Meteorite Impacts in the Midwest
Nelson Shaffer (Indiana Geological Survey, Bloomington, Indiana, USA)
2009 Midwest Chapter of the Friends of Mineralogy Symposium and Field Conference (Geology Department of Miami University, Oxford, Ohio, USA)
5 September 2009
See the book Killer Rocks from Outer Space.
Earth-crossing asteroids are traveling at orbital speed (dozens of kilometers per second) - they can be slowed down by atmospheric friction, though. This leads to enormous force/energy/pressure (the consequence of ½mV2).
So, get unusual mineral formation - rocks and minerals change as a result of impacts.
Most meteorites are stony (84% - chondrites + achondrites), not irons (8%).
Iron meteorites cause the most trouble, apparently (Ex: Arizona’s Meteor Crater).
Lafayette Meteorite - an oriented meteorite with flow lines.
Irons are more likely to form craters. Irons are from the cores of differentiated asteroids.
Diogenite meteorites are from the mantle of a differentiated asteroid. Eucrite and howardite meteorites are from the crust of a differentiated asteroid.
There’s a correlation between major impacts and mass extinctions. Ex: K-T, Devonian, & back further.
Earth has many identified impact sites despite plate tectonics and erosion and burial.
Exs: an old impact structure at Kentland, Indiana and an old impact structure at Serpent Mound, Ohio - both are complex craters with central uplifts.
Meteor Crater in Arizona is a simple crater without a central uplift.
See the Baranoski, Schumacher, Watts, Carlson, El-Saiti book on Serpent Mound.
Material gets ejected even slightly before actual contact between the impactor and the substrate due to the presence of a shock wave in front of the impactor.
Much of the impact site gets vaporized. Close to the impact site, get melting. Near to impact site, get a shock metamorphism zone. Far from the impact site, get fracturing & brecciation.
Impactors that retain their cosmic velocity upon impact with Earth are once every few million years.
Most meteorites are dust-sized.
Tektites occur around the largest impacts - melting of country rock + some impactor material sometimes. Most meteorites impact at sea - will generate tsunamis, resulting in “seismite” beds.
Meteorites hitting at terminal velocity make very small craters.
Look for shattercones at a putative impact site, the result of pressure waves going through the rocks. Shattercones often have a skin of blackish material.
Kentland has twisted black shales with large pieces of other rocks mixed in, like a blender.
See Bevan French’s book Traces of Catastrophe.
Meteorites can hit each other in space - can get melt pockets, planar features in olivine, plagioclase melt, etc.
Earth impact rocks - have shocked minerals, disfigured rocks, microspherules, iridium, soot, ash, extraterrestrial fullerenes. Very high pressures involved - >10 GPa. Get planar deformation features (PDFs), shattercones (formed at 2-30 GPa), diaplectic glass (incipient melting), and melting.
Diamond formation pressures are at >35 GPa. Diamonds occur in meteorites themselves from impacts in space. Also get diamonds forming on Earth from impacts (microdiamonds).
Kentland, Indiana [info. & map - scroll down] - a unique site for being an impact structure that is being quarried/mined. Has an observation deck.
Kentland Dome has limestone quarrying for aggregate & agrilime.
Kentland has a central uplift, has been planed off by several glaciers, and has injection melts (polymict) there.
Serpent Mound is highly dissected - can see it on Google Earth. It’s a complex crater with a central uplift. There’s an effigy mound on the SW flank. There are no fragments of the meteorite left here due to its age.
Serpent Mound has a “mineral district” - sphalerite and other minerals.
Reidel did an early, detailed map of Serpent Mound.