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Lightning shapes mountain landscapes

- Wits University

Jasper Knight and Stefan Grab are stirring the geomorphology community by overturning ideas that have stood for decades on how mountain landscapes are formed.

Knight and Grab, both Professors in the School of Geography, Archaeology and Environmental Studies, have proven that lightning and not only periglacial climates, may be responsible for forming the angular and jagged nature of mountain landscapes in southern Africa.

Researchers have believed that mountain landscapes worldwide are primarily formed by extreme cold conditions during global glacial periods that caused exposed rocks to fracture.

For many decades it was believed that the same happened to the local mountain landscapes, such as the Drakensberg, as they are relatively high and exposed to extreme cold.

The research has shown that this is not necessarily the case but that these angular fragments of mountain landscapes were caused, at least in part, by lightning strikes.

“When lightning hits the surface of the land from thunder clouds, that lightning is high energy and high temperature. The very high temperatures of the lightning strike on the land surface, which is up to 30 000 degrees, can cause any moisture within fractured surfaces within the exposed rock to evaporate.”

That instantaneous evaporation within cracks in weaknesses in the rock can cause the rock to literally explode, like a bomb,” explains Knight. “That explosion causes the formation of this angular debris.”

Knight and Grab have mapped many lightning strikes in areas of the Drakensberg in eastern Lesotho and have found more than 100 such sites, proving that lightning in the area is common and has the capacity to shape mountain landscapes.

Lightning strikes change the magnetic structure of the rock and ‘re-magnetise’ it as well.

“Very often, if you have an ordinary compass and you pass it over the lightning strike site, the red North arrow of the compass swings erratically. This tells us that there’s been a magnetic disturbance of the rock,” he adds. If the rocks were fractured by ice, there would be no disturbance in the magnetic signature of the rock.

“Our research shows that climate is by no means the only control over mountain landscapes. Many previous ideas of mountain geomorphology are just wrong,” he adds.

These findings have roused the geological community in South Africa and inform a larger debate on how South Africa’s landscape was formed.

“The geomorphological community in South Africa is very conservative. Many viewpoints that have been put forward have been overturned and many decades of previous thinking have been disturbed, which makes it difficult for some to accept,” says Knight.

“This is part of a much bigger discussion whereby geologists are trying to understand the time periods over which the landscape has evolved and the relative interplay between different types of processes, such as tectonic processes, climate processes and weathering processes on the land surfaces and the role of factors such as rivers and wind in redistributing sediments.”

Knight and Grab have together published a book, titled: Landscapes and Landforms of South Africa in 2015. 

They are continuing their research to identify the impact of lightning strikes to get a better understanding of their spatial distribution and impact on other types of landscapes.