De archeoloog in mij dwingt mij iets over de geologie van Auckland te vertellen, het is een heuse natte droom voor iedere geoloog!
Ik was er mij niet van bewust voor ik voet zette op Nieuw-Zeelandselanse bodem maar ... het deugt hier niet! Vooral in de Auckland-regio, waar ik sinds enkele weken een permanente residentie heb.
Het was mij al eerder opgevallen dat Auckland niet bepaald 'plat' is (ik heb er sinds enkele dagen zelfs een spier door verrokken waardoor ik nu al hinken go&see's mag gaan doen). Wat blijkt: het is hier 'nogal' vulkanisch: in totaal maken -geen zever- 49 vulkanen het gebied onveilig.
Ik heb er iemand bijgehaald die het piekfijn kan uitleggen:
Volcanoes are a conspicuous feature of the Auckland city landscape. In some cases their form is emphasised by their preservation as reserves and parks, while in others they have been quarried to meet the city’s demand for building materials. Within a radius of about 20km centred on Auckland city there are 49 discrete volcanoes; this is the area referred to as the Auckland volcanic field. The distribution of the volcanoes is shown in figure 1.
This booklet is concerned with the nature of Auckland’s volcanoes and with what may happen when the next volcanic eruption occurs in the field. The risk of this happening is small but significant and since there is likely to be only a very short warning period when it does occur it is important to be prepared.
There are no written records of the most recent eruption in the Auckland volcanic field about 600 years ago and there are only tantalising fragments of oral history from the legends of the Maori known to have been living nearby. Nonetheless there is a detailed geological record to be gleamed from this volcano and from many others in the Auckland region which shows that there have been repeated and varied volcanic eruptions within the area in the recent past. This is the basis for predicting that there will be another eruption in Auckland at some time in the future; the question is when?
Volcanic activity from an eruption at a site within the Auckland volcanic field could produce serious problems in the day to day existence for those who live in the Auckland region and for New Zealanders supported by Auckland’s business and industry. The degree of destruction will depend on the size and location of the eruption, type of activity, warning time and preparedness planning. Knowledge of the type and effects of volcanic activity are based on studies of volcanic deposits in the Auckland area and on analogues of similar eruptions from similar types of volcanoes elsewhere in the world.
Auckland’s volcanoes are small by comparison with most of the world’s volcanoes. The size of a volcano is usually measured in terms of the volume of material erupted during its active life, and individual eruptions are measured in terms of the volume of material erupted during a particular period of activity. The volume units used are cubic metres (m3) and cubic kilometres (1 km3 = 1,000 million m3. Try imagining a box 1 kilometre high and one kilometre on each side). The cumulative volume of all the material erupted from Auckland’s 49 volcanoes is approximately 4.1km3. To put this figure into a global context the total volume of volcanic deposits in the Auckland volcanic field is only about the size of one average eruption such as the 1980 eruption of the American volcano, Mount St Helens.
Most of the volcanoes in Auckland are small cones less than 150m in height. These grew by eruptions, which lasted only a few months or possibly a few years. In some cases only a single cone resulted from the eruption but there is also evidence that some eruptions have built several adjacent cones. The type of volcanic activity which has created the Auckland volcanic field is referred to as monogenetic which means that each time there has been an eruption it has occurred at a new location and that each eruption is the result of a single batch of magma which rises from its source in the mantle about 100km beneath the city.
The monogenetic nature of Auckland’s volcanoes has particular implications for volcanic hazards because in the event of an eruption, rather than one of the existing volcanoes becoming active, a new volcano will form. Because of this situation, a hazard map based on any one location cannot be drawn and the entire field has to be considered as under a threat of a future volcanic eruption.
Although it is at least 600 years since the last eruption in the Auckland volcanic field, there is every reason to expect eruptions in the future.
These eruptions are likely to be on a small scale compared with some recent overseas eruptions, but because the city of Auckland is built on and around potential eruption sites their effects are likely to be serious.
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