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
Hell's Half Acre Lava Field few as two fissure vents. Hell's Half Acre was created when basaltic pāhoehoe quickly flowed out of the volcanic rift. At least eight lava lobes have been identified by geologists. As magma and volcanic gases drained from underneath the lava field, the field subsided—leaving behinds hummocks, or "hills" of lava (a feature which Hell's Half Acre retains today). There is evidence that lava filled and drained the lava lake numerous times, and that lava repeatedly overflowed the lake. Lava from the fissure vent tended to flow downhill toward the southeast, covering part of the fissure. Lava tubes extend in a