what is lava composed of?

Lava can be made up of a variety of minerals dependent on the tectonic setting of the volcano from which it erupts,

Basalt is the main constituent of lavas from constructive plate boundary volcanoes like Hekla in Iceland, and hotspot volcanoes like Hawaii while volcanos that derive their magma from remelted subducted plates have more variable chemistry but can often be more Andesitic in nature. Magma that has a high silica content tends to erupt explosively rather than as lava flows, which are typical of magmas with less viscous, less gassy basaltic composition