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Centre for Earth and Environmental Science Research

Palaeointensity investigation of Cenozoic igneous rocks from south-west Australia

Executive Summary
ResearchersDr N. Thomas
Dr A. Garcia
Dr M. Hill: (The University of Liverpool)
Funding Body/SourceKingston University
Duration2002 -
Project SummaryThis ongoing project investigates well-dated Cenozoic ingeous rocks from south west Australia, using both conventional thermal Coe-Thellier and microwave palaeointensity methods.


The intensity of the Earth's magnetic field has varied over geological time. The Earth's magnetic field is generated and maintained predominantly by convection processes operating in the outer core which in turn are thought to be governed by convective processes within the mantle as a whole. The links between processes occurring deep within the core, mantle and crust suggest the existence of a global geodynamic system, the evolution of which could possibly be documented by long-term intensity variations.

The palaeointensity record while containing data spanning over 400 million years, does not unambiguously reflect the nature of intensity variations as data is temporally biased. The database has limited temporal coverage, with some age windows containing a dense data population while others containing sparse or no data. Deficiencies in the database reduce the overall confidence in assessing field variations and prevent thorough testing of the hypotheses linking long-term variations with the evolution of the geodynamic system. Hence, enhancing the quality and quantity of data in key sparsely covered time windows is a priority. Palaeointensity data are deficient in many parts of the Cenozoic record. Additionally, pre-existing Cenozoic estimates are predominantly from northern hemisphere sources, creating a spatially biased dataset.

Cenozoic intrusive and extrusive rocks are distributed along the length of the eastern margin of Australia, from Tasmania, through New South Wales, into Queensland. The temporal and spatial distribution of these igneous rocks makes them attractive sources, which could contribute additional data to the palaeointensity record. Well-dated igneous rocks have been investigated as these are direct products of the geodynamic processes and can yield reliable spot readings of absolute field intensity.

Fig 1: Sampling with a portable petrol drill in a remote quarry]



Fresh un-weathered rock outcrops are targeted. Working quarries, exposed wave cut platforms or recently excavated road cuttings provide excellent sampling sites. Samples commonly take the form of 2.5cm diameter cores, extracted using a portable gasoline powered drill with water-cooled diamond tipped bit. Ideally cores are sufficiently long (>8cm) to avoid surface weathering and positioned away from fractured portions of rock. Core samples are orientated preferably using a compass sun-compass or a standard magnetic.

Fig 2: orientating a drilled core
sample, extracted from an exposed basalt flow]


In the laboratory samples are subject to investigations that determine both the palaeo-magnetic field direction and intensity. The conventional thermal Coe-modified-Thellier method is employed to obtain palaeointensity estimates in all thermal palaeointensity investigations. This is an established and accepted method of palaeointensity acquisition within the palaeomagnetic community. A second method, the 'microwave technique' has also been used. Microwave technology was originally developed for use with archaeological ceramics, but its use has been extended to determine palaeointensity estimates from young and ancient igneous rocks.


Thomas, D.N., Hill, J.M. and Garcia, A.S. (2004), Comparison of the Coe-Thellier-Thellier and microwave palaeointensity techniques using high-titanium titanomagnetites: results from a Tertiary basaltic intrusion from the Sydney Basin, New South Wales. Earth and Planetary Science Letters, 229, 15-29.

Hill, J.M., Thomas, D.N. and Garcia, A.S. (2003), Palaeointensity results from the Tertiary intrusive rocks from the Sydney basin: A comparison of the modified - conventional and microwave Thellier methods. IUGG 2003 Conference, Sapporo, Japan. (Abstract, GAI.05/01P/A08-010)

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