Stefan Grab has long been familiar with the power of the lightning bolts that regularly strike the Drakensberg Mountains of southern Africa. Nineteen years ago, the geomorphologist was caught in a storm of such electric ferocity that he vowed never again to camp there in the summer.
But if you had told him at the time that lightning played a major role in shaping those mountains, he observes, "I would have said, 'You must be joking—what nonsense.'"
Not anymore. Grab and a colleague at Johannesburg's University of the Witwatersrand, geologist Jasper Knight, have just given a jolt of their own to conventional notions about the forces that shape mountains. In research published in the January 1 issue of the journal Geomorphology, they present evidence that lightning—rather than ice or heat—is the main force shattering rocks on Drakensberg summits.
Grab and Knight surveyed a quarter square mile amid Drakensberg peaks in Lesotho and found 90 sites where lightning strikes had blasted apart the basalt rock face, scattering up to ten tons of debris a dozen feet or more. The electric impacts leave behind pits up to three feet deep and can shift a boulder the size of a small truck.
The Drakensberg—like nearly all other ranges—were generally thought to have been chiseled by the weathering effects of ice, with smaller contributions from heat and naturally occurring chemicals. Turns out, "that's not the case," says Knight.
Lightning splits rock in much the same way as the better-known cause of weathering: frost shattering. Just as water expands when frozen, it also expands if it's been vaporized by lightning. This expansion occurs within cracks in the rocks, wedging blocks apart. But frost shattering generally occurs over thousands of years. Lightning, at temperatures of up to 54,000°F (almost 30,000°C), can burst rocks in milliseconds. "It basically causes a bomb to explode on the rock surface," explains Knight.
The Signs of Lightning
The fact that a lightning strike will partially melt basalt in an instant allowed Knight and Grab to develop a diagnostic "tool kit" to distinguish the sites of lightning strikes from other rock fragments. A key piece of evidence: The tremendous amount of electricity in lightning leaves a magnetic signature so strong that the needle of a compass passed over a strike site often swings wildly. And compared with other rocks in the area, lightning-blasted fragments are harder, smoother, and more often free of lichens because they are freshly broken and sheared off in an instant.
Powerful electric currents also spread tiny cracks deep within rocks. This sets the stage for water, plants, ice, and heat to weather rocks further. Lightning, says Grab, is "part of the much bigger jigsaw," a puzzle piece that had been passed over by geoscientists for decades. The impact is most obvious up close, but peaks shaped by lightning will likely look more jagged when viewed from afar, Knight notes.
The two researchers believe that further exploration will reveal other examples of mountain ranges shaped in large part by lightning—especially in warmer regions of Australia, Africa, and Asia that were mostly passed over by the Ice Age. Unlike the Rockies and Appalachians, the Drakensberg Mountains were never heavily scoured by glaciers.
No one disputes that the northern ranges were shaped largely by rivers of ice. But where glaciers have now melted and lightning storms are common, such as the southern Rockies, lightning could play an important role in the ongoing weathering of peaks. And in a warming world, that role is likely to grow.