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Dacite is an igneous, volcanic rock. The word dacite comes from Dacia, a
province of the Roman Empire which lay between the Danube River and Carpathian
Mountains (now modern Romania) where the rock was first described.
usually forms as an intrusive rock such as a dike or sill. Examples of this type
of dacite outcrop are found in northwestern Montana and northeastern Bulgaria.
Nevertheless, because of the moderate silica content, dacitic magma is quite
viscous and therefore prone to explosive eruption. A notorious example of this
is Mount St. Helens in which dacite domes formed from previous
Dacitic magma is formed by the subduction of young oceanic crust under a thick
felsic continental plate. Oceanic crust is hydrothermally altered causing
addition of quartz and sodium. As the young, hot oceanic plate is subducted
under continental crust, the subducted slab partially melts and interacts with
the upper mantle through convection and dehydration reactions. Once at the cold
surface, the sodium rich magma crystallizes plagioclase, quartz and hornblende.
Accessory elements like pyroxenes provide insight to the history of the
The formation of dacite provides a great deal of information
about the connection between oceanic crust and continental crust. It provides a
model for the generation of felsic, buoyant, perennial rock from a mafic, dense,
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