Achievement Standard 91412
Auckland Geological History
* Nature of Auckland’s Volcanoes
* Eruption History
Volcanoes are an important feature of the Auckland city landscape. In most cases they are emphasized by their different forms such as reserves and parks, while in the other locations, they have been quarried away to meet the city’s demand for construction material. According to research, distributed over a 20km radius are approximately about 49 volcanoes in the Auckland region, this area is referred to as the Auckland volcanic field. The distribution of the volcanoes is as shown in the following figure Source: http://gns.cri.nz Page 1 0f 19
Majority of the volcanoes in Auckland are small coned and less than 150 meters in height. Most of these extinct volcanoes grew by eruptions, which lasted only for a few months or possibly a few years. In some cases, only a single cone resulted from the eruption but there is also proof that some eruptions have built several cones, forming multiple domes. The type of volcanic activity which has created the Auckland volcanic field is referred to as the monogenetic which means that each time a volcanoe has erupted, it has occurred at a new location. Each of these eruptions is the result of a single batch of magma which rises from its source, the magma chamber in the mantle about 100km beneath the land.
Source: http://gns.cri.nz The Auckland volcanic field from Mt Eden
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However, this monogenetic nature of Auckland’s volcanic lanscape has particular concerns for volcanic hazards because in the event of an eruption, rather than one of the existing volcanoes becoming active, a new volcano is bound to erupt and settle over time in its dormant form. Because of this situation, a hazard map based on any of the locations cannot be drawn as the entire field has to be considered under the threat of a future volcanic eruption.
Nature of Auckland’s Volcanoes
The Auckland volcanic field originates to the presence of a region of hot rock known as a hot spot or plume located approximately 100km beneath the field. In this hot spot, temperatures get high enough so that the rock begins to melt. When a certain amount of molten rock has accumulated, it separates itself from the solid component and rises towards the surface. This melted rock is known as basalt magma and one of its most important characteristics is that it has a very low viscosity, meaning the thickness of the substance is low causing it to be runny. This causes it to force its way through the upper crust quite quickly. Each of the volcanoes in the Auckland volcanic field has originated from a deep source of magma and everytime there has been an eruption, it has been of a new batch of basaltic magma. A crucial aspect of this particular style of volcanism is that there is no crustal magma reservoir present between the eruptions so there is no source of heat to drive the geothermal systems as there is in the central North Island.
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This lack of surface activity in the Auckland field leads to the mistaken fact and confirmation that the field is extinct whereas nothing could be certainly concluded. The only other active basaltic volcano zones in New Zealand are found in the Northland region near Whangarei, around Kaikohe and in the Bay of Islands. It seems to be that these are the areas where the crust is stretching gradually which causes the magma to rise through the cracks. Millions of years ago, similar activity lead to the formation of the volcanoes in the Banks of Peninsular and Dunedin regoin, but eventually causing them to be extinct.
One of the most serious obstacles there is in working out the...