New Zealand is sitting on a plate boundary, so two tectonic plates are
colliding on a line that runs right through New Zealand. One thing to help us
understand the plate boundary is that a tectonic plate can be made up of both
oceanic crust and continental crust. Now the difference of those two types of
crust which are represented if you like by these two rocks here, is that this
oceanic crust is basalt which has a volcanic origin, it's more heavy than
typical continental crust which is represented by this little piece of
granite. And so let's have a look along the plate boundary to see what that actually
looks like as we passed through New Zealand and beyond. So the very dark blue
colour on Google Earth here is the deep ocean. This is probably averaging about
4000 meters deep. But as you can see around New Zealand it's a paler blue
colour which is because the floor of the ocean is shallower and so that whole
area there is the underwater immersed continent of Zealandia made up of the
lighter continental rock compared to the deeper areas which consists of the heavy
dense oceanic basalt rock. So what we'll do is we will zoom in a little bit first
of all to the south of New Zealand and on the western side we have the deep
ocean and on the eastern side we have the paler coloured continental rock.
Although it's underwater it's still actually submerged continent. Now this
trench here indicates the point of collision of the two plates in this area.
Here we have the heavier oceanic rock of the Australian Plate diving down beneath
the lighter continental rock to the east - the Pacific Plate. As we come up towards
the South Island the plate boundary basically passes up along a line just
right sort of along the length of the South Island. This part of the plate
boundary is the Alpine Fault, but here we do not have oceanic crust involved we
only have continental rocks colliding with continental rocks and this means
that neither side goes under the other and the collision is basically a compression of the crust
that forces up the mountains of the Southern Alps. There is a little bit of a lean to the fault - it doesn't go straight down.
What it means is we have uplifted mountains - the huge spectacular Southern Alps being
pushed up on one side of the plate boundary. Over the last 20 million years
or so the crust in this area of the of the South Island has actually been
squeezed and squashed by about a hundred kilometers of compression over all those
millions of years . One thing to understand also is that most of the
movement along the plate boundary in the South Island is a sideways movement.
It's moving about three centimetres a year on average and the uplift is about one
centimetre a year. But this doesn't happen continuously it happens
occasionally, roughly every 300 years in major earthquakes all of a sudden,and
then following that there's a period of no movement. So when we pass a bit
further to the north we come to this area of Marlborough where the plate
boundary is not quite so clear the Alpine Fault continues but there are
also some other major faults in this area. You can see them here and so this
complex of faults is the Marlborough Fault System.
The Marlborough Fault System is what ruptured during the Kaikoura Earthquake in 2016
and as you may well know at that time many many faults ruptured at the same
time so it's a complex area of faulting. Some of those fault ruptures were really
spectacular. - they were globally at the top end of the scale in terms of the amount
of movement that was produced. And so the plate boundary from Marlborough goes out to sea.
And as we pass to the north from here you can see that the plate
boundary is now east of the North Island and the whole of the North Island is
sitting on the Australian Plate. So here we have a plate boundary with subduction
that means the the Pacific Plate is going underneath the North Island at a
low angle initially and then getting steeper and steeper as it goes deeper
and deeper down. So that's a bit of a description of the plate boundary as it
runs through New Zealand and why it is different in different parts of the
country. OK so are there any places in New Zealand where you could actually put
your finger on the plate boundary or is it all hidden under the ground? Well it
turns out that in the South Island this is a cliff section at Gaunt Creek and
you can see on one side there is this pale coloured rock and this is the
Pacific Plate. It's actually been pushed up from many kilometres deep in the
earth and it's kind of riding over the younger
rocks to the right here which are on the Australian Plate and so here in this
photo you can actually see someone putting their finger on the plate
boundary in the South Island at Gaunt Creek. So that gives you a picture of the
fact that New Zealand sits right across this plate boundary. it's quite a
complicated plate boundary, it's different in different places, and one
thing to bear in mind of course is that earthquakes are not just produced along
that plate boundary although that sometimes happens, but the Earth's crust on
either side is broken up by many faults which is why we often get earthquakes on
fault lines in other parts of the country at some distance from the plate boundary itself.