Vertical Variation of Lithologic Characters and Intensity of Natural Remanent Magnetization of the Aso Pyroclastic Flow Deposit

Abstract

The Aso-IIIA pyroclastic flow deposit presents distinct variation of lithologic characters. Fig. 1 shows variation of lithologic and petrographic characters and the intensity of natural remanent magnitization (NRM) depending on the height of an outcrop of one cooling unit from the non-welded base to the zone of dense welding. Some magnetic characters are summarized as follows. [table] From the results following a few points are discussed. 1. The intensity of TRM of the rock specimens and its variation with the height is very similar to that of NRM. It shows that NRM of this column is due to TRM and it corresponds to the fact that the outcrop belongs to one cooling unit. 2. The variation of NRM does not correspond to the variation of abundance of magnetite phenocryst. It is thought that the increase of the intensity of NRM from the B part to the C part is due to formation of submicroscopic magnetite by devitrification. 3. The Curie temperature of the ferromagnetic mineral separated from the B part is lower than them from the A and the C parts. It seems that the Curie temperature from the B part is near to the original one and the higher Curie temperature is caused by oxidation just after the emplacement of the pyroclastic flow.