October 15, 2013
How Lightning Strikes Shape Stoney Mountain Tops
[ Watch the Video: Lightning Strikes Can Change Mountains ]
Brett Smith for redOrbit.com - Your Universe Online
During powerful thunderstorms, lightning strikes can hit rocks and cause them to explode. When this happens it causes a disruption in the magnetic field of the rocks and this change can easily be detected with a magnetic compass.
“A compass needle always points to magnetic north,” explained Jasper Knight, from the School of Geography, Archaeology and Environmental Studies at Wits University. “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 Drakensburg, there are a lot of basalt rocks which contain a lot of magnetic minerals, so they’ve got a very strong magnetic signal,” Knight said about the area where the study was conducted.
The study researchers said that passing a compass over an area hit by a lighting strike will cause its magnetic needle to swing around in a complete circle.
“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,” Knight said.
Earth's magnetic field and the location of magnetic north has shifted through geological history, and as a result any newly formed rock will have a different magnetic signature than the older rock around it.
“You have two superimposed geomagnetic signatures,” Knight said. “It’s a very useful indicator for identifying the precise location of where the lightning struck.”
Along with his co-author Stefan Grab, Knight mapped the delivery of lightning strikes in the Drakensburg mountain range of South Africa and found that lightning plays a major role in the evolution of the landscape because it effectively shapes the summit areas.
Popular theories have stated that the landscape of this region of South Africa was shaped by glacier-related events and fracturing due to frost.
“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,” 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."
The South African scientists said their research could be used to quantify how much rock is moved by lightning strikes.
“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,” Knight said. “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 kilometer per year on average.”
The environmental scientist noted that the movement of rock down the mountainsides has a significant effect on the water supply and potential for landslides.
“The more we increase our understanding, the more we are able to do something about it,” he added.