There are a limited number of ways to determine where a meteorite has landed/crashed. A crater is the obvious sign, unusual minerals is another, while a woman in British Colombia knew from the hole in her ceiling and the hot rock on her pillow. However, lacking those obvious signs, geologists, archeologists and historians are at a loss to find evidence of meteorite strikes … until now. A University of Alaska Fairbanks (UAF) scientist has discovered that rocks in New Mexico hit by meteorites were magnetically changed forever and that lack of natural magnetism can lead hunters to other missing meteorites.
“When you have an impact, it's at a tremendous velocity. And as soon as there is a contact with that velocity, there is a change of the kinetic energy into heat and vapor and plasma. A lot of people understand that there is heat, maybe some melting and evaporation, but people don't think about plasma. We were able to detect in the rocks that a plasma was created during the impact.”
Associate research professor at the UAF Geophysical Institute Gunther Kletetschka led a team to New Mexico to study a known meteor impact site – the Santa Fe impact structure, a 1.4 billion-year-old bolide crater in the Sangre de Cristo Mountains northeast of Santa Fe discovered in 2005. Because of its age, the crater has eroded into a structure made of shatter cones – rare geological features caused only by the massive pressure of meteorite impacts or underground nuclear explosions. While most rocks have a 2% to 3% natural magnetization, Kletetschka found that the human-sized shatter cones contained less than 0.1% magnetism. What changed it?
"We present a support for a newly proposed mechanism where the shock wave appearance can generate magnetic shielding that allow keeping the magnetic grains in a superparamagnetic-like state shortly after the shock's exposure, and leaves the individual magnetized grains in random orientations, significantly lowering the overall magnetic intensity."
In his paper published in the journal Scientific Reports, Kletetschka explains that a meteor impact sends a shockwave through the rocks beneath it – blasting the magnetic grains inside them and altering their magnetism. Under most circumstances, the rocks return to their normal magnetism right away. However, massive impacts like the one in Santa Fe create plasma – a gas containing gas free-floating negative electrons and positive ions – inside the rocks that could be detected by the researchers. The plasma increased the rocks’ electrical conductivity and formed a shield which blocked the natural magnetism from returning. And that, according to the study, is a big deal for meteorite hunters.
“This represents a new mechanism and new indicator that can be used for identifying the substrate rock affected by an impact in the absence of diagnostic indicators, such as crater morphology or shatter cones. The degree of DS we found cannot be achieved by regular igneous/metamorphic rock terrestrial processes within the geomagnetic field.”
DS is rock that is “Demagnetization by Shock,” so the new sign that an area was once impacted by a massive meteorite is rocks that have an unnaturally low level of magnetism -- .01% or less – rather than the normal 2% to 3%.
How do you detect a lack of magnetism in a rock – press it against a refrigerator door? Or send it on a blind date?