Dacite

Dacite is a fine-grained extrusive (volcanic) igneous rock with a composition between andesite and rhyolite. It is rich in silica and plagioclase feldspar, contains free quartz, and often holds visible crystals (phenocrysts) of plagioclase, hornblende, biotite, or pyroxene set in a fine groundmass.

It is usually light to medium grey, tan, or pinkish. Because dacitic magma is viscous and gas-rich, it tends to erupt explosively, building lava domes and producing pyroclastic flows.

Dacite is the rock of many famous and dangerous volcanoes, making it important in volcanic hazard studies.

Dacite forms from viscous, silica-rich magma generated mainly at subduction zones, where melting of crust and mantle and the mixing or fractionation of magmas produce intermediate-to-felsic compositions.

Its high viscosity traps gases, so eruptions are often explosive, building steep lava domes and generating pyroclastic flows. It is the volcanic equivalent of granodiorite. Famous dacitic volcanoes include Mount St. Helens, Mount Pinatubo, Mount Unzen, and Santorini in Greece.

Look for a light to medium grey, tan, or pinkish fine-grained volcanic rock, commonly with visible white plagioclase crystals and dark needles or flakes of hornblende and biotite scattered through a dense groundmass.

It contains quartz (unlike andesite) but is finer-grained and lighter than basalt. A hand lens helps spot the phenocrysts and any tiny glassy quartz.

Look-alikes include andesite (darker, little or no quartz), rhyolite (lighter, more silica, often flow-banded), and trachyte (alkali feldspar dominated). Composition often requires lab work to confirm.

Dacite is used as crushed stone for road base, concrete aggregate, and railway ballast where it is locally abundant. Some dense, attractively colored dacite is used as dimension and landscaping stone.

It is significant chiefly for volcanic hazard research, since dacitic eruptions are among the most explosive and dangerous; studying dacite helps predict eruptions at volcanoes like Mount St. Helens. It has no gemstone value, though porphyritic dacite specimens with visible crystals appeal to rock collectors.