Spherulites and lithophysae-200 years of investigation on high-temperature crystallization domains in silica-rich volcanic rocks

High-temperature crystallization domains (HTCDs) including spherulites and lithophysae form during cooling of silica-rich lava and welded ignimbrites. Spherulites grow in silicate melts or hot glass and they display a radiating or microcrystalline texture, typically consisting of cristobalite, tridymite, and sanidine. Lithophysae are HTCDs comprising one or more cavities. This contribution reviews the research and discussions on HTCDs carried out over the last 200 years. The emphasis, here, is on lithophysae and summarizes current knowledge of their formation. A number of parameters influence the initiation and growth of lithophysae, as well as, their shapes and internal textures. The most likely cause of cavity formation is transient tensional stress that produces a mechanical opening and widening at the interface between the crystallization front and the host melt (e.g., where T>Tg). Cavity growth and expansion forced by rising vapor pressure is considered less important. In some cases, further growth of HTCD cavities results from vapor phase corrosion and brecciation.