Short answers, pulled from the story.
Lava describes molten rock expelled by a volcano during an eruption that emerges at temperatures ranging from about 700 to 1,250 degrees Celsius depending on its chemical makeup. Solid volcanic rock resulting from subsequent cooling is often also called lava by geologists.
Petrologists divide these mixtures into four groups based on silica content: felsic, intermediate, mafic, and ultramafic. Felsic lavas contain more than 63% silica while mafic lavas range from 45% to 52% silica and typify basaltic flows that create shield volcanoes.
Lava flow speeds vary based primarily on viscosity and slope with Hawaiian basaltic flows moving slowly but reaching maximum speeds up to 9 kilometers per hour on steep slopes. A typical lava behaves as a Bingham fluid showing considerable resistance until a stress threshold called yield stress is crossed.
Permanent lakes of lava exist only at specific sites including Mount Erebus in Antarctica Erta Ale in Ethiopia Nyiragongo in the Democratic Republic of Congo and Ambrym in Vanuau. These locations maintain active molten reservoirs unlike other volcanic features.
On the night of the 10th of January 1977 a crater wall was breached at Mount Nyiragongo causing a fluid lava lake to drain out in under an hour. The resulting flow sped down steep slopes at up to 60 miles per hour overwhelming several villages while residents were asleep.