The Collaborative International Dictionary of English v.0.48:

Rhyolite \Rhy"o*lite\, n. [Gr. "rei^n to flow + -lite.] (Min.)
   A quartzose trachyte, an igneous rock often showing a fluidal
   structure. -- Rhy`o*lit"ic,, a.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rhyolite
    n 1: very acid volcanic rock