Dacite is an extrusive igneous rock. Dacite lava is most often light gray, but can be dark gray to black. Dacite lava consists of about 63 to 68 percent silica (SiO2). The principle minerals that make up dacite are plagioclase, quartz, pyroxene, or hornblende. Dacite generally erupts at temperatures between 800 and 1000 degrees C., and is one of the most common rock types associated with enormous Plinian-style eruptions.
Dacia, a Roman Province:
The word dacite comes from Dacia, a Roman province found between the Danube River and Carpathian Mountains, where the rock was first described.
When relatively gas-poor dacite erupts onto a volcano's surface, it typically forms thick rounded lava flow in the shape of a dome.
-- Excerpts from:
Volcano World Website, August 2001, and USGS Volcano Hazards Program Website Photoglossary, August 2001