WorldWide Drilling Resource

Frequently-Used Geological Terms Part 48 Glossary Adapted from the Dictionary of Geological Terms M Meridian - An imaginary great circle on the surface of the earth which passes through the north and south poles perpendicular to the equator. This line connects all points of equal longitude. Mesa - A tableland; flat-topped mountain; or plateau bounded on at least one side by a steep cliff. Mesh - The openings in a sieve or screen. The value of the mesh is usually given as the number of openings per linear inch. Mesocratic - Composed of almost equal amounts of light and dark constituents; applied to igneous rocks intermediate in color between leucocratic (light-colored) and melanocratic (dark-colored) containing 30-65% of mafic (mainly of dark ferromagnesian) minerals. Mesocrystalline - Refers to the texture of a rock consisting of midsize crystals. Mesonorm - Theoretical calculation of minerals in metamorphic rocks of the mesozone as indicated by chemical analy- ses. Mesosiderite - A stony-iron meteorite in which the silicates are mainly pyroxene and calcic plagioclase. They often appear to be breccias made of fragments of differ- ent chemical and mineralogical compo- sition, in a nickel-iron matrix. Olivine is sometimes present. Mesothermal - A hydrothermal mineral deposit formed at considerable depth and in the temperature range of around 390- 570ºF (200-300ºC); it is also used to de- scribe that environment. Also refers to an organism which prefers moderate temper- atures and climate characterized by those temperatures. Mesozoic - An era of geological time, from the end of the Paleozoic to the beginning of the Cenozoic, or from about 225 to about 65 million years ago. This term is also used to describe rocks formed during that era. This includes the Triassic, Jurassic, and Cretaceous periods. Mesozone - The intermediate depth zone of metamorphism, characterized by tem- peratures of 570-930ºF (300-500º) and moderate hydrostat ic pressure and shearing stress. Image courtesy of astronomy.ohio-state.edu Look for more terms next month! 33 WorldWide Drilling Resource ® MARCH 2016