Abstract
Anisotropy of magnetic susceptibility (AMS) analyses have been used widely in many applications that include studying lava flows. In this paper, we introduce an auxiliary parameter, i.e., preferred pore orientation, on the use of AMS for lava flow studies on the basaltic lava samples from Batur Volcano in Bali Indonesia. We also examine the effect of sample position in lava flow outcrop to the relationship between preferred pore orientation and AMS. The samples are subjected to petrographic analyses as well as to magnetic measurements and micro-computed tomography (μCT) imaging. Preferred pore orientations were obtained by quantified the long-axis of the vesicles from the images. The correlation was evaluated by measuring the angle between the maximum susceptibility axes and the preferred pore orientations. All samples show that the maximum susceptibility axes are parallel with the flow direction. Three out of six samples of two lava flows from the same eruption show a positive correlation between AMS and preferred pore orientation, where both parameters point to the northeast direction. A difference of sample position in the outcrop of lava flow was observed as a possible factor that influenced the results for the preferred pore orientations. Samples which were taken from the summit of the lava flow have pore orientation parallel to the lava flow direction. While samples which were taken from the foot slope of the lava flow have pore orientation perpendicular to the lava flow direction. This study provides further evidence that pore orientation might be positively correlated with the AMS.
Highlights
We investigated the correlation between Anisotropy of magnetic susceptibility (AMS) and preferred pore orientation by measuring the angle between the pore orientation with each susceptibility axis
The preferred pore orientations for all three samples point to the northeast direction, nearly parallel with the maximum susceptibility axes (K1)
The remaining sites have the same direction of their AMS with respect to other sites, their preferred pore orientation aligns with the intermediate axes (K2)
Summary
The use of anisotropy of magnetic susceptibility (AMS) analyses in lava flow studies has been found to have unpredictable results (Khan, 1962; Wing-Fatt and Stacey, 1966; Halvorsen, 1974; Symons, 1975; Kolofikova, 1976; Knight and Walker, 1988; Cañón-Tapia et al, 1995; HerreroBervera et al, 2002; Zhu et al, 2003; Panaiotu et al, 2011; Maggart, 2016; Martin et al, 2018; Atarita et al, 2019). The main difference in the conclusions from these studies was how the lava flow directions aligned with the principal magnetic susceptibility axes. Vesicles are formed when there is a drop in pressure during rock formation, which increases gas content inside lava (Lockwood and Hazlett, 2010). According to Waters (1960) as well as Peterson and Hawkins (1971), the inclination of pipe vesicles inside lava flow and long-axis of ellipsoidal shaped vesicles near base and upper part of lava flow can infer the directions of the flow
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