Reaction of alkalis with strained quartz is thought to be one reactive component of aggregates causing this reaction. A wide variety of quartz-bearing rocks have been found to be reactive including graywackes, argillites, quartzwackes, quartzarenites, quartzites, hornfels, quartz biotite, gneiss, granite, phyllite, arkose and sandstone. This type of reaction is characterised by a delayed onset of expansion and cracking may not become evident for up to 20 years after construction. See alkali-aggregate reaction.