A surface flow of hot, molten lava covered in a carapace of crystalline, angular blocks which tend to be smoothly faceted and may have dimensions up to several metres. The blocks, which have the same composition as the flow interior, are formed by fragmentation of the chilled flow surface as lava continues to move within the flow interior. Blocky lava morphology is usually confined to lavas of high viscosity and intermediate to high silica contents.