Lava Vesuvius between May 14 and June 4, 1737. Serao described "a flow of fiery lava" as an analogy to the flow of water and mud down the flanks of the volcano following heavy rain. The composition of almost all lava of the Earth's crust is dominated by silicate minerals, mostly feldspars, olivine, pyroxenes, amphiboles, micas and quartz. Igneous rocks, which form lava flows when erupted, can be classified into three chemical types: "felsic", "intermediate", and "mafic" (four if one includes the super-heated "ultramafic"). These classes are primarily chemical, however, the chemistry of lava also tends to correlate with the magma
Igneous rock commonly erupt explosively, and rhyolitic lava flows are typically of limited extent and have steep margins, because the magma is so viscous. Felsic and intermediate magmas that erupt often do so violently, with explosions driven by the release of dissolved gases—typically water vapour, but also carbon dioxide. Explosively erupted pyroclastic material is called tephra and includes tuff, agglomerate and ignimbrite. Fine volcanic ash is also erupted and forms ash tuff deposits, which can often cover vast areas. Because lava usually cools and crystallizes rapidly, it is usually fine-grained. If the cooling has been so rapid as to prevent the formation