At a rock outcropping in southern France, a jagged fracture runs along the granite. The surface in and around the crevice is discolored black, as if wet or covered in algae. But, according to a new paper coauthored by the University of Pennsylvania's Reto Gieré, the real explanation for the rock's unusual features is more dramatic: a powerful bolt of lightning.
Using extremely high-resolution microscopy, Gieré, professor and chair of the Department of Earth and Environmental Science in Penn's School of Arts & Sciences, and his coauthors found that not only had the lightning melted the rock's surface, resulting in a distinctive black "glaze," but had transferred enough pressure to deform a thin layer of quartz crystals beneath the surface, resulting in distinct atomic-level structures called shock lamellae.
Shock lamellae are formed when lightning hits rock, the study found. Here, a single lamella is seen under a transmission electron microscope. Lamellae appear as straight, parallel lines and occur when the crystal structure of a mineral deforms in response to a vast wave of pressure.
Credit: University of Pennsylvania
Prior to this study, the only natural events known to create this type of lamellae were meteorite impacts.
"I think the most exciting thing about this study is just to see what lightning can do," Gieré said. "To see that lightning literally melts the surface of a rock and changes crystal structures, to me, is fascinating."
Gieré said the finding serves as a reminder to geologists not to rush to interpret shock lamellae as indicators of a meteorite strike.
"Most geologists are careful; they don't just use one observation," he said, "But this is a good reminder to always use multiple observations to draw big conclusions, that there are multiple mechanisms that can result in a similar effect."
Gieré collaborated on the study with Wolfhard Wimmenauer and Hiltrud Müller-Sigmund of Albert-Ludwigs-Universität, Richard Wirth of GeoForschungsZentrum Potsdam and Gregory R. Lumpkin and Katherine L. Smith of the Australian Nuclear Science and Technology Organization.
The paper was published in the journal American Mineralogist.
Geologists have long known that lightning, through rapid increases in temperature as well as physical and chemical effects, can alter sediments. When it strikes sand, for example, lightning melts the grains, which fuse and form glass tubes known as fulgurites.
Fulgurites can also form when lightning strikes other materials, including rock and soil. The current study examined a rock fulgurite found near Les Pradals, France. Gieré and colleagues took samples from the rock, then cut thin sections and polished them.
Under an optical microscope, they found that the outer black layer -- the fulgurite itself -- appeared shiny, "almost like a ceramic glaze," Gieré said.
The layer was also porous, almost like a foam, due to the lightning's heat vaporizing the rock's surface. A chemical analysis of the fulgurite layer turned up elevated levels of sulfur dioxide and phosphorous pentoxide, which the researchers believe may have derived from lichen living on the rock's surface at the time of the lightning strike.
The team further studied the samples using a transmission electron microscope, which allows users to examine specimens at the atomic level. This revealed that the fulgurite lacked any crystalline structure, consistent with it representing a melt formed through the high heat from the lightning strike.
But, in a layer of the sample immediately adjacent to the fulgurite, slightly deeper in the rock, the researchers spotted an unusual feature: a set of straight, parallel lines known as shock lamellae. This feature occurs when the crystal structure of quartz or other minerals deform in response to a vast wave of pressure.
"It's like if someone pushes you, you rearrange your body to be comfortable," Gieré said. "The mineral does the same thing."
The lamellae were present in a layer of the rock only about three micrometers wide, indicating that the energy of the lightning bolt's impact dissipated over that distance.
This characteristic deformation of crystals had previously only been seen in minerals from sites where meteorites struck. Shock lamellae are believed to form at pressures up to more than 10 gigapascals, or with 20 million times greater force than a boxer's punch.
Gieré and colleagues hope to study rock fulgurites from other sites to understand the physical and chemical effects of lightning bolts on rocks in greater detail.
Another takeaway for geologists, rock climbers and hikers who spend time on rocks in high, exposed places is to beware when they see the tell-tale shiny black glaze of a rock fulgurite, as it might indicate a site prone to lightning strikes.
"Once it was pointed out to me, I started seeing it again and again," he said. "I've had some close calls with thunderstorms in the field, where I've had to throw down my metal instruments and run."
The study was supported in part by the Australian Nuclear Science and Technology Organization.
Katherine Unger Baillie | EurekAlert!
Receding glaciers in Bolivia leave communities at risk
20.10.2016 | European Geosciences Union
UM researchers study vast carbon residue of ocean life
19.10.2016 | University of Miami Rosenstiel School of Marine & Atmospheric Science
Researchers from the Institute for Quantum Computing (IQC) at the University of Waterloo led the development of a new extensible wiring technique capable of controlling superconducting quantum bits, representing a significant step towards to the realization of a scalable quantum computer.
"The quantum socket is a wiring method that uses three-dimensional wires based on spring-loaded pins to address individual qubits," said Jeremy Béjanin, a PhD...
In a paper in Scientific Reports, a research team at Worcester Polytechnic Institute describes a novel light-activated phenomenon that could become the basis for applications as diverse as microscopic robotic grippers and more efficient solar cells.
A research team at Worcester Polytechnic Institute (WPI) has developed a revolutionary, light-activated semiconductor nanocomposite material that can be used...
By forcefully embedding two silicon atoms in a diamond matrix, Sandia researchers have demonstrated for the first time on a single chip all the components needed to create a quantum bridge to link quantum computers together.
"People have already built small quantum computers," says Sandia researcher Ryan Camacho. "Maybe the first useful one won't be a single giant quantum computer...
COMPAMED has become the leading international marketplace for suppliers of medical manufacturing. The trade fair, which takes place every November and is co-located to MEDICA in Dusseldorf, has been steadily growing over the past years and shows that medical technology remains a rapidly growing market.
In 2016, the joint pavilion by the IVAM Microtechnology Network, the Product Market “High-tech for Medical Devices”, will be located in Hall 8a again and will...
'Ferroelectric' materials can switch between different states of electrical polarization in response to an external electric field. This flexibility means they show promise for many applications, for example in electronic devices and computer memory. Current ferroelectric materials are highly valued for their thermal and chemical stability and rapid electro-mechanical responses, but creating a material that is scalable down to the tiny sizes needed for technologies like silicon-based semiconductors (Si-based CMOS) has proven challenging.
Now, Hiroshi Funakubo and co-workers at the Tokyo Institute of Technology, in collaboration with researchers across Japan, have conducted experiments to...
14.10.2016 | Event News
14.10.2016 | Event News
12.10.2016 | Event News
21.10.2016 | Health and Medicine
21.10.2016 | Information Technology
21.10.2016 | Materials Sciences