What does dacite mean?
Dacite is a fine-grained volcanic rock that occupies the middle-to-upper range of silica compositions among lavas, typically containing about 63 to 69 percent silica. It consists chiefly of plagioclase feldspar together with quartz, with smaller amounts of dark minerals such as hornblende or biotite. Chemically, dacite is the extrusive counterpart of granodiorite, meaning it has the same composition as that coarse-grained intrusive rock but cooled rapidly at the surface, giving it a fine texture. Because its lava is highly viscous, dacite tends not to flow far: instead it builds up into steep, bulbous lava domes and is associated with some of the world's most explosive eruptions, including the post-1980 dome growth at Mount St. Helens. The name derives from Dacia, the ancient Roman province corresponding roughly to modern Romania, where early studies of the rock were made. Dacite is a specialist term encountered in geology courses, field guides, and volcanology literature rather than everyday conversation, and the related adjective 'dacitic' frequently appears in technical descriptions of volcanic deposits.
nounA fine-grained, light-coloured extrusive volcanic rock of intermediate to felsic composition, consisting chiefly of plagioclase feldspar with quartz and smaller amounts of mafic minerals; chemically it is the volcanic equivalent of granodiorite.
- A fine-grained volcanic rock of intermediate-to-felsic composition (roughly 63–69% silica), composed mainly of plagioclase feldspar and quartz, formed from viscous lava typical of explosive stratovolcano eruptions.
"The towering lava dome that grew inside Mount St. Helens after the 1980 eruption was built almost entirely of dacite."
"The geologists identified the flow as dacite by its pale colour and high quartz content under the microscope."
"Because dacite lava is so viscous, it tends to pile up into steep-sided domes rather than spreading out as fluid basalt does."
As a countable noun referring to specimens or occurrences of the rock, the plural 'dacites' is standard in geological writing.
"The thin section collection included several dacites from different eruptive phases."
Dacite got its name not from a scientist's whim but from an ancient map label — it was first described in rocks from Dacia, the old Roman name for roughly modern-day Romania.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of dacite
The word 'dacite' was coined in the 19th century from Dacia, the Latin name for the ancient region corresponding approximately to modern Romania and Moldova, where the rock was first systematically described by geologists working in the Carpathian region. It belongs to a family of rock names derived from geographic localities, much like 'basalt' and 'andesite' (the latter named after the Andes). The adjective 'dacitic' developed directly from it to describe rocks of similar composition.
Related word forms
How dacite is actually used
Dacite is a technical term used mainly in geology, volcanology, and earth-science writing; it rarely appears outside specialist or educational contexts. It carries no emotional connotation. The adjective form 'dacitic' is common in technical descriptions of rock composition.
Easily confused with dacite
Andesite is a closely related volcanic rock that contains less quartz than dacite, sitting at a lower silica content on the compositional scale.
Rhyolite is more silica-rich than dacite and contains proportionally more quartz and alkali feldspar.