Palaeomagnetic studies of Eyjafjardardalur, northern Iceland: does the geocentric axial dipole exist during 3-7 Ma?

The geocentric axial dipole (GAD) hypothesis states that when we average the geomagnetic field over sufficient geological time intervals, the time-averaged field (TAF) behaves like a dipole aligned along the Earth’s spin axis and positioned at the Earth’s centre. This hypothesis is crucial in palaeo...

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Main Author: Supakulopas, Radchagrit
Other Authors: Muxworthy, Adrian
Format: Doctoral or Postdoctoral Thesis
Language:unknown
Published: Earth Science & Engineering, Imperial College London 2018
Subjects:
Online Access:http://hdl.handle.net/10044/1/78482
https://doi.org/10.25560/78482
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spelling ftimperialcol:oai:spiral.imperial.ac.uk:10044/1/78482 2023-05-15T16:19:42+02:00 Palaeomagnetic studies of Eyjafjardardalur, northern Iceland: does the geocentric axial dipole exist during 3-7 Ma? Supakulopas, Radchagrit Muxworthy, Adrian 2018-08 http://hdl.handle.net/10044/1/78482 https://doi.org/10.25560/78482 unknown Earth Science & Engineering, Imperial College London http://hdl.handle.net/10044/1/78482 https://doi.org/10.25560/78482 Creative Commons Attribution Non-Commercial No Derivatives licence (CC BY-NC-ND). CC-BY-NC-ND Thesis or dissertation Doctoral Doctor of Philosophy (PhD) 2018 ftimperialcol https://doi.org/10.25560/78482 2020-04-30T22:38:19Z The geocentric axial dipole (GAD) hypothesis states that when we average the geomagnetic field over sufficient geological time intervals, the time-averaged field (TAF) behaves like a dipole aligned along the Earth’s spin axis and positioned at the Earth’s centre. This hypothesis is crucial in palaeomagnetic research, e.g., it is a key to tectonic reconstructions. However, there is some evidence for the persistence of long-term hemispheric asymmetry on time scales of 10^5-10^6 yrs, particularly at high-latitudes. As most palaeomagnetic research is conducted under the GAD hypothesis, this hypothesis needs to be rigorously tested. This thesis investigates the symmetry of the palaeomagnetic field and tests the GAD hypothesis during ~2.6-8.5 Ma using full-vector palaeomagnetic data - including palaeodirection and palaeointensity - from dated lava piles in northern Iceland. Demagnetisation measurements including alternating field (AF) and thermal were made to determine palaeomagnetic directions. A mean declination and inclination of 354.1° and 71.4° were found in this study, with a 95% confidence limit of only 2.4°. The mean virtual geomagnetic pole (VGP) is located at 80.6°N and 184.6°E, which is tilted ~10° from the geographic north; this VGP position does not agree with the GAD hypothesis. It is argued that this shallow inclination is a result of the non- dipole fields which contribute to the GAD field. This was confirmed by the palaeosecular variation (PSV) model compilation of the global dataset during 0-8.5 Ma. The PSV model suggests that 4% of axial quadrupole and 1% of axial octupole contribute to the TAF during 0-8.5 Ma. Palaeointensity data reveals a weak mean intensity of 26.9 ± 1.8 μT, which is lower than the intensity of the expected GAD field (55.9 μT) at 65°N for the current day field. This weak intensity equates to a virtual dipole moment (VDM) of 37.4 ± 2.5 ZAm^2. After combining this study data with 1-3 Ma data also from Iceland to improve temporal resolution, a mean VADM of 44.0 ± 2.2 ZAm^2 was obtained. The direct comparison of the VADM between high-northern and high-southern latitudes (37.3 ± 3.8 ZAm2) was made. This study reveals that there is hemispheric asymmetry of the field between Northern and Southern Hemisphere. Open Access Doctoral or Postdoctoral Thesis Geomagnetic Pole Iceland Imperial College London: Spiral
institution Open Polar
collection Imperial College London: Spiral
op_collection_id ftimperialcol
language unknown
description The geocentric axial dipole (GAD) hypothesis states that when we average the geomagnetic field over sufficient geological time intervals, the time-averaged field (TAF) behaves like a dipole aligned along the Earth’s spin axis and positioned at the Earth’s centre. This hypothesis is crucial in palaeomagnetic research, e.g., it is a key to tectonic reconstructions. However, there is some evidence for the persistence of long-term hemispheric asymmetry on time scales of 10^5-10^6 yrs, particularly at high-latitudes. As most palaeomagnetic research is conducted under the GAD hypothesis, this hypothesis needs to be rigorously tested. This thesis investigates the symmetry of the palaeomagnetic field and tests the GAD hypothesis during ~2.6-8.5 Ma using full-vector palaeomagnetic data - including palaeodirection and palaeointensity - from dated lava piles in northern Iceland. Demagnetisation measurements including alternating field (AF) and thermal were made to determine palaeomagnetic directions. A mean declination and inclination of 354.1° and 71.4° were found in this study, with a 95% confidence limit of only 2.4°. The mean virtual geomagnetic pole (VGP) is located at 80.6°N and 184.6°E, which is tilted ~10° from the geographic north; this VGP position does not agree with the GAD hypothesis. It is argued that this shallow inclination is a result of the non- dipole fields which contribute to the GAD field. This was confirmed by the palaeosecular variation (PSV) model compilation of the global dataset during 0-8.5 Ma. The PSV model suggests that 4% of axial quadrupole and 1% of axial octupole contribute to the TAF during 0-8.5 Ma. Palaeointensity data reveals a weak mean intensity of 26.9 ± 1.8 μT, which is lower than the intensity of the expected GAD field (55.9 μT) at 65°N for the current day field. This weak intensity equates to a virtual dipole moment (VDM) of 37.4 ± 2.5 ZAm^2. After combining this study data with 1-3 Ma data also from Iceland to improve temporal resolution, a mean VADM of 44.0 ± 2.2 ZAm^2 was obtained. The direct comparison of the VADM between high-northern and high-southern latitudes (37.3 ± 3.8 ZAm2) was made. This study reveals that there is hemispheric asymmetry of the field between Northern and Southern Hemisphere. Open Access
author2 Muxworthy, Adrian
format Doctoral or Postdoctoral Thesis
author Supakulopas, Radchagrit
spellingShingle Supakulopas, Radchagrit
Palaeomagnetic studies of Eyjafjardardalur, northern Iceland: does the geocentric axial dipole exist during 3-7 Ma?
author_facet Supakulopas, Radchagrit
author_sort Supakulopas, Radchagrit
title Palaeomagnetic studies of Eyjafjardardalur, northern Iceland: does the geocentric axial dipole exist during 3-7 Ma?
title_short Palaeomagnetic studies of Eyjafjardardalur, northern Iceland: does the geocentric axial dipole exist during 3-7 Ma?
title_full Palaeomagnetic studies of Eyjafjardardalur, northern Iceland: does the geocentric axial dipole exist during 3-7 Ma?
title_fullStr Palaeomagnetic studies of Eyjafjardardalur, northern Iceland: does the geocentric axial dipole exist during 3-7 Ma?
title_full_unstemmed Palaeomagnetic studies of Eyjafjardardalur, northern Iceland: does the geocentric axial dipole exist during 3-7 Ma?
title_sort palaeomagnetic studies of eyjafjardardalur, northern iceland: does the geocentric axial dipole exist during 3-7 ma?
publisher Earth Science & Engineering, Imperial College London
publishDate 2018
url http://hdl.handle.net/10044/1/78482
https://doi.org/10.25560/78482
genre Geomagnetic Pole
Iceland
genre_facet Geomagnetic Pole
Iceland
op_relation http://hdl.handle.net/10044/1/78482
https://doi.org/10.25560/78482
op_rights Creative Commons Attribution Non-Commercial No Derivatives licence (CC BY-NC-ND).
op_rightsnorm CC-BY-NC-ND
op_doi https://doi.org/10.25560/78482
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