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.
Pallasite and mesosiderite meteorites
are from the rind around the core of a differentiated asteroid.
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.
The
Noblesville
Meteorite grazed a boy. A meteorite
hit a woman on the thigh - the story made Life magazine.
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.
