The block and ash flows are generally andestic to dacitic in composition. Andesites are fine-grained extrusive rocks that are rich in silica, composed mainly of plagioclase feldspar, very little quartz, and mafic minerals such as pyroxene, hornblende, and biotite. Dacites contain a bit more quartz and alkali feldspar than an andesite. The high silica content of the lava that these rocks come from means that they are highly viscous and therefore tend to erupt more explosively than lavas which are silica-poor.

The sizes of particles in block and ash flows ranges on average from several centimetres in diameter to less than a millimetre.