How Auckland Volcanoes Erupt

How Auckland Volcanoes Erupt


Auckland City is built on a field of small
volcanoes, about 50 of them. And some of these volcanoes you can see as
you drive around the city – they’re obvious cones. other volcanoes are really just holes
in the ground, and that tells us about the different styles of eruption that have taken place in Auckland during the past quarter of a million years. Beneath Auckland there is a hot-spot in the mantle, we think at a depth of about 80 to
100 kms and in this hot-spot rocks are melting and producing magma. The magma segregates and it rises quickly. We know it must rise quickly, because if it
moved slower, it would cool and solidify before it reached the surface. so there is only days between the time that the magma leaves it’s source and the time it reaches the surface and this is one of the problems we have in predicting eruptions in Auckland. today in this trip around auckland’s volcanoes, we’re going to be able to see the different types of rock that are produced when magma
reaches the surface. Here we are at the base of the North Head
volcano cone, and here we can look at the first of the three main rock types that we have in Auckland. the first thing that can happen is an interaction
of magma with water. Hot magma, maybe 1000 degrees,
encounters cold water, the water turns to steam and the magma fragments into lots of little pieces, and we call the resulting eruption ‘phreatomagmatic’. phreatomagmatic eruptions are the most violent type of eruption we would expect to have in Auckland. Each bed here reflects an exlplosion. Ash is thrown up in the air – up to hundreds of metres, and material falling out of that ashcloud
to make a bed. Another explosion, then another, then another, and so we have built up through time, a phreatomagmatic deposit of ash and small stones. If we look at the whole deposit we can see other changes. Here the beds are small, and repetitive. As we go higher we can see thicker beds of coarse material interspersed with fine material, and further up there are some quite large blocks. This reflects a change in the eruption and an increase in the intensity of the eruption, from repeated smaller explosions to perhaps larger explosions at longer intervals. The magma comes through the earth’s crust and as it does so it can pick up fragments
of the rocks which it passes through and here’s an example. This is a little block of sandstone
which is the rock that is common beneath Auckland. If we look higher we can see other light-coloured blocks, these are fragments of the underlying geology. In addition there are also blocks of lava
such as this one. These are bits of magma which have cooled in the vent system and have been thrown out and broken up as they’ve come. There’s quite a nice one there, and there’s some bigger ones up here too. so this was a point at which the eruption was becoming much more explosive. We’re still on north head volcano but we’ve moved higher on the cone, and here we can see the second kind
of deposit that we find in Auckland. the second type of eruption style
occurs when the magma comes to the surface and is proplled out of the vent by the gases
within the magma itself. but what happens then is the development of
a fountain of fire. that is the process which forms many of Auckland’s little cones. They’re referred to as scoria cones and the material they’re made of is these lumps of basalt that we call scoria. In this deposit we see individual lumps of magma. They’ve fallen out of a fire fountain, they’re red because basalt contains quite a lot of iron, and that iron often becomes oxidised. This is a process that happens at high temperature. So here’s quite a large blob of magma, these holes in the rock are the evidence for the
gases which drove the eruption. So just to recap, what happened here at north head is magma rose through the mantle, through the crust, was erupted at the surface. The first
thing that happened was the mama encountered water and we had that explosive type of activity
that I referred to as phreatomagmatic. That produced a low tuff cone, a low cone
of ash and other fragments. The eruption then became what volcanologists call ‘dry’ the eruption was then solely driven by the gases in the magma itself. And that is the second type of activity
that we see in Auckland’s volcanoes. So now we’ve moved to another nearby volcano, this is Pupuke volcano and we can see the third kind of activity typical
of an Auckland volcano, and this is the eruption of magma
at the earth’s surface, flowing as a lava flow. and at that time at this locality there
was no sea, but there was a forest and we see relics preserved of that forest
a quarter of a million years ago. Here’s an example of a place where a tree has been knocked over by the lava flow, the lava has chilled against the tree and become solid, but the tree has been burned and no longer exists. This lava clearly contained a lot of natural gas, because you can see that it is full of these holes. As well as that there was probably gas released by the decomposition of vegetation that the lava was running over.
And so within the lava flow, we got bubbles pockets of gas creating what we call lava blisters. and here’s a very good example of a lava blister. Here’s another lava blister – a rather bigger one – here’s a tree mold, another tree mold, another lava blister. There’s a lot going on in this outcrop.

15 thoughts on “How Auckland Volcanoes Erupt

  1. Great video, thanks for publishing. I remember these Auckland geology field trips when I studied at the University in Auckland in the early 90's. Would love it though if GNS could add closed captions to your videos, thereby making them accessible to everyone who wants to learn more about geology in New Zealand. Thanks.

  2. The beauty of knowing what happened in the past by looking at the formation and composition rocks is amazing!

  3. How is it that if the earth is filled with melted rock and other materials under the crust why isn’t the ocean and land above incredibly hot and unliveable for plants and animals. Might seem like this idea that’s widely believed is ridiculous. Maybe everyone needs to actually have a proper think as to how the heat manages to stay underground. So by rights the heat that’s down there somehow stays there or somehow dissipates at a lovely fanciful rate. Just because it’s the accepted theory rammed home doesn’t make it true. I think there needs more thought given towards accepting the electric universe models. Same goes for everyone who says and thinks comets have ice jets as a tail as they come towards the sun. Come on people the old models need some rethinking!!!

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