A fissure eruption is a volcanic eruption that occurs in cracks, or fissures, along the sides of the volcano. These eruptions differ from the typical centralized eruption that emerges from the mouth of a crater.
Fissures develop in volcanoes over time based on shifting in the earth's crust and the cumulative effective of lava and steam boiling on the inside. A fissure eruption occurs when the gases, lava and rock fragments inside the crater spill out through the cracks. The result is similar to gushing water that flows out of a crack in a toilet tank or a pitcher of water.
While fissure eruptions take place in many volcanoes throughout the world, they occur especially often in the volcanoes of Iceland and Hawaii. The San Diego State University geology website notes that 1783 fissure eruption in Iceland, called the Laki flow, caused the most expansive recorded lava flow.
In contrast to explosive eruptions, fissure eruptions do not produce much of an ejection through the top of the crater. The pressure pushes the lava and gases out of the cracks instead. Some fissure eruptions have rings of fire that form around the outside as the hot lava and gases pass through rocky mountain terrain.