Lightning as a Geomorphic Agent in Low-Latitude Mountains

It is often assumed that high mountain environments are dominated by the geomorphic imprints of cold-climate weathering and erosion processes, forming angular bedrock fragments that are commonly found across mountain summits. Generation of this loose debris is important because it provides the raw materials for gravity-driven mountain landforms, and contributes to downslope sediment supply. However, low-latitude mountains in particular are also affected by lightning strikes, which are examined here.

Lightning is a climatological phenomenon. It takes place over land in continental interiors where warm (and sometimes moist) air masses are forced to rise up a mountain front, often during afternoons in the hot summer season, leading to atmospheric instability and the formation of thunderclouds (Christian et al., 2003). Thunder, lightning and heavy rain often results. This seasonal climatological pattern is common in many parts of low latitude (15^oN–30^oS) Africa, southern and central Asia, central America, and southeast USA where cloud-to-ground strike rates of up to 150 strikes/(km^2.yr) can be measured. Many ocean and some land areas receive few or no strikes (Figure 1). During thundercloud development a net positive electrostatic charge develops at the top of the cloud and net negative charge at the base. Cloud-to-ground lightning strikes take place as a result of this negative charge traveling from the cloud base to the (positively charged) ground surface. The electrical current of most cloud-to-ground lightning strikes is around 30 kiloampere (kA) and the speed with which this current is discharged means that, in the atmosphere, air molecules are heated and expand very rapidly, causing thunder. When the lightning hits the ground, its short blast of energy can also leave significant surface and subsurface geomorphic and other impacts. These impacts are now explored.

Lightning impacts are most commonly seen on mountain bedrock summits, because mountains are located closest to thunderclouds and minerals within exposed bedrock contain a positive electrostatic charge to which the lightning is attracted. Physical impacts of lightning strikes on exposed bedrock result from the lightning's high temperature which can reach 30,000^oC (54,000^oF). Any moisture on the bedrock surface or within bedrock cracks is immediately superheated and evaporated. Air within the bedrock cracks is also superheated and expands in volume, forcing cracks to widen very rapidly and resulting in the rock exploding apart (Appel et al., 2006; Knight, 2007). The most obvious expression of a lightning impact is therefore 'exploded rock'. This can be identified by the presence of freshly-fractured rock surfaces that may not correspond to bedrock joints, and angular bedrock-derived debris that has been scattered away from a central impact point (Figure 2). Boulders of considerable size can be detached in this manner.

Along the eastern escarpment of the Drakensberg Mountains in southern Africa (around 3200 m elevation, see Figure 3) multiple lightning strikes have fractured through the basalt bedrock to a depth of over 2 m, and displaced boulders of over 4 tonnes in weight. Bedrock fragments have been scattered over a radius of 15-18 m from the central impact point. Where lightning strikes a block field rather than intact bedrock, boulders can be displaced more easily, forming circular pits that can be several metres in diameter and up to 1.5 m depth (e.g. Wilson and Clark, 2001). More subtly, superheating of the rock surface at the point of lightning impact can cause quartz mineral grains to fuse together, forming a tubular aggregate known as fulgarite. The electromagnetic charge of the lightning strike can also cause re-magnetization of any magnetic minerals within the bedrock, giving a locally-remagnetized magnetic field that can vary substantially from the regional background (e.g. Graham, 1961). The greatest re-magnetization takes place at the point of lightning impact, and its influence extends for a few tens of cm away from this point. This can be evaluated in the field simply by moving your compass over this point: the magnetic North arrow will swing erratically, which can be quite disconcerting!

One key issue is how to distinguish lightning strike products from those resulting from cold-climate weathering and erosion processes (such as freeze-thaw and frost shattering) that are considered more typical of high mountain environments worldwide. Frost shattering is a winter process that operates over long time scales and regional spatial scales. Lightning strikes are a summer process that operates over short times scales and very local spatial scales. Although many mountain landscapes cannot be monitored in enough detail and over long enough time periods to definitively identify these two process domains in action, lightning strikes can be distinguished according to these criteria:

· Freshly-fractured rock surfaces caused by a lightning strike will only affect a very small area, maybe around 1 m² or so. Frost shattering will affect a whole mountain summit surface.

· Fractured and detached boulders formed by lightning can be dispersed some distance from source. Frost-shattered boulders will be found right next to their source.

· Lightning strikes will produce lots of fresh rock surfaces of the same age, as can be identified through the study of lichen size (lichenometry), rock surface hardness and other methods (Figure 4). Frost shattering will produce rock surfaces of varying ages.

The precise role of lightning strikes in the formation of low-latitude mountain landscapes is difficult to evaluate. In part this is due to the imprecise nature of lightning monitoring systems; for example, the South African Weather Service's monitoring network comprises only 19 detectors across the entire country (which is about twice the size of the state of Texas) and strike locations have a spatial accuracy of 500 m (Gijben, 2012). Even if strike locations correspond spatially with features observed in the field, a causal relationship is difficult to establish: it is very dangerous to try and observe lightning in action! More widely, lightning may be an important long-term process of mountain weathering and denudation, and in slope sediment supply, especially if the mountain climate is not cold enough for frost shattering to commonly occur, as in the Drakensberg Mountains. This means that lightning as a geomorphic agent may be more important than previously thought, particularly on low-latitude mountains.

• Appel, P.W.U., Abrahamsen, N. and Rasmussen, T.M. 2006. Unusual features caused by lightning impact in West Greenland. Geological Magazine, 143, 737-741.
• Christian, H.J., Blakeslee, R.J., Boccippio, D.J., Boeck, W.L., Buechler, D.E., Driscoll, K.T., Goodman, S.J., Hall, J.M., Koshak, W.J., Mach, D.M. and Stewart, M.F. 2003. Global frequency and distribution of lightning as observed from space by the Optical Transient Detector. Journal of Geophysical Research, 108, 4005, doi:10.1029/2002JD002347.
• Gijben, M. 2012. The lightning climatology of South Africa. South African Journal of Science, 108, Article 740, 10pp, doi:10.4102/sajs.v108i3/4.740.
• Graham, K.W.T. 1961. The re-magnetization of a surface outcrop by lightning currents. Geophysical Journal of the Royal Astronomical Society, 6, 85-102.
• Knight, J. 2007. Impact of a lightning strike on a tor summit, County Waterford, Ireland. Geology Today, 23, 11-12.
• Wilson, P. and Clark, R. 2001. Unusual events: impacts of a possible lightning strike in Burtness Comb, Lake District. Geology Today, 17, 213-214.