Study of the Evolutional History of the Earth based on the Paleomagnetic Observations of the Archean Geomagnetic variations

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Observational and theoretical investigations were made, aiming for the evolutional history of the Earth. Observational study includes the paleomagnetic study of Archean rocks sampled from Pilbara district, Western Australia. Rock sampling was made during July-August, 1995, and sedimentary rocks (Banded lron Formation, chert, etc) and igneous rocks (pillow basalts, bastlt flows, komatiite, dacite, etc) with the Archean ages (3.3-2.5 by) were sampled from 4 areas (Cleaverville, Marble Bar, North Pole, and Beaseley River) at the Western Australia. Paleomagnetic study of the banded iron formation (BIF) revealed a reversal sequence of the geomagnetic field during the Archean period, although the frequency of the reversals are not clear yet. Paleointensity study of the igneous rocks indicates that the strength of the geomagnetic field during ca. 33 by ago is much higher than the present field, and the filed strength decreases around the archean-Proterozoic boundary (-2.5 by ago). Results of the simulation study on the growth of the inner core due to secular cooling of the earth, indicate some relations between the physical properties of the inner core (heterogeneity and anisotorpy of the seimic P wave, radial distribution of the density, etc) and the cooling rate of the core, which can be used to infer the evolution of the earth's core from the seismic observations.