Dacite is an igneous, volcanic rock with a high iron content found at many lava-domes.

Dacite (pronounced /deɪsaɪt/) is an igneous, volcanic rock with a high iron content. It is intermediate in compositions between andesite and rhyolite, and, like andesite, it consists mostly of plagioclase feldspar with biotite, hornblende, and pyroxene (augite and/or enstatite). It has an aphanitic to porphyritic texture with quartz as rounded, corroded phenocrysts, or as an element of t... -> See whole entry

A sudden collapse of volcanic material from an unstable side of a volcano. Debris avalanches are a particularly violent type of pyroclastic flows (in its broader meaning).