Magnetic evidence for lightning strikes on mountains: a case study from Lesotho

Abstract:

Recent work in Lesotho, based on the identification and mapping of freshly-fractured, unweathered angular basalt blocks, shows that lightning, and not water-related freeze-thaw cycles, is the dominant contributor to weathering in low-latitude mountains (Knight & Grab 2014, Geomorphology). Lightning strikes generate a distinctive geomagnetic remanent field due to the strong magnetic pulse associated with lightning discharge. This effect can be identified qualitatively with a compass. Here we confirm this interpretation with the use of total magnetic intensity and magnetic susceptibility measurements. The field site is located in the high Drakensberg of eastern Lesotho, southern Africa. The summit studied is ~3350 m asl, comprising a series of Jurassic basalt flows in step-like plateaus separated by steep scarps that demarcate individual flow units. Using a Geometrics Walkmag with built in GPS, we collected detailed total magnetic intensity data on a grid of ~1 m x 1 m in a region ~500 m x 500 m where geomorphologic evidence indicates a high concentration of lightning strikes. In addition we collected over 1500 magnetic susceptibility measurements using a ZH Instruments SM-30 susceptibility meter. The susceptibility data show limited variability (from ~0.001 – 0.02 SI), although the magnetic intensity data show tremendous variation from less than ~20,000 to over ~40,000 nT. The most variable data are closest to the steep scarps, with intensity and variation falling off dramatically in the slope regions. Forward modeling of the susceptibility data demonstrates that the variations in measured susceptibilities cannot be responsible for the magnetic intensity variations. This indicates that strong, lightning-induced remanent magnetization is the cause. Detailed magnetic observations indicate that the most intensely remanently magnetized region is not necessarily where the rock has fractured, indicating that the magnetization process is complicated and likely related to preferential lightning energy transfer along rock microfractures away from the strike location. This study provides conclusive evidence that lightning is a dominant process affecting low-latitude mountains, and that this can be identified using geomagnetic techniques.