Molten rock, normally a silicate, erupted by a volcano. It may be vesicular, glassy, or porphyritic in texture, and varies between acidic and basic in composition. Its behaviour on extrusion and its relief-forming capacity depend largely on its viscosity, which is affected by silica content, temperature, and amount of dissolved gases and solids. Generally, the less viscous the lava the faster the flow, and the more viscous the lava the greater the tendency towards explosive eruption. Two varieties of basaltic lava surface are recognized: ‘aa’, a jagged, stony clinker, bristling with sharp points; and ‘pahoehoe’, characterized by a smooth, ropy appearance. Andesitic and rhyolitic lavas tend to have ‘blocky’ surfaces, characterized by smooth-faceted blocks, 1–5 m in diameter.