New Zealand's Twisted Plate Boundary | Discovery Geology

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 pass through New Zealand and beyond. So the very dark blue colour on Google Earth here is the deep ocean. This is probably averaging about 4,000 metres 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 consist 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, and here we have the 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 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 100 kilometres of compression over all those millions of years. One thing to understand 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 you 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. Okay, 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 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.