How is shock in a meteorite measured?
Scientists examine minerals under a polarising microscope, looking for features such as fractures, mosaic patterns, glass and high-pressure minerals, and assign a shock stage from S1 to S6.
Most meteorites were knocked off their parent bodies by collisions, and many experienced several impacts before that. Each collision sends a shock wave through the rock, leaving distinctive marks. Scientists measure the intensity of shock by examining these features.
The standard method for ordinary chondrites, published in 1991, uses thin sections viewed under a polarising microscope. Petrologists look especially at olivine and plagioclase grains:
- S1, unshocked: crystals extinguish sharply. - S2, very weakly shocked: olivine shows undulatory extinction. - S3, weakly shocked: olivine shows planar fractures; small shock veins may appear. - S4, moderately shocked: olivine shows weak mosaicism, and plagioclase is partly turned to glass. - S5, strongly shocked: strong mosaicism and plagioclase converted to maskelynite, a glass formed without melting. - S6, very strongly shocked: olivine recrystallises or transforms to high-pressure minerals such as ringwoodite; parts of the rock melt.
These stages correspond approximately to peak pressures, from below about 5 gigapascals for S1 to roughly 75 to 90 gigapascals for S6. Above that, the whole rock melts, forming an impact melt rock.
Shock also produces black melt veins, darkening of the rock and loss of gases. Shock stages help reconstruct collision histories. Many L chondrites are heavily shocked, consistent with a major break-up of their parent body about 470 million years ago. Similar schemes exist for other meteorite types, and shock stage is often given in classification, such as L6 S4.
More about the science of meteorites
- What does a meteorite's weathering grade mean?
- How do scientists tell which parent body a meteorite came from?
- How many different parent bodies do meteorites come from?
- Why do iron meteorites come from so many parent bodies?
- Can meteorites tell us what is inside Earth?
- How do scientists cut and study a meteorite?