The minerals contained in rocks record physicochemical information from the time when the rocks were formed. The thermal history and origin of rocks can be determined by a detailed investigation of their elemental distribution. Using an electron probe microanalyzer (EPMA), here, element mapping of volcanic rock from Mt. Asama in Japan was carried out, and a phase analysis of minerals were conducted. The results of detailed element mapping of orthopyroxene revealed that portions with high concentrations of Al and Ti existed in a lumpy form in the core part of the rock. Thus, it is possible to clarify the physicochemical process of mineral growth and elucidate the origins of rocks by investigating the distributions of their constituent elements.