Close view of dacite lava from
the May 1915 eruption of Lassen Peak, California
Dacite
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). Common minerals include plagioclase feldspar,
pyroxene, and amphibole. Dacite generally erupts at
temperatures between 800 and 1000#176;C.
It is one of the most common rock types associated with enormous
Plinian-style eruptions. When relatively gas-poor dacite erupts
onto a volcano's surface, it typically forms thick rounded lava
flow in the shape of a dome.
Even though it contains less silica than rhyolite, dacite can be even
more viscous (resistant to flow) and just as dangerous as rhyolites.
These characteristics are a result of the high crystal content of many
dacites, within a relatively high-silica melt matrix. Dacite was
erupted from Mount St. Helens 1980-86, Mount Pinatubo in 1991, and
Mount Unzen 1991-1996.
The word dacite comes from Dacia, a Roman province found between the
Danube River and Carpathian Mountains, where the rock was first
described.
