New evidence on lightning strikes

LIGHTNING strikes causing rocks to explode have for the first time been shown to play a huge role in shaping mountain landscapes in southern Africa, debunking previous assumptions that angular rock formations were necessarily caused by cold temperatures, and proving that mountains are a lot less stable than we think.

In a world where mountains are crucial to food security and water supply, this has vast implications, especially in the context of climate change.

Professors Jasper Knight and Stefan Grab from the School of Geography, Archaeology and Environmental Studies at Wits University, used a compass to prove – for the first time ever – that lightning is responsible for some of the angular rock formations in the Drakensberg.

“A compass needle always points to magnetic north. 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. In the Drakensberg, here are a lot of basalt rocks which contain a lot of magnetic minerals, so they’ve got a very strong magnetic signal,” said Knight.

If you pass a compass over an area where a lightning strike occurred, the needle will suddenly swing through 360 degrees.

“The energy of the lightning hitting the land’s surface 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,” said Knight.

Because of the movement of continents, magnetic north for the newly formed rock will be different from that of the older rock around it. “You have two superimposed geomagnetic signatures. It’s a very useful indicator for identifying the precise location of where the lightning struck.”

Knight and Grab mapped out the distribution of lightning strikes in the Drakensberg and discovered that lightning significantly controls the evolution of the mountain landscapes, because it helps to shape the summit areas – the highest areas – with this blasting effect.

Previously, angular debris was assumed to have been created by changes typical of cold, periglacial environments, such as fracturing due to frost. Water enters cracks in rocksand when it freezes, it expands, causing the rocks to split apart.

Knight and Grab are challenging centuries old assumptions about what causes mountains to change shape. “Many people have considered mountains to be pretty passive agents, just sitting there to be affected by cold climates over these long periods of time.

“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 environmentsand we need to know more about them.”

It is also useful to try and quantify how much debris is moved by these blasts which can cause boulders weighing several tonnes to move tens of metres.

“We can identify where the angular, broken up material has come from, trace it back tosource, 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, so we can estimate how much volume is moved per square kilometre per year on average,” said Knight.