Download AS 90952 Student 1 The volcanoes of Auckland The volcanoes of

AS 90952
Student 1
Student 1: Low Excellence
The volcanoes of Auckland
The volcanoes of Auckland we see today are a combination of internal forces and external
forces the cause the volcanoes and shape the volcanoes. There are about 50 volcanic
cones in the Auckland district. In the last 20 000years there have been 19 confirmed
eruptions, about 1 every 1000 years. They seem to be increasing in frequency and size.
Internal Forces
Auckland city sits atop a pool of magma. This magma occasionally makes it way to the
surface and erupts a small volcano. This type of volcano is called a “hot spot” volcano,
named by J Tuzo Wilson in 1965. In this type of volcano heat from the core of the Earth rises
directly through the mantle as a plume until it meets the colder solid crust. Here, 100km
down, it melts the upper mantle layer and a pool of molten magma forms. As this pool grows
in size it tries to find its way to the surface and when it does this it forms a “hot spot” volcano.
They last for months or even a few years. This is being happening in Auckland for over 250
000 years. The first eruption was the Onepoto eruption. The magma is basalt and its
derivatives like scoria. Mt Wellington is made of scoria. The basalt is primitive basalt and
tends to only erupt once from a vent. Rangitoto is an exception to this as it erupted twice
using the same vent 600 years ago. If the vent is into water an explosive eruption occurs like
Lake Pupuki. Hot spot volcanoes tend to stay in one place and the plates move over them.
(1) This is why there is 50+ volcanoes in the Auckland area.
External Forces
Erosional forces that contribute to the shape of volcano we see today are primarily stream
erosion and erosion due to sea-level change. Stream erosion attacks the weaker rocks and
ash deposits and in wearing these away changes the shape of a volcano. The sides get
worn down and hard rocks are left standing. Volcanoes that erupt solid magma flows wear
away the slowest. The longest lava flow is over 10km from the Mt Saint John vent. Over the
last 100 000 years the world has been subjected to numerous ice ages and these have
changed the slope of the land and effected erosion. Human activity also affects the shape of
a volcano. Early Maori erected pa and kumara pits on volcanoes, especially One Tree Hill
and helped change the shape of the volcano. More lately quarrying has changed the shape
of the volcanoes. In fact the Onepoto rocks have been used to build the northern motorway.
(2)