Rhyodacite

Rhyodacites form from rapid cooling of lava relatively rich in silica and low in alkali metal oxides.

However, the IUGS allows the use of the term to describe rocks close to the boundary between the rhyolite and dacite fields in each classification scheme.

Rhyodacite then describes a fine-grained igneous rock containing between 20% and 60% quartz and in which plagioclase makes up about two-thirds of the total feldspar content.

[3] The U.S. Geological Survey defines rhyodacite as volcanic rock containing 20% to 60% quartz and with plagioclase making up 35% to 65% of the total feldspar content.

[5] Rhyodacite lava flows occur in northwestern Ferry County (Washington),[4] and at An Sgùrr on the island of Eigg in Scotland.

A sample of rhyodacite from Slovakia
QAPF diagram with the approximate location of rhyodacite highlighted
TAS diagram with the approximate position of rhyodacite highlighted in yellow (T=trachyte or trachydacite; R=rhyolite; O3=dacite)
Rhyodacite pumice from the late August 1883 eruption of Krakatoa . This rock floated across the Indian Ocean for almost a year before it washed ashore at Takwa Beach , Kenya, East Africa.