I am walking in the hills - the Western Hills near Wellington in New Zealand.
That's the harbour over there behind me. What I want to do is tell you what these
hills are actually made of. Mostly you can't really see because they're covered
in grass but in places there are rock outcrops and what you see when you come
to those is very weathered bedrock - brown - grey coloured sand stone, sometimes mudstone.
And it's the story of that rock that I want to tell you in this video.
So I think the best way to explain these rocks and their history is to show you
some pictures and talk you through them. So first of all this is a map of the
same rocks that we've just seen on the hills, across New Zealand. So the
geologists have mapped this over many years and they find that this greywacke,
this sandstone and siltstone, extends right through the country . So it's if you
like the backbone or the bedrock of New Zealand. It's New Zealand's National Rock
you could say. So it's a very important rock and it's not only on the land of
New Zealand -if we were to sort of dig under the surface and look beneath the
vegetation we would find it in these areas - but also we can actually know that
it extends out into the ocean as well. This image here shows a kind of
impression of what you would see if you stripped away all of the ocean around
New Zealand. So there's New Zealand in the middle but this orange coloured
shape - it looks like a sort of maybe a wing or a triangular shape - around New
Zealand, is an area of rock which includes a lot of that similar sandstone
and mudstone. So in other words New Zealand is the tip of a huge area of
this material and this is continental rock it is not the same
as the deeper crust here in the oceans which is the blue colour which is a
different type of material -its basalt. So where did this huge area of rock come
from? Because when we answer that question we'll answer the question where
did that individual rock come from on the Western Hills. Firstly we know that
it's continental rock because these red marks on the map here are samples that
have been taken from under the ocean by drill ships or sometimes from little
islands around the place ,which show this particular type of rock. So we know that
this rock extends beyond New Zealand. Another view: you might just see a pink
outline that shows you the outline of this area. It is about 17 times the size
of the land of New Zealand so it's a hidden continent. And how did that arise?
So this this map shows the situation when Australia and Antarctica were joined up
as part of Gondwana or Gondwanaland and you can see that out here in the Pacific
on the eastern side there's this huge kind of greenish colour and it's named
Zealandia. So what this area is is all the mud and sand that has been washed
off the coast of Gondwana is washed out into the ocean and it's sitting if you
like on the ocean floor. And at this time Australia is joined to Antarctica and
this material is being washed off the coast and this situation lasted for
something like 400 million years. Along this line on the on the coast of
Gondwana there was a subduction margin. In other words the Pacific Ocean was
being pushed into Gondwana and underneath it and so there was a lot of
uplift along the coast there was erosion of the mountains,
there was lots of sediment pouring into the ocean and it was all being pushed
and scrunched up against the Gondwana as well so these rocks tend to be highly
deformed when we find them. So this image here shows the subduction happening
before it stopped. So this is the Pacific Plate pushing into Gondwana - the rocks
getting deformed, some of them being dragged underneath, and then on Gondwana
there are mountains,there are volcanoes and this is a highly active subduction
margin very similar to what we might see today on the North Island on the east
coast of New Zealand. So over time this long-term situation of the past it
actually eventually stopped and instead of the Pacific Ocean pushing
into Gondwana in this way which it had been doing for so long, it actually
stopped and instead it started to split apart. So this area called Zealandia, this
ribbon of continental sort of sediments, started to drift away from from Gondwana.
So I'll show you a series of little maps which just shows how Zealandia separates
from Gondwana. The blue line represents a line where the separation is going to
start happening. So you can see that the Tasman Sea starts to open up and
gradually gets wider and wider, and then you might notice this shape here which
is if you like bits of modern New Zealand. They're not quite joined up in
the same way that they are now, but you can see a black line coming through and
that is the modern plate boundary developing. So if I keep going we're now
in a situation about 20 million years ago where this plate boundary is now
pretty much fully developed. The South Island is up against the North Island
and it is going to dragged southwards along the Alpine
Fault so that the east and the west coast of the South Island line up as
they do today. So that's the final modern situation: so we have a new plate
boundary, Australia and Antarctica have now separated as Zealandia has been
pulled away and separated from what had been Gondwana and is now Australia and
Antarctica. So that's our modern situation and if I just show you a
little bit of a detailed map of what New Zealand looks like in the setting of the
remains of Zealandia around it, you can see we've got the plate boundary coming
right through the country, the Alpine Fault and then it becomes a Hikurangi
Trough, and the Kermadec Trench that's all the plate boundary. The North Island is
on the Australian Plate, most of the South Island is on the Pacific plate and
these two plates are colliding with each other and grinding past each other along
that boundary. And that's our modern situation, it gives us uplifted mountain
ranges ,it gives us earthquakes, we still have active volcanoes, so we have this huge
active landscape in continual formation. It's not stopped, it's been doing this
for millions of years and it's still in that process today. So that is a summary
of this epic history if the rocks that we have right through New Zealand.
That's the story of our New Zealand greywacke and the story
it contains - its deep history from the past in Gondwana through to the
separation of Zealandia, the drowning of Zealandia in the ocean and then it's
uplift to form the landmass of New Zealand that we have today.