WorldWide Drilling Resource

Frequently-Used Geological Terms Part 67 Glossary Adapted from the Dictionary of Geological Terms P Plutonic - Pertaining to igneous rocks formed at great depth. Also describes rocks formed by any process at great depth. Pneumatogenic - Used to describe a rock or mineral deposit formed by a gaseous agent. Pneumatolysis - Alteration of a rock or crystallization of minerals by gaseous releases from solidifying magma. Pocket - A small body of ore in a mineralized crevice, a locally rich part of an ore deposit. May also refer to a small body of groundwater. Poikilitic - Said of a structural pattern in igneous rocks in which a crystal of one mineral en- closes smaller unoriented grains of another mineral so a lustrous mottling effect is produced. Alternate spelling is poecilitic. Poikiloblastic - Said of a texture of metamorphic rock in which small grains of one mineral lie within larger metacrysts of another. May also be spelled poeciloblastic. Polar Glacier - A glacier with a below freezing temperature at considerable depth, or through- out and which there is no melting, even in summer. Polarity - The magnetically positive (north) or negative (south) character of a magnetic pole. Polarity Zone - A unit of rock characterized by its polarity signa- ture. The fundamental unit of polarity lithostratigraphic classifi- cation. Polish - An attribute of surface texture of a rock or particle char- acterized by high luster and strong reflected light. Polished Section - A section of rock or mineral which has been highly polished. It is used for the study of opaque minerals by plane or polarized reflective light. Polycrystal - An assemblage of crystal grains of a mineral, of unspecified number, shape, size, orientation, or bonding which forms a solid body. Polygenetic - Consisting of more than one type of material, or having a heterogenous composi- tion, such as a conglomerate composed of materials from several sources. Also used to describe mountain ranges resulting from several orogenic episodes. Look for more terms next month! Plutonic igneous rocks form when magma is trapped deep inside the earth. Molten rock rises toward the surface, and while some of the magma may feed volcanoes on the surface, most remains trapped below, where it cools very slowly over thousands or millions of years until it solidifies. Slow cooling means the individual mineral grains have a very long time to grow. 20 DECEMBER 2017 WorldWide Drilling Resource ®