Uses of anisotropy of magnetic susceptibility in the study of emplacement processes of lava flows
Edgardo Cañón-Tapia, 2005. "Uses of anisotropy of magnetic susceptibility in the study of emplacement processes of lava flows", Kinematics and dynamics of lava flows, Michael Manga, Guido Ventura
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The anisotropy of magnetic susceptibility (AMS) is a powerful technique that can be used to explore in detail the mineral fabric of many types of rocks. In particular, it is well suited to determine mineral fabric of massive, otherwise featureless rocks, like the internal parts of many lava flows and dikes. Although the AMS technique relies on the use of an external magnetic field for its measurement, the methodology and general assumptions behind the AMS technique are more akin to other traditional petrofabric techniques than they are to paleomagnetic works. Furthermore, like most other mineral fabrics, the AMS is mainly acquired at a stage when flow-related deformation promotes a mineral array within the liquid (albeit viscous and probably with a yield strength) lava. The effort required to obtain three-dimensional information of such a mineral array using AMS, however, is less than the effort required by other more traditional methods of fabric analysis. It must be noted that due to differences in the shape of various minerals it is possible to find some differences between magnetic and optically determined mineral fabrics. When attention is given to the systematic variations of the AMS within a lava flow or dike, however, the AMS method allows us to infer aspects of lava (magma) emplacement that are not easy to study through other traditional petrofabric techniques, like mapping the regions of a lava flow that experienced late shearing during emplacement. Detailed knowledge of such deformation partition within a lava flow can be used in turn as a useful criterion to determine whether the flow grew by endogenous or exogenous processes. Also, in some circumstances it may be possible to delineate zones that experienced the same deformation regime (i.e., pure or simple shear) using the information provided by the AMS method alone, or in combination with other petrofabric techniques, therefore allowing us to obtain a detailed record of the internal deformation of one flow unit. For these reasons, the AMS method is an efficient means for collecting a large number of observations, which, in turn, are required to fully characterize lava flow fabric and kinematics.