Lava is molten rock that has been extruded onto Earth's surface. Before it reaches the surface, lava is called magma. Magma contains crystals, unmelted rock, and dissolved gasses, but it is primarily a liquid. Oxygen, silica, aluminum, iron, magnesium, calcium, sodium, potassium, titanium, and manganese are the primary elements found in magma, but other trace elements may be present in small amounts.

Viscosity of lava, or its resistance to flow, is determined by its temperature and chemical composition. In general, hotter lava that is low in silica flows much more readily than

Lava from Mt. Kilauea. This kind of lava has little silica in it, and is not explosive. When it dries, it will be referred to as pahoehoe lava. JLM Visuals. Reproduced by permission.

cooler, high-silica lava. The composition and viscosity of magma determines both its eruptive style and the rock type that will be formed when...