IMPACT CRATER/STRUCTURE GLOSSARY
by: Charles O’Dale
[see – PLANAR DEFORMATION FEATURES.]
Ground which has been disturbed by impact, thrust or nappe displacement, but where the displacement is small enough that the rocks are still in contact with their source (moved but appear to be in place).
[see – CRATON, AUTOCHTHONOUS, ALLOCHTHONOUS, PARAUTOCHTHONOUS.]
PEAK RING IMPACT CRATER/STRUCTURE
Peak ring craters develop within the rim of larger complex craters. The ring structure forms as the central peak collapses and creates a peak ring before all motion stops (Melosh 1989).
[see – CRATER CLASSIFICATIONS]
[see- CRATER FORMATION]
Bodies ranging in size from meters up to hundreds of kilometers in diameter that formed during the process that formed the planets by accretion. Most planetesimals accreted to form the planets. A rocky and/or icy body, a few to several tens of kilometers in size, that was produced in the solar nebula.
PLANAR DEFORMATION FEATURES
Upon bolide impact, the passage of the resultant shock wave through the rock changes the structure of some of the enclosed minerals.
Planar deformation features (PDFs) are not cracks in quartz, but are “… multiple sets of closed, extremely narrow, parallel planar regions …” that are typically less than 2-3 μm wide and spaced around 2-10 μm apart (French, 1998: 42).
PRESSURE-TEMPERATURE CONDITIONS for SHOCK METAMORPHISM
[see – SHOCK METAMORPHISM]
PSEUDOTACHYLITE (friction melt)
Pseudotachylite is formed by frictional effects within the crater floor and below the crater during the initial compression phase of the impact and the subsequent formation of the central uplift. It may contain unshocked and shocked mineral and lithic clasts in a fine-grained aphanatic [aphanatic = very fine-grained], crystalline texture matrix. (A tachylite is a black volcanic glass formed by the chilling of basaltic magmas.)
RADIOMETRIC DATING Reports of the National Center for Science Education – Brent Dalrymple,
[see – DATING – RADIOMETRIC]
REIDITE (METAMORPHIC ZIRCON)
Reidite is a rare mineral, a dense form (polymorph) of the fairly tough gemstone zircon, which is produced when the latter is subjected to very high pressures. Reidite has been found only in four crater impacts: the Chesapeake Bay Crater in Virginia, Ries Crater in Germany, Xiuyan Crater in China, and Rock Elm Crater in Wisconsin in the United States (Wiki).
Zircon transforms into reidite when meteorites slam into the ground because shock waves from the impact cause a dramatic increase in temperature and pressure at the site. The high pressures cause the building blocks of the mineral to rearrange, becoming tightly repacked. The resulting mineral is similar in composition to zircon, but around 10% more dense. Reidite can also be formed under high-pressure or shock recovery laboratory experiments. In fact, reidite was only known from lab-made samples for around 30 years before it was first discovered in nature in 2001 (Reidite was finally identified in nature starting in 2001, at three impact sites: the Chesapeake Bay Crater in Virginia, Ries Crater in Germany and Xiuyan Crater in China.).