The use of the cooling-history of thick intrusive sheets for the study of the secular variation of the earth's magnetic field
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Numerical results are given which show the way in which thc Curie temperature moves inwards from the margin during the cooling of a thick intrusive sheet. The time taken to move from the margin to the centre varies as the square of the thickness of the sheet, being about 6500 years for a sheet 1000 m thick on reasonable assumptions as to the thermal properties of the material. This implies that the secular variation of the Earth's magnetic field over periods of time of 1000 years or more may be studied by measuring the directions of magnetisation at a series of points across the thickness of such a sheet.
Some measurements on a Tasmanian sill about 430 m thick of Jurassic or Cretaceous age suggest a change from maximum to minimum inclination in a time of about 200 years. Since the margins of the sill are not available this figure may be considerably in error but the results suggest the general practicability of the method.
KeywordsMagnetic Field Cretaceous General Practicability Jurassic Thermal Property
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