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Paleomagnetisk undersökning av vulkanen Rangitoto, Nya Zeeland, för att bestämma dess utbrottshistoria

Hjorth, Ingeborg LU (2019) In Examensarbeten i geologi vid Lunds universitet GEOL01 20191
Department of Geology
Abstract (Swedish)
Rangitoto är en vulkanö som ligger nordost om Auckland, Nya Zeeland, och tillhör Aucklands vulkaniska fält (AVF) som består av ca 50 utdöda monogenetiska vulkaner. Eftersom Rangitoto har egenskaper som skiljer sig från de andra vulkanerna i området tror man att området genomgått en förändring och gått in i en ny fas. Det var ca 500 år sedan Rangitoto senast hade utbrott och eftersom storstadsområdet Auckland är beläget intill Rangitoto och ovanpå AVF är det viktigt att undersöka hur utbrottshistorien ser ut för att kunna bedöma riskerna för nya utbrott i framtiden. Det finns olika hypoteser om hur utbrottshistorien ser ut och de som utvärderas i denna studie är att de flesta av utbrotten, som omfattar den så kallade skölduppbyggande fasen,... (More)
Rangitoto är en vulkanö som ligger nordost om Auckland, Nya Zeeland, och tillhör Aucklands vulkaniska fält (AVF) som består av ca 50 utdöda monogenetiska vulkaner. Eftersom Rangitoto har egenskaper som skiljer sig från de andra vulkanerna i området tror man att området genomgått en förändring och gått in i en ny fas. Det var ca 500 år sedan Rangitoto senast hade utbrott och eftersom storstadsområdet Auckland är beläget intill Rangitoto och ovanpå AVF är det viktigt att undersöka hur utbrottshistorien ser ut för att kunna bedöma riskerna för nya utbrott i framtiden. Det finns olika hypoteser om hur utbrottshistorien ser ut och de som utvärderas i denna studie är att de flesta av utbrotten, som omfattar den så kallade skölduppbyggande fasen, har skett under en period på 1) mindre än 100 år, eller 2) upp emot 500 år. För att undersöka hypoteserna har, i denna studie, 157 basaltprover från 35 av Rangitotos lavaflöden analyserats med paleomagnetiska metoder för att fastställa eventuella variationer i magnetfältets inklination mellan de olika utbrotten. Lavaflödena har delats in i 6 grupper med liknande inklinationsvärden. Variationer i inklination mellan de olika grupperna visar på relativt stora förändringar i magnetfältet, vilket är mest förenligt med hypotes 2. Jämförelser med oberoende referenskurvor antyder dock ett utbrottsförlopp på över tusen år, vilket är oförenligt med båda hypoteserna. Av detta kan följande slutsatser dras: i) att inklinationsvariationerna inte bara reflekterar förändringar i jordens magnetfält, eller ii) att referenskurvorna inte är tillförlitliga inom sina konfidensintervall, eller iii) att dateringarna som ligger till grund för de ursprungliga hypoteserna är felaktiga. (Less)
Abstract
Rangitoto is a volcanic island, located northeast of Auckland, New Zealand, and belongs to Auckland Volcanic field (AVF), which consists of c. 50 extinct monogenetic volcanoes. Since Rangitoto has characteristics that makes it different from the other volcanoes in the area, it is believed that the volcanic field has undergone a change and entered a new phase. Rangitoto had its latest eruption about 500 years ago and since the metropolitan area of Auckland is situated close to Rangitoto and on top of the AVF it is important to examine the eruption rate to be able to evaluate the risks for new eruptions in the future. There are different hypotheses about the eruption history of Rangitoto and those that are evaluated in this study are that... (More)
Rangitoto is a volcanic island, located northeast of Auckland, New Zealand, and belongs to Auckland Volcanic field (AVF), which consists of c. 50 extinct monogenetic volcanoes. Since Rangitoto has characteristics that makes it different from the other volcanoes in the area, it is believed that the volcanic field has undergone a change and entered a new phase. Rangitoto had its latest eruption about 500 years ago and since the metropolitan area of Auckland is situated close to Rangitoto and on top of the AVF it is important to examine the eruption rate to be able to evaluate the risks for new eruptions in the future. There are different hypotheses about the eruption history of Rangitoto and those that are evaluated in this study are that most of the eruptions, which comprise the shield building phase, occurred during a period of 1) about 100 years, or 2) up to 500 years. To test these hypotheses, 157 samples of basalt from 35 of Rangitoto’s lava flows have been analysed to determine their paleomagnetic directions (or more specifically the inclinations). The lava flows have been divided into 6 groups with similar inclination values. The variations in inclination between the different groups indicate that the magnetic field has changed relatively much, which is more compatible with hypothesis 2. However, comparison with independent reference curves indicates an eruption sequence of over thousand years, which is incompatible with both hypotheses. From this the following conclusions can be drawn: i) that the inclination variations do not only reflect changes in earth’s magnetic field, or ii) that the reference curves are not reliable within their confidence intervals, or iii) that the datings, which the hypotheses are based on, are incorrect. (Less)
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author
Hjorth, Ingeborg LU
supervisor
organization
course
GEOL01 20191
year
type
M2 - Bachelor Degree
subject
keywords
paleomagnetism, Aucklands vulkaniska fält, Rangitoto, naturlig remanent magnetisering, magnetisk inklination
publication/series
Examensarbeten i geologi vid Lunds universitet
report number
559
language
Swedish
id
8981621
date added to LUP
2019-06-10 23:47:54
date last changed
2019-06-10 23:47:54
@misc{8981621,
  abstract     = {Rangitoto is a volcanic island, located northeast of Auckland, New Zealand, and belongs to Auckland Volcanic field (AVF), which consists of c. 50 extinct monogenetic volcanoes. Since Rangitoto has characteristics that makes it different from the other volcanoes in the area, it is believed that the volcanic field has undergone a change and entered a new phase. Rangitoto had its latest eruption about 500 years ago and since the metropolitan area of Auckland is situated close to Rangitoto and on top of the AVF it is important to examine the eruption rate to be able to evaluate the risks for new eruptions in the future. There are different hypotheses about the eruption history of Rangitoto and those that are evaluated in this study are that most of the eruptions, which comprise the shield building phase, occurred during a period of 1) about 100 years, or 2) up to 500 years. To test these hypotheses, 157 samples of basalt from 35 of Rangitoto’s lava flows have been analysed to determine their paleomagnetic directions (or more specifically the inclinations). The lava flows have been divided into 6 groups with similar inclination values. The variations in inclination between the different groups indicate that the magnetic field has changed relatively much, which is more compatible with hypothesis 2. However, comparison with independent reference curves indicates an eruption sequence of over thousand years, which is incompatible with both hypotheses. From this the following conclusions can be drawn: i) that the inclination variations do not only reflect changes in earth’s magnetic field, or ii) that the reference curves are not reliable within their confidence intervals, or iii) that the datings, which the hypotheses are based on, are incorrect.},
  author       = {Hjorth, Ingeborg},
  keyword      = {paleomagnetism,Aucklands vulkaniska fält,Rangitoto,naturlig remanent magnetisering,magnetisk inklination},
  language     = {swe},
  note         = {Student Paper},
  series       = {Examensarbeten i geologi vid Lunds universitet},
  title        = {Paleomagnetisk undersökning av vulkanen Rangitoto, Nya Zeeland, för att bestämma dess utbrottshistoria},
  year         = {2019},
}