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Lightning helps shape mountains, study finds

Oct. 15, 2013
Courtesy of Wits University
and World Science staff

Light­ning strikes cause rocks to ex­plode, a pro­cess that helps shape moun­tain land­scapes in south­ern Af­ri­ca and per­haps else­where, a study has found.

Its au­thors say the find­ings prove moun­tains are a lot less sta­ble than we think, and over­turn pre­vi­ous as­sump­tions that icy con­di­tions cre­at­ed an­gu­lar rock forma­t­ions. In a world where moun­tains are cru­cial to food se­cur­ity and wa­ter sup­ply, this has ma­jor im­plica­t­ions, es­pe­cially in the con­text of glob­al warm­ing, they add.

The find­ings are pub­lished in the ad­vance on­line edi­tion of the re­search jour­nal Ge­o­mor­phol­ogy. Re­search­ers Jas­per Knight and Ste­fan Grab from Wits Uni­vers­ity in South Af­ri­ca said they used a com­pass to prove light­ning caused some of the an­gu­lar rock forma­t­ions in the Drak­ens­burg, South­ern Af­ri­ca’s high­est moun­tain range.

Nor­mal­ly, a com­pass nee­dle points to near the North Pole. “But when you pass a com­pass over a land’s sur­face, if the min­er­als in the rock have a strong enough mag­net­ic field, the com­pass will read the mag­net­ic field of the rock, which cor­re­sponds to when it was formed,” said Knight.

“In the Drak­ens­burg, there are a lot of bas­alt rocks which con­tain a lot of mag­net­ic min­er­als, so they’ve got a very strong mag­net­ic sig­nal,” he added. And if you pass a com­pass over an ar­ea where a light­ning strike oc­curred, the nee­dle will sud­denly swing around.

Image: Prof. Jasper Knight


Light­ning “can, for a short time, par­tially melt the rock and when the rock cools down again, it takes on the mag­net­ic im­print of to­day’s mag­net­ic field, not the mag­net­ic field of mil­lions of years ago when the rock was orig­i­nally formed,” Knight ex­plained. “It’s a very use­ful in­di­ca­tor” to iden­ti­fy ex­actly where light­ning struck.

Knight and Grab mapped out where light­ning struck the Drak­ens­burg and found that light­ning con­trols the ev­o­lu­tion of the moun­tain land­scapes be­cause it helps to shape the sum­mit ar­e­as – the high­est ar­e­as – with a blast­ing ef­fect. Pre­vi­ously, an­gu­lar forma­t­ions were as­sumed to re­sult from changes typ­i­cal of cold en­vi­ron­ments, such as frac­tur­ing due to frost. Wa­ter en­ters cracks in rocks and when it freezes, it ex­pands, split­ting the rocks.

Knight and Grab said they’re chal­leng­ing cen­turies-old as­sump­tions about what causes moun­tains to change shape. Many peo­ple have con­sid­ered moun­tains to just sit there “to be af­fect­ed by cold cli­mates over these long pe­ri­ods of time,” Knight said. 

“This ev­i­dence sug­gests that that is com­pletely wrong. Af­ri­can moun­tain land­scapes some­times evolve very quickly and very dra­mat­ic­ally over short pe­ri­ods of time. These are ac­tu­ally very sen­si­tive en­vi­ron­ments and we need to know more about them.”

It’s al­so use­ful to try to de­ter­mine how much de­bris these blasts move around, which can shift huge boul­ders by tens of me­ters or yards, he added. “We can iden­ti­fy where the an­gu­lar, bro­ken-up ma­te­ri­al has come from, trace it back to source, and de­ter­mine the di­rec­tion and ex­tent to which the de­bris has been blast­ed on ei­ther side. Of course we know from the South Af­ri­can Weath­er Serv­ice how many strikes hit the land’s sur­face,” al­low­ing an es­ti­mate over how much ma­te­ri­al moves, over what ar­ea and how of­ten, he added.

The sta­bil­ity of the land’s sur­face has im­por­tant im­plica­t­ions for the peo­ple liv­ing in the val­leys be­low the moun­tain, he went on. “If we have lots of de­bris be­ing gen­er­at­ed it’s go­ing to flow down slope and this is as­so­ci­at­ed with haz­ards such as land­slides,” said Knight.

Moun­tains are al­so linked to food se­cur­ity and wa­ter sup­ply. In Le­so­tho, a coun­try cru­cial to South Af­ri­ca’s wa­ter sup­ply, food short­ages are lead­ing to overgrazing, ex­pos­ing the rock sur­face and mak­ing moun­tain land­scapes even more vul­ner­a­ble to weath­er­ing by light­ning and oth­er pro­cesses. Knight hopes the new work will help to put in place mon­i­tor­ing and sys­tems to coun­ter­act some of the ef­fects. “The more we in­crease our un­der­stand­ing, the more we are able to do some­thing about it.”


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Lightning strikes cause rocks to explode, a process that helps shape mountain landscapes in southern Africa and perhaps elsewhere, a study has found. Its authors say the findings prove mountains are a lot less stable than we think, and overturn previous assumptions that icy conditions created angular rock formations. In a world where mountains are crucial to food security and water supply, this has major implications, especially in the context of global warming, they add. The findings are published in the advance online edition of the research journal Geomorphology. Researchers Jasper Knight and Stefan Grab from Wits University in South Africa said they used a compass to prove lightning caused some of the angular rock formations in the Drakensburg, the highest mountain rang in Southern Africa. Normally, a compass needle points to near the North Pole. “But when you pass a compass over a land’s surface, if the minerals in the rock have a strong enough magnetic field, the compass will read the magnetic field of the rock, which corresponds to when it was formed,” said Knight. “In the Drakensburg, there are a lot of basalt rocks which contain a lot of magnetic minerals, so they’ve got a very strong magnetic signal,” he added. And if you pass a compass over an area where a lightning strike occurred, the needle will suddenly swing around. Lightning “can, for a short time, partially melt the rock and when the rock cools down again, it takes on the magnetic imprint of today’s magnetic field, not the magnetic field of millions of years ago when the rock was originally formed,” Knight explained. “It’s a very useful indicator” to identify exactly where lightning struck. Knight and Grab mapped out where lightning struck the Drakensburg and found that lightning controls the evolution of the mountain landscapes because it helps to shape the summit areas – the highest areas – with a blasting effect. Previously, angular formations were assumed to result from changes typical of cold environments, such as fracturing due to frost. Water enters cracks in rocks and when it freezes, it expands, splitting the rocks. Knight and Grab said they’re challenging centuries old assumptions about what causes mountains to change shape. Many people have considered mountains to just sit there “to be affected by cold climates over these long periods of time,” Knight said. “This evidence suggests that that is completely wrong. African mountain landscapes sometimes evolve very quickly and very dramatically over short periods of time. These are actually very sensitive environments and we need to know more about them.” It’s also useful to try to determine how much debris these blasts move around, which can shift huge boulders by tens of meters or yards, he added. “We can identify where the angular, broken up material has come from, trace it back to source, and determine the direction and extent to which the debris has been blasted on either side. Of course we know from the South African Weather Service how many strikes hit the land’s surface,” allowing an estimate over how much material moves, over what area and how often, he added. The stability of the land’s surface has important implications for the people living in the valleys below the mountain, he went on. “If we have lots of debris being generated it’s going to flow down slope and this is associated with hazards such as landslides,” said Knight. Mountains are also linked to food security and water supply. In Lesotho, a country crucial to South Africa’s water supply, food shortages are leading to overgrazing, exposing the rock surface and making mountain landscapes even more vulnerable to weathering by lightning and other processes. Knight hopes the new work will help to put in place monitoring and systems to counteract some of the effects. “The more we increase our understanding, the more we are able to do something about it.”