Volcanism magma produces volcanoes characterised by explosive eruptions, while non-viscous (runny) magma produce volcanoes characterised by effusive eruptions pouring large amounts of lava onto the surface. In some cases, rising magma can cool and solidify without reaching the surface. Instead, the cooled and solidified igneous mass crystallises within the crust to form an igneous intrusion. As magma cools the chemicals in the crystals formed are effectively removed from the main mix of the magma (by a process known as fractional crystallization), so the chemical content of the remaining magma evolves as it solidifies slowly. Fresh unevolved magma injections can remobilise more
Lava to have reached at least . Lava fountains may occur as a series of short pulses, or a continuous jet of lava. They are commonly associated with Hawaiian eruptions. Rarely, a volcanic cone may fill with lava but not erupt. Lava which pools within the caldera is known as a lava lake. Lava lakes do not usually persist for long, either draining back into the magma chamber once pressure is relieved (usually by venting of gases through the caldera), or by draining via eruption of lava flows or pyroclastic explosion. There are only a few sites in the world where