GLOSSARY – P

GLOSSARY – P

by: Charles O’Dale

PDFs
Upon bolide impact, the passage of the shock wave through the rock changes the structure of some of the enclosed minerals. IE: change is possible in the feldspar mineral plagioclase. The shock wave can break down the structure of the mineral, changing parts of it into a diapletic glass (glass formed at high-pressure in the solid-state) which is isotropic, or uniform in all directions.
Decorated PDFs – Planar deformation features (PDFs) decorated by fluid and/or mineral inclusions as the result of annealing.
[see – SHOCK METAMORPHISM– Planar Deformaion Features]

PARAUTOCHTHONOUS
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 – ALLOCHTHONOUS, AUTOCHTHONOUS]
[see – IMPACT CRATER EJECTA]

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 CLASSIFICATION]
[see –  CRATER FORMATION]
[see – CRATER IDENTIFICATION]

PETROGRAPHY
Systematic description of rock texture and composition.

PLANETESIMALS
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 – PDF
Upon bolide impact, the passage of the shock wave through the rock changes the structure of some of the enclosed minerals. IE: change is possible in the feldspar mineral plagioclase. The shock wave can break down the structure of the mineral, changing parts of it into a diapletic glass (glass formed at high-pressure in the solid-state) which is isotropic, or uniform in all directions.
[see – SHOCK METAMORPHISM– Planar Deformaion Features]

PLANAR FEATURES – PF
Cleavage. Fractures in minerals following crystallographic orientations. In quartz crystals, PFs (cleavage) is practically unknown and originates in rare instances from tectonicdeformation in very strong regional metamorphism. On the other hand, PFs in quartz are an indicator of shock metamorphism.

PLATINUM GROUP ELEMENTS (PGE)
Trace element geochemistry of impact melt rocks, especially the abundances and inter-element ratios of PGE is used to unravel the origin of a meteorite crater projectile.
The platinum group elements (PGE) comprise platinum (Pt), palladium (Pd), iridium (Ir), osmium(Os), rhodium (Rh) and ruthenium (Ru).

Three elements form a lighter group including:

  • Ru, ruthenium
  • Rh, rhodium
  • Pd, palladium

Three elements form a heavier group:

  • Os, osmium
  • Ir, iridium
  • Pt, platinum

POLYMICTIC
A clastic rock composed of many rock types, e.g. a graywacke; also, said of the clasts of such a rock.
[see – MONOMICTIC]

POLYMORPH
Two or more minerals that contain the same chemical composition but differ in their atomical arrangement and crystal structure. A well-known example is Diamond and Graphite, which are two different minerals composed of the same exact substance, though they crystallize distinctly.

PRESSURE-TEMPERATURE CONDITIONS for SHOCK METAMORPHISM
[see – SHOCK METAMORPHISM]
[see – SHOCK METAMORPHISM – PRESSURE-TEMPERATURE CONDITIONS]

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.)

Sudbury pseudotachylite dikes range from veins less than 1 mm thick to massive zones measuring up to 1 km thick and extending for approximately 45 km. Formations of SB are found up to 100 km north of the SIC . The pseudotachylite here is injected into the pink gneiss country rock (the toe of my boot is for scale).

[see – SHOCK METAMORPHISM – pseudotachylite]