The anisotropy of magnetic susceptibility of lava flows: an experimental approach

Measurements of the anisotropy of magnetic susceptibility (AMS) of natural lavas have shown that AMS varies with depth within a lava flow. We have inv estigated the reasons for such variation by studying the effects of tempera ture and strain rate on the AMS of recent lava in the laboratory. Samples o f lava from Kilauea were melted and subjected to a range of strain rate and cooling histories. The results show that the degree of anisotropy is a fun ction of both the thermal and shearing history of a sample. High degrees of anisotropy were found only in samples that were deformed at temperatures c lose to those encountered during eruption and then rapidly quenched. Lavas subjected to similar shear stresses at high temperatures had low degrees of anisotropy if allowed to cool down slowly without further deformation. Add itionally, lava subjected to complex shearing yield a lower degree of aniso tropy even when high strain rates were imposed on it. These results lead to the conclusion that only the last phase of deformation is detectable using AMS and that high strain rates will not result in high degrees of anisotro py if either deformation ends while lava is still fluid or if the orientati on of the maximum shear stress varies with time. The relation between the o rientation of the principal susceptibilities and that of shear is less sens itive to variation on shear with time. Consequently, Row directions can be inferred confidently with this type of measurements. (C) 2000 Elsevier Scie nce B.V. All rights reserved.