more calcic core surrounded by progressively more sodic rinds. This zoning reflects the change in magma composition as crystallization progresses. This is described as normal zoning if the rim of the crystal shows a lower-temperature composition than the core of the crystal. Reverse zoning describes the more unusual case where the rim shows a higher-temperature composition than the core. Oscillatory zoning shows period fluctuations between low- and high-temperature compositions. In rapakivi granites, phenocrysts of orthoclase are enveloped within rinds of sodic plagioclase such as oligoclase. In shallow intrusives or volcanic flows phenocrysts which formed before eruption or shallow emplacement are surrounded by a fine-grained to glassy matrix. These volcanic phenocrysts often show flow banding, a parallel arrangement of lath-shaped crystals. These characteristics provide clues to the rocks' origins. Similarly, intragranular microfractures and any intergrowth among crystals provide additional clues. See also Amygdule Notes References The Integrated Ocean Drilling Program (IODP). (2001) Proceedings of the Ocean Drilling Program, Vol. 187 Initial Reports. Petrology Rocks