us the results of this extreme erosion.
I'm on the track leading up to the Manganui Ski Field and it takes us right past this huge lava
flow. But if we take a close look at what's underneath the lava flow you can see here
that there's the very fine sandy ash which is incredibly weak, and on top of it is this
huge mass of very rigid solid lava. So this is a perfect example of how Taranaki is made: weak
layers with very strong solid layers above them adding up to a kind of unstable pile of material.
So when we look at the material that's deposited in the side of this gully you can see that it is
very chaotic. The big pieces are mixed up with the very fine material and there doesn't seem
to be any order. Over time this boulder will get rolled and washed down this steep valley
and break into pieces so that when it reaches the foot of the mountain
typically the particle size will be much smaller than this.
So we have this very dynamic, very rapid erosion of this mountain but where does it go, where does it
finally end up? And so where we'll go to next is we'll find some places lower down on the slopes
of the mountain and beyond to see if we can find and recognize any of this volcanic debris.
All around Mount Taranaki there is a huge plain extending in all directions
and it's called the ring plain. And the ring plain is really thick layers of debris that have washed
off the mountain over time , and so all of these boulders tell us that they've been rolled down
the river over a long period of time. The sharp angles have been taken off, the edges have been
broken away. This is lava - little white crystals of feldspar in a black sort of ground mass
of darker crystals. Most of Mount Taranaki is actually in the ring plane. This is it. The bit
that we see, the very spectacular peak that we see in the distance is the small remaining part
of this enormous quantity of material that's been washed in all directions into the ring plain. If
you travel around Mount Taranaki, around the ring plain, there are places where you see hummucks in
the landscape. They're quite curious formations. They're like lots of isolated, separate hills.
And these are the remains of a lahar flow that has ended up at a distance from the volcano. So
it looks like a hummocky landscape and if you look close up at some of the boulders in these lahar
mounds, they're still angular and have got sharp edges to them. They've not been rounded by flowing down
a river because they've been carried along and transported in one very sudden event taking them
a long way from the mountain but still retaining their sort of crude, if you like, broken shapes.
We are now at Airedale Reef and this is over 40 kilometres from Taranaki Volcano.
Where I'm standing there is a dark layer and you can see bits of wood sticking out and so this
layer is a very peaty layer. It's full of carbon, there are some sprinkles of volcanic ash in it,
but it's telling us that once upon a time there was a forest here.
So all over this beach and all around these rocks there is this layer with
quite large pieces of wood, tree trunks and tree stumps in life position.
Right above this layer you can see these rocks here behind me. They are jumbled,
chaotic, they don't really have any layers to them, a little bit rounded and a little bit
smaller than the ones we saw on the mountain, but they are the same material. They're made of
very crystalline rock so we can recognize it's definitely lava. And so within this
very chaotic layer there are pieces of wood also which have obviously been caught up in the layer
before it finally came to rest in this position. So this chaotic layer is about four meters
thick in this area, and it is believed to represent a collapse of Taranaki Volcano.
The extent of this layer can be found over 250 square kilometres of the ring
plain around to the north of Mount Taranaki, and it is believed to consist of a total of
over 3.6 cubic kilometres of material. All of that material represents a huge amount of the total
size of the mountain that collapsed about a hundred thousand years ago.
And the mountain would have subsequently continued as an active volcano, erupted more material,
become a bigger mountain again, and then collapsed another time - and later on, and another time
and here over 40 kilometres away we can see the results of one of these enormous events:
a forest obliterated by a huge debris flow of a collapsing volcano.
We're at Opunaki which is on the coast at about something like 25 kilometres from the
peak of the volcano, and what we have here is a really clean vertical section through the cliff
showing what the ring plain looks like below our feet. And you can see that there are layers
and layers of different sorts. There are big boulders, there are small particles,
there are sandy layers and they've all been brought here by a variety of different
processes. Some of them show that they're river deposits, some of them are lahar deposits
or debris flows. There are channels cut through some of the layers by overlying layers
and the story here it's a bit like a history book - the oldest page is at the bottom of
the cliff and as you go up the cliff each layer was added later and later through time.
So the story is telling us of different processes to move all that material from the peak
of the mountain until we end up with this massive thickness of sediment eroded from the volcano.
So places where sediments end up, we call them sedimentary basins.
And those are very interesting places because when we look at the rocks in sedimentary basins
we find all sorts of clues about where that material came from.
So we don't want to go necessarily to the mountain to find the long detailed historic record of
what's happened. We actually want to go to the surrounding area where all that material has piled
up in one layer after another, and each of those layers telling the story of change through time
as we work upwards from the oldest layer at the bottom to the higher layers above.