lightning as a geomorphic agent on mountain summits ... · lightning asa geomorphic agent...

Geomorphology xxx (2013) xxx–xxx

GEOMOR-04438; No of Pages 10

Contents lists available at ScienceDirect

Geomorphology

j ourna l homepage: www.e lsev ie r .com/ locate /geomorph

Lightning as a geomorphic agent on mountain summits: Evidence from southern Africa

Jasper Knight ⁎, Stefan W. GrabSchool of Geography, Archaeology and Environmental Studies, University of the Witwatersrand, Private Bag 3, Wits 2050, Johannesburg, South Africa

⁎ Corresponding author. Tel.: +27 117176508; fax: +2E-mail address: [email protected] (J. Knight).

0169-555X/$ – see front matter © 2013 Elsevier B.V. All rhttp://dx.doi.org/10.1016/j.geomorph.2013.07.029

Please cite this article as: Knight, J., Grab, S.Wphology (2013), http://dx.doi.org/10.1016/j.

a b s t r a c t
a r t i c l e i n f o
Article history:Received 24 January 2013Received in revised form 13 May 2013Accepted 25 July 2013Available online xxxx

Keywords:Lightning strikesDebris productionPeriglacial processesMountainsFrost shatteringLesotho

The presence of angular bedrock-derived debris on mountain summits worldwide has usually been associatedwith present or past periglacial frost shattering, thermal fracturing and other climatically-mediated weatheringprocesses. Climatic inferences are commonly made based on such geomorphological evidence, even if frostshattering and other processes are unlikely under present climatic conditions. This paper questions this assumedgenetic link between present/past climate and production of angular bedrock-derived debris by describing thegeomorphological impacts of lightning strikes on exposed mountain summits. Using examples from the highDrakensberg of eastern Lesotho, southern Africa, the impacts of lightning strikes are described, which includethe generation of angular, fractured bedrock-derived debris. These impacts are identified in the field based onclear and unambiguous criteria that can be used to distinguish between lightning-induced weathering processesand those processes associatedwith ‘more typical’ alpineweathering. This paper argues that lightning strikes arean important geomorphic agent of, in particular, low-latitudemountain summits, and that to make uncritical cli-matic inferences based on the presence of ‘frost shattered debris’ on mountain summits is wholly erroneous.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

The geomorphological evolution and morphological properties ofmountain landscapes worldwide have been most commonly linkedto weathering and erosion processes under glacial, paraglacial andperiglacial climatic regimes (Owens and Slaymaker, 2004; Knight andHarrison, 2009). Although glacierised mountains have the highestrates of sediment yield into outflowing rivers (e.g., Brardononi et al.,2009; Schiefer et al., 2010; Van den Berg and Schlunegger, 2012), sub-aerial weathering and erosion processes typical of cold, non-glacial(periglacial) environments are geomorphologicallymore significant be-cause they operate on larger spatial scales and longer time scales thanglacial processes alone (Pawelec, 2011; Verleysdonk et al., 2011). Assuch, the geomorphology of mountain summits is most commonlyviewed as a product of past and/or present periglacial weathering anderosion (e.g., Nelson et al., 2007; Goodfellow et al., 2009; Ballantyne,2010; Hall and Thorn, 2011). The physical (mechanical), chemical andbiological weathering processes most commonly cited as important inperiglacial environments are, in no particular order, frost shatteringthrough ice crystal growth (gelivation), porewater migration, thermalexpansion, and biochemical dissolution (formation of tafoni) (e.g.,Hoch et al., 1999; Hall and André, 2001; Matsuoka, 2001;Boelhouwers, 2004; Egli et al., 2004; Sumner et al., 2004; Darmodyet al., 2005; Dixon and Thorn, 2005; Hall and Thorn, 2011; Matthewsand Owen, 2011; Hall et al., 2012). The unifying theme of theseweathering processes is that their occurrence and rate of operation

7 117176529.

ights reserved.

., Lightning as a geomorphicgeomorph.2013.07.029

are strongly climatically-mediated (Rea et al., 1996; Boelhouwers,2004; Paasche et al., 2006). The relative importance of each process atany one location, and the interplay between processes, depends onthe absolute values and the diurnal/seasonal ranges of temperature,precipitation and relative humidity. These variables, their interplayand relative importance also change with respect to elevation, rocktype, aspect, soil/snow cover and other antecedent, environmentaland edaphic factors (André, 2003; Egli et al., 2006).

Views of the relationship between climate and development ofmountain summit geomorphology have recently been informed bystudies that have examined independent lines of evidence for the lon-gevity, and thus climatic control, of summit geomorphological features,in particular blockfields. For example, the presence of gibbsite and otherminerals within mountain summit soils and sub-blockfield weatheringprofiles has been used as an indicator of long-term subaerial weatheringunder variable and warm past climatic regimes (e.g., Marquette et al.,2004; Paasche et al., 2006; Munroe et al., 2007; Goodfellow et al.,2009; Strømsøe and Paasche, 2011; Betard, 2012). The preservation ofsuch weathering products on mountain summits has been used asevidence to suggest that these summits were not glaciated duringthe late Quaternary (Ballantyne et al., 1998), or that mountainsummits were preserved beneath cold-based ice (Kleman et al., 1999).Supporting evidence for such partial preservation of mountain summitgeomorphology over one ormore glacial cycles comesmainly from cos-mogenic dating of intact bedrock surfaces (not loose surface boulders).These studies show that adjacent rock surfaces can have markedlydifferent radiometric ages (Stroeven et al., 2002; Goodfellow et al.,2008), and thus that the effects of glacial erosion in shaping the macro-scale geomorphology of mountain summits have high spatial variability

agent on mountain summits: Evidence from southern Africa, Geomor-

http://dx.doi.org/10.1016/j.geomorph.2013.07.029

mailto:[email protected]

Unlabelled image


Unlabelled image

http://www.sciencedirect.com/science/journal/0169555X


2 J. Knight, S.W. Grab / Geomorphology xxx (2013) xxx–xxx

(Kleman andBorgström, 1996; Kleman et al., 1999). Collectively, the ev-idence from weathering minerals and cosmogenic ages shows that thedevelopment and preservation of mountain summit geomorphologydoes not follow a single climate forcing–response relationship, andthus the presence of certain summit geomorphological features cannotbe used uncritically as palaeoclimatic indicators (Fjellanger et al., 2006).

Although the role of climatically-mediated weathering processescontributing to the formation of mountain geomorphology has been re-cently questioned (Hall et al., 2012), the prevailing view is that climateis the primary driving factor for all physical, chemical and bioticweathering processes that affect all exposed land surfaces (Dixon andThorn, 2005; Hall and Thorn, 2011). This relationship is one foundedon decades of observational-based research on different scales and indifferent climatic, altitudinal and geomorphic settings (Hall et al.,2012). Apart from cryospheric (glacial and periglacial) processes them-selves, it is usually assumed that the geomorphological evolution ofmountain summits results from subaerial physical (mechanical) andchemical weathering under a cold-climate regime (Hoch et al., 1999;Hall et al., 2002; Darmody et al., 2005; Nicholson, 2008; Matthewsand Owen, 2011), in particular through the process of frost shattering(Matsuoka, 2001; Matsuoka and Murton, 2008). Evidence for this pre-vailing viewpoint comes mainly from the presence of angular detachedbedrock debris that is found across mountain summits worldwide andwhich forms block fields on plateaus and screes/talus cones and fansmantling steep bedrock slopes (e.g., Ballantyne, 1998; Boelhouwers,2004; Ballantyne, 2010). Generation of this angular surficial debris isimportant because it provides the raw materials that can be moved byglacial, periglacial and slope processes to form moraines, rock glaciers,blockfields, block streams, debris lobes or cones, and contribute todownslope sediment supply and the formation of solifluction lobesand valley-fills (Grab, 1999; Boelhouwers et al., 2002; Slaymaker et al.,2003; Sumner, 2004; Gordon and Ballantyne, 2006). This paradigm ofclimatically-mediated mountain weathering processes is an importanttenet of palaeoclimate reconstruction in mountain environmentsworldwide (e.g., Hall et al., 2002; Nelson et al., 2007).

Whilst the role of cold-climate weathering is certainly of global im-portance in mountain geomorphology, low-latitude mountains in par-ticular are affected by another significant geomorphic agent, namelylightning strikes. The aimof this paper is to examine the role of lightningstrikes in the formation of angular, bedrock-derived mountain summitdebris which, geomorphically, looks very similar to ‘frost-shattered de-bris’. The paper briefly reviews the processes by which lightning occursover mountain blocks and the surface evidence for lightning strikes(Section 2). This provides the context for describing the regional geolog-ical and climatic setting of the study area (Fig. 1) in the high Drakens-berg of eastern Lesotho, southern Africa (Section 3), methods of datacollection and analysis used in this study (Section 4.1), and the criteriaused to distinguish unequivocally between the agencies of lightningand more typical cold-climate mountain weathering processes inthe formation of angular, bedrock-derived mountain summit debris(Section 4.2). The paper then describes field evidence for lightningstrikes (Section 5), and discusses the implications of this evidence forthe geomorphic evolution of mountain summits and the climatic inter-pretation of such apparent ‘frost-shattered angular debris’ (Section 6). Acritical outcome of this study is that lightning strikes have beenneglected as a geomorphic agent in mountains, and that the viewpointthatmountain summit debris is produced dominantly bypast or presentfrost-shattering and other climatically-mediatedprocesses is erroneous.

2. Climatology and effects of lightning strikes on mountains

Over land, cloud-to-ground lightning strikes are most commonwhere warm air masses rise orographically up a mountain front,resulting in atmospheric instability, latent heat release, and thunder-cloud development (Christian et al., 2003; Williams, 2005). Thunder,lightning and heavy rain are therefore commonly triggered over or

Please cite this article as: Knight, J., Grab, S.W., Lightning as a geomorphicphology (2013), http://dx.doi.org/10.1016/j.geomorph.2013.07.029

around mountain blocks, particularly during summer months. Cloud-to-ground strike rates in the order of b150 strikes km−2 yr−1 arerecorded across many low latitude (15°N–30°S) continental areas ofAfrica, southern and central Asia, central America and southeast USA(Christian et al., 2003; Collier et al., 2006). As storm clouds develop, apositive electrostatic charge of water molecules progressively accumu-lates at the top of the cloud, with a negative charge at the base ofthe cloud. Cloud-to-ground lightning takes place as the negatively-charged lower cloud is discharged against the positively-chargedground surface. Fig. 2 shows an example of a lightning strike impactingon the ground surface in eastern Lesotho. Uniquely, this photo capturesa bright blast generated directly by the lightning strike at themoment ofimpact. The very short time duration of lightning strikesmeans that thisbright blast, coincidingwith the lightning flashmaking contact with theground surface, cannot be a post-event fire. The bright blast is thereforeinterpreted as an explosive event taking place on the ground surface atthe very moment and location of strike impact.

Generally, lightning strike frequency increases with increased landsurface elevation (i.e., mountain height) but declines with elevationabove around 1500–1800 m (Bhavika, 2007), and shows strong season-al and diurnal patterns related to the timing of themost intense convec-tive storms (Rivas Soriano et al., 2005; Collier et al., 2006; Santos et al.,2012). The electrical current produced by most cloud-to-ground light-ning strikes is highly variable, from 10 kA to 300 kA (Verrier andRochette, 2002;Wakasa et al., 2012) with an instantaneous ground sur-face heating of up to 30,000 °C over a time period of ~b1 ms (Grapesand Müller-Sigmund, 2010). Such conditions can cause instantaneousheating and expansion of air and moisture on and within the groundsurface, and can yield a range of physical impacts. The most commonphysical impacts of lightning strikes on exposed rock surfaces include:

• Incineration of organic materials on the rock surface (Appel et al.,2006);

• Formation of fulgurite (Pasek et al., 2012) through very rapid selectivemelting and fusion of pre-existing minerals within host rocks, or for-mation of new minerals (Rietmeijer et al., 1999; Grapes and Müller-Sigmund, 2010). Fulgurite can also form within loose sediments orthin soils above the rock surface (Navarro-González et al., 2007;Longinelli et al., 2012);

• Formation of localised geomagnetic anomalies developed within therocks' minerals (Cox, 1961; Graham, 1961; Beard et al., 2009). Thisarises from the selective melting and subsequent cooling of pre-existing minerals within the host rock (see previous bullet point),with an induced contemporary remanent magnetic field beingsuperimposed upon the regional geomagnetic background (Sakaiet al., 1998; Verrier and Rochette, 2002; Beard et al., 2009). On amesoscale, the induced field can be readily identified using a magne-tometer, and can extend spatially over b20 m2 (S.Webb, pers. comm.,2012). On a microscale, the induced field can be identified using acompass. When the compass is slowly moved over the bedrock sur-face, the induced field will reorientate the compass needle awayfrom the regional background field. The degree of reorientation re-flects the strength of the induced field, which is highest at the positionof the lightning strike. In extreme cases, the compass needle spinsquickly through 360° and over a distance of a few cm around the po-sition of the lightning strike;

• ‘Explosive blasting’ of intact rock surfaces causedmainly by very rapidheat-expansion of air and/or moisture on the rock surface, within therock matrix, or within cracks or fractures (Barnett, 1908; Knight,2007; Wakasa et al., 2012; see Fig. 2). Pre-existing cracks or fracturescan be widened or new cracks developed. ‘Explosive blasting’ of rocksis the primary mechanism by which angular bedrock-derived debriscan form (Knight, 2007);

• Formation of pits or enclosed depressions within a boulder orblockfield, in which weathered, lichen-covered boulders have beenmoved some metres of distance away from the pit centre, revealing



Fig. 1. Location of the study area discussed in this paper. (A) Location of eastern Lesotho, southern Africa. (B) Location of the study area showing the position of the eight sites described inthis study (numbered dark circles) and other lightning strike sites identified in the study area though field walking.

3J. Knight, S.W. Grab / Geomorphology xxx (2013) xxx–xxx

Please cite this article as: Knight, J., Grab, S.W., Lightning as a geomorphic agent on mountain summits: Evidence from southern Africa, Geomor-phology (2013), http://dx.doi.org/10.1016/j.geomorph.2013.07.029

image of Fig.�1


Fig. 2. Photo of themoment of a lightning strike's impact on the ground surface in eastern Lesotho (area highlighted in black box is shown in inset in the bottom left). The bright blast at theend of the lightning flash is interpreted as an explosive event taking place on the ground surface at the very moment and location of strike impact.Photo by Brian Mallinson.


less weathered, lichen-free boulders beneath (Wilson and Clark,2001).

3. Study area

The study area is located in the highDrakensberg of eastern Lesotho,southern Africa (Fig. 1). This region (summits generally 3200–3400 masl) is underlain by Jurassic basalts (Mitchell et al., 1996), andthe flat-lying nature of the basalt flows has meant that the summitscomprise a step-like series of plateau segments separated by contour-parallel vertical cliffs (2–25 m high) that are generally laterally exten-sive around mountain summits (Grab et al., 2005). The study area(around 29°30′50″S, 29°13′00″E) is located 10.1 km NW of Sani Passand encompasses several high summits (maximum elevation of3395 masl) and has a maximum (valley bottom to summit) relief of~530 m (Fig. 1). Bedrock is exposed over most slope segments, particu-lar near scarp faces,where present soil is only a few cm thick andmainlycomprises weathered mineral grains. Previous investigations havelargely attributed mountain top weathering in this region to frostshattering (e.g., Boelhouwers et al., 2002; Sumner, 2004; Sumneret al., 2009) or thermal stress (thermoclastis) Grab, 2007). A thicker so-lifluction mantle (b16 m thick) covers lower slope segments adjacentto valley bottoms (Grab and Mills, 2011). Vegetation comprises alpinetussock grasses (predominantly Merxmuellera) and interspersed withlow shrubs (predominantly Helichrysum).

The high Drakensberg region has strongly seasonal precipitation,with ~80% falling betweenOctober and April and total annual precipita-tion of b1600 mm (Sene et al., 1998). Approximately eight snowfallsoccur per year and snow may last for several months on shadedsouth-facing slopes (Mulder and Grab, 2009). Summers are cool (aver-age temperature 11 °C) and winters cold (average temperature 0 °C) at~3000 masl, whilst the highest summits N3400 masl have an estimatedmean annual air temperature of 4 °C (Grab, 1999). At this elevation, ap-proximately 200–225 annual frost days occur (Schulze, 1997). Groundfreezing to N50 cm depth and over a 3-month period has been locallyrecorded (Grab, 2004). As such the highDrakensberg has been classified


as marginally periglacial, and a range of active and inactive periglaciallandforms has been identified including earth hummocks (thufur),sorted patterned ground and stone- and turf-banked lobes (see Grabet al., 2012 for a review).

4. Methodology

4.1. Distinguishing between different processes of angular debris production

An important issue is how to distinguish unequivocally betweenangular debris produced by frost shattering and other cold-climateweathering processes (e.g., dilatation, thermoclastis, and wetting/drying cycles), and geomorphically-similar debris produced by light-ning strikes. This issue can be resolved from several independent setsof field data. Frost shattering and related mechanical weathering pro-cesses are predominantly cold-season phenomena that operate overlong time scales and regional spatial scales. Lightning strikes are pre-dominantly a summer phenomenon that operates over short timescales and very local spatial scales. If frost shattering (and similarclimatically-driven) processes are dominant, then entire mountainsummits should have similar geomorphological properties (size andshape of detached clasts) and severity of surface weathering, becausethe majority of surfaces will be of similar age. If lightning is an impor-tant agent, fresh lightning-struck surfaces and angular debris will beless highly weathered than surrounding surfaces and debris. Outsideof the strike zone, different lightning-struck surfaces will also be of dif-ferent ages but will be considerably younger than adjacent non-strucksurfaces. Although many mountain landscapes cannot be monitored inenough detail and over long enough time periods to unequivocallyidentify these two process domains in action, this study identifies light-ning strikes in the field, and distinguishes them from ‘frost shattering’and other typically climatically-mediated mountain weathering pro-cesses, according to the following criteria:

• The presence of freshly-fractured, unweathered and lichen-free bed-rock surfaces that cover a relatively small area (around 1–4 m2).


image of Fig.�2


Scarp cliff face

Basalt plateau top

Type 1 Type2

Debris scatter, including upslope

Debris cone extending downslope

Fig. 3. Schematic diagram of a basalt scarp face and plateau surface showing the two typesof lightning strike impact depressions identified in this paper. Type 1 depressions result ina scatter of angular debris around the depression, including upslope. Type 2 depressionsare associated with formation of a debris cone of angular debris.

Table 1Geometric properties of type 1 and type 2 depressions.

Depressiontypes

Site # Estimated depressiondimensions (cm)

Estimated volume ofdisplaced material (m3)

Type 1 2 80 × 50 × 20 0.085 120 × 100 × 40 0.486 40 × 25 × 20 0.02

Type 2 1 240 × 160 × 100 3.843 240 × 220 × 160 8.454 100 × 80 × 50 0.407 160 × 150 × 150 3.608 70 × 70 × 40 0.20


These fresh surfaces contrast with older, more highly weathered sur-faces outside of this zone;

• The intact bedrock shows evidence of recent fracturing. The fracturesmay be intersecting, conjugate to zigzag in shape, and may not followpre-existing joints or fracture patterns;

• The presence of angular, bedrock-derived debris that is ‘fresh’ in ap-pearance, relatively ‘unweathered’, lichen-free, and which has beendetached and dispersed some distance (from tens of cm to severalm) from source. This dispersal direction may be upslope;

• Evidence for induced remagnetisation of bedrock andwith amagneticpeak that lies adjacent to the area with the greatest recent fracturing.If there is no remagnetisation then the bedrock has not been affectedby lightning.

4.2. Field data collection

Geomorphological features of the study areawere investigatedusingstandard field observation and mapping methods. Lightning strike siteswere identified across the study based on the criteria described above.Spatial data were positioned using a handheld GPS. A sample of eightlightning strike sites was investigated in detail (shown in Fig. 1B). Geo-morphological, geomagnetic and surface hardness measurements weremade at each of these sites. Geomorphological and sedimentary fea-tures, including strike impact size, nature of detached clasts, fracturepatterns and distribution, were measured using a compass and tape.Geomagnetic changes as a consequence of lightning-induced remanentmagnetisation were observed qualitatively using a compass, as de-scribed below. Rock surface hardness inside and outside of the strikeimpact-affected areas was measured using a digital Proceq Equotip 3,which is a more precise instrument than a Schmidt hammer (Aokiand Matsukura, 2007; Viles et al., 2011), and results were evaluatedstatistically.

5. Results

Locations that show evidence of lightning impact, based on thecriteria outlined in Section 4.1, were identified using systematic fieldwalking across summits in the region shown in Fig. 1B. Over 80 siteswere identified. The eight sites described in detail here are representa-tive of the general distribution and properties of all strike sites identi-fied. The eight sites are all located either on the edge or on the verticalcliff faces of scarps representing individual basalt lava flows, or a fewmetres back from the scarp edge (site 6).

5.1. Geomorphological evidence

Formation of angular bedrock-derived fragments as a result of light-ning strikes creates an excavated depression in the intact bedrocksurface that may be one of two geomorphic forms, depending onwhether the strike is located on a scarp plateau top or scarp face(Fig. 3). Type 1 depressions are located on scarp plateau tops and aregenerally relatively shallow and broad, either as a rectangular-shapeddepression with a flat floor and relatively well defined sides (asshown in Fig. 3), or a shallow dish-shaped depression in which the ex-cavated layer thins to a more poorly-demarcated margin. Type 2 de-pressions are located on the steep scarp cliff face. They may eitherintersect the cliff edge (as shown in Fig. 3) or be located part waydown the cliff face. Properties of these two types are described inTable 1. Some strike sites may also show several depressions of thesame apparent age (based on the extent of surface weathering) thatare located up to 3 m apart. This may suggest that the lightning strikeconsisted of multiple forks. There is considerable variability in thetotal volume of material that has been displaced from the depressions,but type 2 depressions are usually larger than those of type 1. These de-pression types are also associated with distinctive distribution patternsof detached angular debris that is derived from these depressions. Type


1 depressions generally result in a relatively uniform distribution of de-bris, including in an upslope direction. Type 2 depressions result in de-bris being scattered mainly downslope, commonly forming a debriscone where the material is laterally constrained, or a high-density litter(Fig. 3).

Examples are shown in Fig. 4 of the different types of lightning-induced geomorphological impacts observed across the eight sites. Atsite 1, a pre-existing vertically-aligned weakness has been fractureddown to 2.2 m depth and has laterally displaced and rotated bouldersof over 2.5 tonnes in weight (Fig. 4A). Intersecting and zigzag-shapedfractures which do not exploit pre-existing weaknesses are developedin the intact bedrock. Fracture width decreases with depth. Smaller de-bris (individually b1.03 m3) has been scattered up to 18.50 m distancefrom source. Some of this debris shows curved fracture surfaces but alldebris is fresh in surface appearance, very angular, and based on mor-phology the transport and fracturing history can in many instances beidentified. The size, shape and angularity of debris depend on extentof geologic control (presence of pre-existing weaknesses and bedrocktexture) and the size, and thus magnitude, of the strike. At site 2, pre-existing parallel-aligned bedrock weaknesses have controlled theblocky morphology of resultant debris (Fig. 4B). A conjugate fracturepattern results from energy transfer along and across these weaknesses(Fig. 4C). Fracture patterns at site 3 reflect both geologic control(Fig. 4D) and granular disintegration by subaerial weathering, inwhich energy transfer exploits not mesoscale weaknesses associatedwith basalt cooling, but rather inter-crystal weaknesses caused byweathering (Fig. 4E). Thus, a combination of processes may result inbedrockweaknesses at different spatial scales that can later be exploitedby high-energy lightning. Moreover, the effects of normal subaerialweathering and lightning-induced fracture can be distinguished alongthese planes of weakness. For example, at site 8, subaerial weatheringalong a pre-existing bedrock crack results in a rough-textured surfacethat has developed over a long time period. Exploitation by lightning


image of Fig.�3


Fig. 4. Field photos of geomorphological impacts of lightning strikes in the study area (see Fig. 1B for site locations). Rock hammer is 36 cm in length in all photos. (A) View of verticallyaligned fracture (f) and large detached block (d) that has been rotated in a clockwise direction, site 1. (B) Fresh surface fracture patterns controlled by bedrock structural weakness, site 2.(C) Conjugate and intersecting fracture patterns controlled by the interplay between geologic structure and lightning energy intensity, site 2. (D) Parallel sets of vertically-aligned fracturesthat reflect mesoscale bedrock weaknesses, site 3. (E) Microscale patterns of rock fracture that reflect the interplay between subaerial weathering and inter-crystal weaknesses, site 3.(F) View of a pre-existing bedrock weakness that has been exploited by subaerial weathering (sw) and lightning (l), site 8. Note that these origins can be distinguished based on surfaceroughness properties. (G) Debris cone of angular, interlocking debris, site 3. (H) Part of debris cone of angular, interlocking debris, site 7.


Please cite this article as: Knight, J., Grab, S.W., Lightning as a geomorphic agent on mountain summits: Evidence from southern Africa, Geomor-phology (2013), http://dx.doi.org/10.1016/j.geomorph.2013.07.029

image of Fig.�4


Table 2Equotip hardness values (relative scale) at lightning strike sites 1–8.

Site # Samplelocation

Mean Standarddeviation

Max–min(range)

Number ofobservations

1 Inside 159.95 52.06 277–87 (190) 37Outside 125.60 45.92 261–82 (179) 50

2 Inside 174.66 70.92 346–82 (264) 50Outside 145.50 53.73 296–81 (215) 50

3 Inside 178.46 51.32 335–88 (247) 52Outside 163.70 69.29 376–83 (293) 52

4 Inside 184.78 83.12 405–53 (352) 53Outside 157.40 86.68 414–53 (361) 53

5 Inside 122.06 28.75 207–82 (125) 52Outside 151.40 50.34 293–82 (211) 48

6 Inside 160.04 65.22 330–53 (277) 53Outside 145.40 67.66 399–81 (318) 48

7 Inside 172.57 63.30 331–55 (276) 55Outside 129.62 45.16 247–82 (165) 47

8 Inside 423.04 111.86 568–97 (471) 50Outside 206.50 104.60 413–82 (331) 51


of such a pre-existing weakness has resulted in the remainder of the at-tached blockbeing sheared off, with the lightning-affected surface beingsmooth and uniform in texture (Fig. 4F). This evidence at site 8 showshow different genetic origins for the creation and exploitation of rockweaknesses can be identified.

The large amount of angular debris produced at the largest strikesites (sites 3, 7; Table 1) forms a blocky debris cone that can extend co-herently for up to 8 mdownslope and attain awidth of 6 m. Angular de-bris is interlocking and has similar appearance and morphometricproperties throughout its extent (Fig. 4G, H). Material that makes up adebris cone can also be affected by lightning strikes, with the loose de-bris capable of being displaced more easily by subsequent strikes.Such lightning pits are generally circular, can be severalmetres in diam-eter and up to 1.5 m deep (e.g., Wilson and Clark, 2001), and can beidentified on the basis of their morphology as well as the surface ap-pearance of the material contained within the centre of the pit. Light-ning pits are therefore formed by the displacement of loose debris,and are not excavated in bedrock. The lightning strike causes thehighly-weathered surface debris to be displaced, revealing less highlyweathered debris below. Especially where debris is coveredwith lichen,this contrast in surface appearance is very pronounced, and thus thelightning pit can be readily identified. Several lightning pits of a similarsize and shape to those described from the literature are identified ineastern Lesotho (Fig. 5).

5.2. Surface hardness evidence

A Proceq Equotip was used to systematically measure rock surfacehardness inside and outside of the lightning-affected rock surfaces. Re-sults are shown in Table 2 and Fig. 6. There are several important ele-ments to note. (1) In almost all cases (other than site 5) rock surfacehardness inside of the lightning-affected area is greater than that out-side of this area. (2) There is no consistent pattern with respect to stan-dard deviation and range. (3) Site 8 has significantly higher rock surfacehardness values both inside and outside of the lightning-affected area.This may be because this site is located on an adjacent summit to theother seven sites, and thus may be positioned in a basalt flow bandwith significantly different mineral compositions (McClintock et al.,2008).

Excluding site 8, it is notable that rock surface hardness values out-side of the lighting-affected areas are very similar. This suggests thatsubaerial weathering during the late Holocene has affected the entireregion equally, yielding rock surfaces with very similar features, agesand surface hardness values. By contrast, surface hardness valueswithin

Fig. 5. Photo of a lightning pit (centre of photo) developed in loose debris, in easternLesotho. Note that debris has been scattered from the pit centre, revealing less weathereddebris below.


the lightning-affected areas are different from each other, suggestingthat the lightning strikes are of different ages.

5.3. Evidence for induced remagnetisation of bedrock

A compass was used to qualitatively evaluate the strength of the in-duced field around the lightning strike sites, based on the extent towhich themagnetic arrow swings when drawn across the site. (Quanti-tative geomagnetic data using a magnetometer are not presented here,because this is not the focus of this study.) The highest induced field oc-curs within a few cm of the position of the lightning strike (Verrier andRochette, 2002). Qualitatively, there is considerable spatial variability inthe induced field around the strike sites. It is notable that the precise po-sition of the greatest induced signal does not correspond with thegreatest geomorphological lightning impact. Based on these qualitativeobservations across the study sites, it is most commonly the case thatthe strike location is a flat bedrock surface located 10–30 cm awayfrom the primary lightning-induced fracture. The inducedfield intensityvaries vertically and laterally across the fracture plane and does not de-cline uniformly with distance away from the strike location. This maysuggest that in the area of the highest induced field intensity, the directlightning strike acted as a ‘chisel’ to wedge apart the rock, yielding azone of lower field intensity immediately adjacent to it. Suchmicroscalevariations in induced field are unlikely to be recorded by standard geo-magnetic surveys. When comparing the intensity of the induced fieldfrom strike sites of different apparent ages according to their degree ofsurface weathering, the induced field appears to decline in amplitudeover time. This may suggest that older strike impacts are more difficultto identify than younger ones when based on the induced geomagneticfield alone. In addition, it appears that the extent of remagnetisationvaries between strikes, and does not clearly correspond with the sizeof the strike (i.e., some large strikes appear to have little remagnetisedsignature, and vice versa). The reason for this is unclear, but it may bedue to mineralogical variations within and between different basaltflow bands (McClintock et al., 2008).

6. Discussion

The presence of angular bedrock-derived debris on mountain sum-mits has been used uncritically as a criterion for identifying the role ofpresent or past cold-climate weathering processes, principally frost-shattering (cf., Hall et al., 2002). Several studies have described theboulder deposits found on mountain summits and flanks in easternLesotho (e.g., Boelhouwers et al., 2002; Sumner, 2004), and the role ofclimatically-mediated weathering processes in their formation (Grab,1999; Sumner and Nel, 2006; Grab, 2007; Grab et al., 2009; Sumneret al., 2009). These studies, however, have been concerned exclusively


image of Fig.�5


0

50

100

150

200

250

300

350

400

450

500

550

600

1 in

side

1 ou

tsid

e

Rel

ativ

e re

boun

d sc

ale

Site: 1 2 3 4 5 6 7 8

5 in

side

5 ou

tsid

e

4 in

side

4 ou

tsid

e

3 in

side

3 ou

tsid

e

2 in

side

2 ou

tsid

e

8 in

side

8 ou

tsid

e

7 in

side

7 ou

tsid

e

6 in

side

6 ou

tsid

e

Fig. 6.Whisker plot of numerical results obtained from the Equotip inside and outside of the lightning-affected rock surfaces at sites 1–8, showing the highest and lowest values recorded(crosses), mean value (open circle) and one standard deviation of these values (whisker).


with attempts to prove or disprove the role of cold-climate processesin landscape evolution. Moreover, the arguments developed in thesestudies have often been based on themorphological similarity betweensurface debris observed in Lesotho and those of mountain summitblockfields elsewhere in the world where Quaternary climate changeswere more profound and geomorphologically more significant, and inwhich evidence for periglacial frost-shattering is unambiguous (e.g.,Nelson et al., 2007; Ballantyne, 2010). Evidence presented in this papershows that lightning strikes can yield morphological forms that arevery similar to those produced by cold-climate mountain weatheringprocesses. This means that to make climatic and environmental infer-ences based on surfacemorphology alone (e.g., Sumner, 2004) is entirelyfalse. Such circular reasoning over the last decades has paralysed anyprogressive debate on the interpretation of mountain landform evolu-tion in southern Africa.

The three independent sets of field evidence used in this study ascriteria to identify the impacts of lightning strikes (geomorphology,rock surface hardness and induced magnetisation) can clearly distin-guish between those rock surfaces that have been affected by lightningstrikes and those that have not. The spatial pattern of such strike evi-dence (Fig. 1B), and the spatial and temporal variability of lightningstrike rates across the Drakensberg and adjacent areas (Gijben, 2012),show that there are non-uniform rates of scarp retreat and debris pro-duction associated with lightning strikes. Previous studies focusing onmacroscale rock mass strength as a control on scarp retreat patterns(Moon and Selby, 1983; Grab et al., 2005) do not capture any of thisvariability. However, microscale density and geometry of pre-existingfractures or microcracks within the host rock will be significant vari-ables in the production of lightning-related debris, and increased densi-ty of microcracks through lightning-induced subsurface brecciationwillhave a positive feedback effect on subsequent subaerial weathering(Karfunkel et al., 2001).

Discussion of the sediment budgets of mountain catchments, intowhich sediments derived from mountain summits are moved, hasfocused onmacroscale climatic events and cold-climate geomorpholog-ical agents, including glaciation, periglaciation, mass wasting and floods(Stoffel et al., 2005; Brardononi et al., 2009; Schiefer et al., 2010; Beylichet al., 2011; Huggel et al., 2012). Evidence from this study showsthat lightning strikes are a significant point source of debris on low-latitude mountains, where cold-climate events and processes aremuch less likely to take place or, even if they do, operate at slowcontemporary rates. Although the calculated debris volume is highly


variable (Table 1), the relatively high lightning strike frequency onlow-latitude mountains suggests that it is a significant factor in loosedebris generation, slope sediment supply, and mountain sedimentyield. During the late Holocene, inwhich the climate of the Drakensbergand similar low-latitude mountain blocks was too warm to experienceappreciable cold-climate weathering, lightning strikes were probablythe most significant agent of debris generation.

7. Conclusions

This study describes geomorphological, rock surface hardnessand induced magnetisation evidence from mountain summits in theDrakensberg of eastern Lesotho. This evidence shows that lightningstrikes are an important agent of rock surface weathering and debrisgeneration. From this evidence, the following conclusions can be made:

• Angular, fractured bedrock-derived debris, which is similar in appear-ance to the debris produced by more typical mountain weatheringprocesses such as frost shattering, is produced by lightning strikes;

• Lightning-derived debris can be readily distinguished from thatresulting from more commonly documented mountain weatheringprocesses on the basis of their rock surface hardness and inducedmagnetisation signatures;

• Climatic inferences based on the presence of ‘frost-shattered debris’on mountain summits, particularly in the low-latitudes, are whollyerroneous. The presence of such debris cannot be used uncriticallyto calculate palaeotemperatures or as evidence for regional-scaleperiglacial climates.

Important future research directionsmay include (1) confirming therole of lightning strikes on other mountain summits worldwide usingmultidisciplinary diagnostic criteria including geophysical methods(Section 4.1); (2) using cosmogenic and radiocarbon dating methodsto evaluate the time periods over which mountain surfaces evolve;and (3) quantifying the volume and rate of debris production bylightning.

Acknowledgements

We thank Lothar Schrott, an anonymous reviewer and editorTakashi Oguchi for their comments.


image of Fig.�6



References

André, M.-F., 2003. Do periglacial landscapes evolve under periglacial conditions?Geomorphology 52, 149–164.

Aoki, H., Matsukura, Y., 2007. A new technique for non-destructive field measurement ofrock-surface strength: an application of the Equotip hardness tester to weatheringstudies. Earth Surface Processes and Landforms 32, 1759–1769.

Appel, P.W.U., Abrahamsen, N., Rasmussen, T.M., 2006. Unusual features caused by light-ning impact in West Greenland. Geological Magazine 143, 737–741.

Ballantyne, C.K., 1998. Age and significance of mountain-top detritus. Permafrost andPeriglacial Processes 9, 327–345.

Ballantyne, C.K., 2010. A general model of autochthonous blockfield evolution. Permafrostand Periglacial Processes 21, 289–300.

Ballantyne, C.K., McCarroll, D., Nesje, A., Dahl, S.O., Stone, J.O., 1998. The last ice sheet innorth-west Scotland: reconstruction and implications. Quaternary Science Reviews17, 1149–1184.

Barnett, V.H., 1908. An example of disruption of rock by lightning on one of the LuciteHills in Wyoming. Journal of Geology 16, 568–571.

Beard, L.P., Norton, J., Sheehan, J.R., 2009. Lightning-induced remanent magnetic anoma-lies in low-altitude aeromagnetic data. Journal of Environmental and EngineeringGeophysics 14, 155–161.

Betard, F., 2012. Spatial variations of soil weathering processes in a tropical mountain en-vironment: the Baturite massif and its piedmont (Ceara, NE Brazil). Catena 93, 18–28.

Beylich, A.A., Lamoureux, S.F., Decaulne, A., 2011. Developing frameworks for studies onsedimentary fluxes and budgets in changing cold environments. QuaestionesGeographicae 30, 5–18.

Bhavika, B., 2007. The Influence of Terrain Evaluation on Lightning Density in SouthAfrica. (MSc thesis) University of Johannesburg.

Boelhouwers, J., 2004. New perspectives on autochthonous blockfield development. PolarGeography 28, 133–146.

Boelhouwers, J., Holness, S., Meiklejohn, I., Sumner, P., 2002. Observations on ablockstream in the vicinity of Sani Pass, Lesotho Highlands, Southern Africa. Perma-frost and Periglacial Processes 13, 251–257.

Brardononi, F., Hassan, M.A., Rollerson, T., Maynard, D., 2009. Colluvial sediment dynam-ics in mountain drainage basins. Earth and Planetary Science Letters 284, 310–319.

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., Stewart, M.F., 2003. Global frequencyand distribution of lightning as observed from space by the Optical Transient Detector.Journal of Geophysical Research 108, 4005. http://dx.doi.org/10.1029/2002JD002347.

Collier, A.B., Hughes, A.R.W., Lichtenberger, J., Steinbach, P., 2006. Seasonal and diurnalvariation of lightning activity over southern Africa and correlation with Europeanwhistler observations. Annales Geophysicae 24, 529–542.

Cox, A., 1961. Anomalous remanent magnetization of basalt. United States Geological Sur-vey Bulletin 1083-E.US Government Printing Office, Washington DC.

Darmody, R.G., Thorn, C.E., Allen, C.E., 2005. Chemical weathering and boulder mantles,Kärkevagge, Swedish Lapland. Geomorphology 67, 159–170.

Dixon, J.C., Thorn, C.E., 2005. Chemical weathering and landscape development in mid-latitude alpine environments. Geomorphology 67, 127–145.

Egli, M., Mirabella, A., Mancabelli, A., Sartori, G., 2004. Weathering of soils in alpine areasas influenced by climate and parent material. Clays and Clay Minerals 52, 287–303.

Egli, M., Mirabella, A., Sartori, G., Zanelli, R., Bischof, S., 2006. Effect of north and south ex-posure on weathering rates and clay mineral formation in Alpine soils. Catena 67,155–174.

Fjellanger, J., Sørbel, L., Linge, H., Brook, E.J., Raisbeck, G.M., Yiou, F., 2006. Glacial survivalof blockfields on the Varanger Peninsula, northern Norway. Geomorphology 82,255–272.

Gijben, M., 2012. The lightning climatology of South Africa. South African Journal ofScience 108. http://dx.doi.org/10.4102/sajs.v108i3/4.740 (Article 740, 10 pp.).

Goodfellow, B.W., Stroeven, A.P., Hättestrand, C., Kleman, J., Jansson, K.N., 2008.Deciphering a non-glacial/glacial landscape mosaic in the northern Swedish moun-tains. Geomorphology 93, 213–232.

Goodfellow, B.W., Fredin, O., Derron, M.-H., Stroeven, A.P., 2009. Weathering processesand Quaternary origin of an alpine blockfield in Arctic Sweden. Boreas 38, 379–398.

Gordon, L.S., Ballantyne, C.K., 2006. ‘Protalus Ramparts’ on Navajo Mountain, Utah, USA:reinterpretation as blockslope-sourced rock glaciers. Permafrost and Periglacial Pro-cesses 17, 179–187.

Grab, S., 1999. Block and debris deposits in the high Drakensberg, Lesotho, SouthernAfrica: implications for high altitude slope processes. Geografiska Annaler 81A, 1–16.

Grab, S.W., 2004. Thermal regime through a sorted circle and stone-banked lobe, Dra-kensberg, southern Africa. Zeitschrift für Geomorphologie 48, 501–518.

Grab, S., 2007. Rock-surface temperatures of basalt in the Drakensberg alpine environ-ment, Lesotho. Geografiska Annaler 89A, 185–193.

Grab, S.W., Mills, S.C., 2011. Late Quaternary slope environments in the upperSehonghong Valley, eastern Lesotho. Proceedings of the Geologists' Association 122,179–185.

Grab, S., van Zyl, C., Mulder, N., 2005. Controls on basalt terrace formation in the easternLesotho highlands. Geomorphology 67, 473–485.

Grab, S.W., Mulder, N.A., Mills, S.C., 2009. Spatial associations between longest-lastingwinter snow cover and cold region landforms in the high Drakensberg, southernAfrica. Geografiska Annaler 91A, 83–97.

Grab, S.W., Mills, S.C., Carr, S.J., 2012. Periglacial and glacial geomorphology. In: Holmes,P., Meadows, M.E. (Eds.), The Geomorphology of Southern Africa. Sun Press,Bloemfontein, pp. 233–265.

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.


Grapes, R.H., Müller-Sigmund, H., 2010. Lightning-strike fusion of gabbro and formationof magnetite-bearing fulgurite, Cornone di Blumone, Adamello, Western Alps, Italy.Mineralogy and Petrology 99, 67–74.

Hall, K., André, M.-F., 2001. New insights into rock weathering from high-frequency rocktemperature data: an Antarctic study of weathering by thermal stress. Geomorphol-ogy 41, 23–35.

Hall, K., Thorn, C.E., 2011. The historical legacy of spatial scales in freeze–thawweathering:misrepresentation and resulting misdirection. Geomorphology 130, 83–90.

Hall, K., Thorn, C.E., Matsuoka, N., Prick, A., 2002. Weathering in cold regions: somethoughts and perspectives. Progress in Physical Geography 26, 577–603.

Hall, K., Thorn, C.E., Sumner, P., 2012. On the persistence of ‘weathering’. Geomorphology149–150, 1–10.

Hoch, A.R., Reddy, M.M., Drever, J.I., 1999. Importance of mechanical disaggregation inchemical weathering in a cold alpine environment, San Juan Mountains, Colorado.GSA Bulletin 111, 304–314.

Huggel, C., Clague, J.J., Korup, O., 2012. Is climate change responsible for changing land-slide activity in high mountains? Earth Surface Processes and Landforms 37, 77–91.

Karfunkel, J., Addad, J., Banko, A.G., Hadrian, W., Hoover, D.B., 2001. Electromechanicaldisintegration — an important weathering process. Zeitschrift fur Geomorphologie,NF 45, 345–357.

Kleman, J., Borgström, I., 1996. Reconstruction of palaeo-ice sheets: the use of geomor-phological data. Earth Surface Processes and Landforms 21, 893–909.

Kleman, J., Hättestrand, C., Clarhäll, A., 1999. Zooming in on frozen-bed patches: scale-dependent controls on Fennoscandian ice sheet basal thermal zonation. Annals ofGlaciology 28, 189–194.

Knight, J., 2007. Impact of a lightning strike on a tor summit, County Waterford, Ireland.Geology Today 23, 11–12.

Knight, J., Harrison, S. (Eds.), 2009. Periglacial and paraglacial processes and environ-ments. Geological Society Special Publication, 320. Geological Society of London Pub-lishing House, Bath.

Longinelli, A., Serra, R., Sighinolfi, G., Selmo, E., Sgavetti, M., 2012. Oxygen isotopic compo-sition of fulgurites from the Egyptian Sahara and other locations. Rapid Communica-tions in Mass Spectrometry 26, 1980–1984.

Marquette, G.C., Gray, J.T., Gosse, J.C., Courchesne, F., Stockli, L., Macpherson, G., Finkel, R.,2004. Felsenmeer persistence under non-erosive ice in the Torngat and Kaumajetmountains, Quebec and Labrador, as determined by soil weathering and cosmogenicnuclide exposure dating. Canadian Journal of Earth Sciences 41, 19–38.

Matsuoka, N., 2001. Direct observation of frost wedging in alpine bedrock. Earth SurfaceProcesses and Landforms 26, 601–614.

Matsuoka, N., Murton, J., 2008. Frost weathering: recent advances and future directions.Permafrost and Periglacial Processes 19, 195–210.

Matthews, J.A., Owen, G., 2011. Holocene chemical weathering, surface lowering and rockweakening rates on glacially eroded bedrock surfaces in an Alpine periglacial envi-ronment, Jotunheimen, Southern Norway. Permafrost and Periglacial Processes 22,279–290.

McClintock, M., Marsh, J.S., White, J.D.L., 2008. Compositionally diverse magmas eruptedclose together in space and time within a Karoo flood basalt crater complex. Bulletinof Volcanology 70, 923–946.

Mitchell, A.A., Ramluckan, V.R., Dunlevey, J.N., Eglington, B.M., 1996. The basalt stratigra-phy of the Sani Pass, Kwazulu/Natal Drakensberg. South African Journal of Geology99, 251–262.

Moon, B.P., Selby, M.J., 1983. Rock mass strength and scarp forms in southern Africa.Geografiska Annaler 65A, 135–145.

Mulder, N., Grab, S.W., 2009. Contemporary spatio-temporal patterns of snow cover overthe Drakensberg. South African Journal of Science 105, 228–233.

Munroe, J.S., Farrugia, G., Ryan, P.C., 2007. Parent material and chemical weathering in al-pine soils on Mt. Mansfield, Vermont, USA. Catena 70, 39–48.

Navarro-González, R., Mahan, S.A., Singhvi, A.K., Navarro-Aceves, R., Rajot, J.-L., McKay,C.P., Coll, P., Raulin, F., 2007. Paleoecology reconstruction from trapped gases in a ful-gurite from the late Pleistocene of the Libyan Desert. Geology 35, 171–174.

Nelson, K.J.P., Nelson, F.E., Walegur, M.T., 2007. Periglacial Appalachia: palaeoclimatic sig-nificance of blockfield elevation gradients, eastern USA. Permafrost and PeriglacialProcesses 18, 61–73.

Nicholson, D.T., 2008. Rock control on microweathering of bedrock surfaces in aperiglacial environment. Geomorphology 101, 655–665.

Owens, P.N., Slaymaker, O. (Eds.), 2004. Mountain Geomorphology. Arnold, London.Paasche, Ø., Strømsøe, J.R., Dahl, S.O., Linge, H., 2006. Weathering characteristics of arctic

islands in northern Norway. Geomorphology 82, 430–452.Pasek, M.A., Block, K., Pasek, V., 2012. Fulgurite morphology: a classification scheme and

clues to formation. Contributions to Mineralogy and Petrology 164, 477–492.Pawelec, H., 2011. Periglacial evolution of slopes — rock control versus climate factors

(Cracow Upland, S. Poland). Geomorphology 132, 139–152.Rea, B.R., Whalley, W.B., Rainey, M.M., Gordon, J.E., 1996. Blockfields, old or new? Evi-

dence and implications from some plateaus in northern Norway. Geomorphology 5,109–121.

Rietmeijer, F.J.M., Karner, J.M., Nuthi, J.A., Wasilewski, P.J., 1999. Nanoscale phase equilib-rium in a triggered lightning-strike experiment. European Journal of Mineralogy 11,181–186.

Rivas Soriano, L., de Pablo, F., Tomas, C., 2005. Ten-year study of cloud-to-ground light-ning activity in the Iberian Peninsula. Journal of Atmospheric and Solar-TerrestrialPhysics 67, 1632–1639.

Sakai, H., Sunada, S., Sakurano, H., 1998. Study of lightning current by remanent magne-tization. Electrical Engineering in Japan 123, 41–47.

Santos, J.A., Reis, M.A., Sousa, J., Leite, S.M., Correia, S., Janeira, M., Fragoso, M., 2012. Cloud-to-ground lightning in Portugal: patterns and dynamical forcing. Natural Hazards andEarth System Sciences 12, 639–649.


http://refhub.elsevier.com/S0169-555X(13)00392-9/rf0005

































http://dx.doi.org/10.1029/2002JD002347


















http://dx.doi.org/10.4102/sajs.v108i3/4.740

















































































































Schiefer, E., Hassan, M.A., Menounos, B., Pelpola, C.P., Slaymaker, O., 2010. Interdecadalpatterns of total; sediment yield from a montane catchment, southern Coast Moun-tains, British Columbia, Canada. Geomorphology 118, 207–212.

Schulze, R.E., 1997. Climate. In: Cowling, R.M., Richardson, D.M., Pierce, S.M. (Eds.), Vege-tation of Southern Africa. Cambridge University Press, Cambridge, pp. 21–42.

Sene, K.J., Jones, D.A., Meigh, J.R., Farquharson, F.A.K., 1998. Rainfall and flow variations inthe Lesotho Highlands. International Journal of Climatology 18, 329–345.

Slaymaker, O., Souch, C., Menounos, B., Filippelli, G., 2003. Advances in Holocene moun-tain geomorphology inspired by sediment budget methodology. Geomorphology55, 305–316.

Stoffel, M., Lièvre, I., Conus, D., Grichting, M.A., Raetzo, H., Gärtner, H.W., Monbaron, M.,2005. 400 years of debris-flow activity and triggeringweather conditions: Ritigraben,Valais, Switzerland. Arctic, Antarctic, and Alpine Research 37, 387–395.

Stroeven, A.P., Fabel, D., Hättestrand, C., Harbor, J., 2002. A relict landscape in the centre ofFennoscandian glaciation: cosmogenic radionuclide evidence of tors preservedthrough multiple glacial cycles. Geomorphology 44, 145–154.

Strømsøe, J.R., Paasche, Ø., 2011. Weathering patterns in high‐latitude regolith. Journal ofGeophysical Research 116, F03021. http://dx.doi.org/10.1029/2010JF001954.

Sumner, P.D., 2004. Geomorphic and climatic implications of relict openwork block accu-mulations near Thabana–Ntlenyana, Lesotho. Geografiska Annaler 86A, 289–302.

Sumner, P.D., Nel, W., 2006. Surface-climate attributes at Injisuthi Outpost, Drakensberg,and possible ramifications for weathering. Earth Surface Processes and Landforms 31,1445–1451.


Sumner, P.D., Nel, W., Hedding, D.W., 2004. Thermal attributes of rock weathering: zonalor azonal? A comparison of rock temperatures in different environments. Polar Geog-raphy 28, 79–92.

Sumner, P.D., Hall, K.J., van Rooy, J.L., Meiklejohn, K.I., 2009. Rock weathering on the east-ern mountains of southern Africa: review and insights from case studies. Journal ofAfrican Earth Sciences 55, 236–244.

Van den Berg, F., Schlunegger, F., 2012. Alluvial cover dynamics in response to floods ofvarious magnitudes: the effect of the release of glaciogenic material in a Swiss Alpinecatchment. Geomorphology 141–142, 121–133.

Verleysdonk, S., Krautblatter, M., Dikau, R., 2011. Sensitivity and path dependence ofmountain permafrost systems. Geografiska Annaler 93A, 113–135.

Verrier, V., Rochette, P., 2002. Estimating peak currents at ground lightning impacts usingremanent magnetization. Geophysical Research Letters 29 (18), 1867. http://dx.doi.org/10.1029/2002GL015207.

Viles, H., Goudie, A., Gradb, S., Lalley, J., 2011. The use of the Schmidt Hammer and Equotipfor rock hardness assessment in geomorphology and heritage science: a comparativeanalysis. Earth Surface Processes and Landforms 36, 320–333.

Wakasa, S.A., Nishimura, S., Shimizu, H., Matsukura, Y., 2012. Does lightning destroyrocks?: results from a laboratory lightning experiment using an impulse high-current generator. Geomorphology 161 (162), 110–114.

Williams, E.R., 2005. Lightning and climate: a review. Atmospheric Research 76, 272–287.Wilson, P., Clark, R., 2001. Unusual events: impacts of a possible lightning strike in

Burtness Comb, Lake District. Geology Today 17, 213–214.

















http://dx.doi.org/10.1029/2010JF001954

















http://dx.doi.org/10.1029/2002GL015207











lightning as a geomorphic agent on mountain summits ... · lightning asa geomorphic agent...

Documents