Abstract: Spherulitic structures are classically defined as concentrically zoned crystal clusters that form in igneoussystems (especially in alkaline igneous rocks), and usually occur as different mineral layers ,or shells.The volcano-sedimentary series observed in the Kırka region, south of Eskişehir, within the Kırka-Afyon-IspartaVolcanic Province, Western Anatolia, consists predominantly of Lower Miocene volcanic ,and pyroclastic sequenceswith rhyodacitic-rhyolitic composition, lacustrine limestones, borate-bearing claystones, tuff, stromatoliticlimestones ,and rhyolites ,and basalts. The formation mechanisms of the spherulitic structures observed in themarginal zone of the sub-alkaline rhyolitic lava dome outcropping within the Neogene series west of İdrisyayla in Kırka (Eskişehir) region are described for the first time with this study. The rhyolites are grayish light brown in color,have hypocrystalline hyalopilitic-holohyaline texture ,and have spherical structures defined as spherulites. Theserocks have a completely flowing structure ,and are composed of volcanic glass containing rare euhedral plagioclase,sanidine ,and quartz microcrystals, the spherical structures observed on the surface of the rhyolites are up to 10-15cm in size. The spherulites, which are almost round in shape, appear as concentric rings within the lava flow bands.The flow band texture of the volcanic glass phase is continuous with the inner texture of the spherulitic structures.This study suggests that the spherulitic structures in the İdrisyayla (Kırka-Eskişehir) region formed by thedevitrification mechanism as a result of high temperature ,and rapid cooling. The petro-mineralogical features of thespherulitic rhyolites show that during the early magmatic stage, the oversaturated liquid crystallized in silica-richfluid phases with relatively different densities, ,and radial quartz-feldspar crystal growths from a center caused theformation of the spherulitic structure. It is also suggested that the lithophysae-like structures formed by the crystalgrowths representing the solid phase in axial cavities due to the local folding of flow bands ,and the later radialmineral growths representing the liquid phase also changed the laminated texture in the marginal phase of the rhyolite.