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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)