BUILDING RISK KNOWLEDGE
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Lava flows destroy everything in their path.
Lava type dictates flow shape and travel distance.
Lava flows are streams of molten rock that spill out. Lava is ejected either by non-explosive activity or by explosive lava fountains. The speed at which lava moves over the ground depends on several factors, including its type and viscosity; the slope of the ground over which it moves; whether the lava flows in a broad sheet, in a confined channel or in a lava tube; and the rate of lava production at the vent.
Fluid basalt flows can extend for tens of kilometres from an erupting vent. The leading edges of basalt flows can reach speeds of more than 10 km/h on steep slopes, but they generally advance at less than 1 km/h on gentle slopes. But when basalt lava flows are confined within a lava channel or tube on a steep slope, the main body of the flow can reach speeds of over 30 km/h.
Viscous andesite flows move only a few kilometers per hour (couple feet per second) and rarely extend more than 8 km from their vents. Viscous dacite and rhyolite flows often form steep-sided mounds called lava domes over an erupting vent. Lava domes often grow by the extrusion of many individual flows >30 m thick over a period of several months or years. Such flows will overlap one another and typically move less than a few meters per hour.
Lava fountain and ‘a‘ā flow during Pu‘u ‘Ō‘ō eruptive episode 21, Kīlauea Volcano, Hawai‘i
Lava flows can create other types of hazard.
Anything in the path of an advancing lava flow will be knocked over, surrounded, buried or ignited by the extremely high temperature of the lava. When lava erupts under a glacier or flows over snow and ice, the meltwater from the ice and snow can cause large-scale lahars. If it enters a body of water or a lava tube, the water can boil violently and cause an explosive rain of molten splash over a wide area. Methane gas, produced by the vegetation of sinks, can migrate into underground voids and explode when heated. Thick, viscous lava flows, particularly those that form a dome, can collapse to form fast-moving pyroclastic flows.
Deaths attributed to lava flows are often due to related causes, such as explosions when lava interacts with water, collapse of an active lava delta that forms when lava enters a body of water, asphyxiation due to accompanying toxic gases, pyroclastic flows from dome collapse and lahars from meltwater.
Other natural phenomena such as hurricanes, tornadoes, tsunami, fires, and earthquakes often destroy buildings, agricultural crops, and homes, but the owner(s) can usually rebuild or repair structures and their businesses in the same location. Lava flows, however, can bury homes and agricultural land under tens of meters of hardened black rock; landmarks and property lines become obscured by a vast, new hummocky landscape. People are rarely able to use land buried by lava flows or sell it for more than a small fraction of its previous worth.
Lava flow moving into the town of Kalapana, Hawaii Island. Flow front is center, silver-black mass with rising fume. Buildings and lagoon in this photograph were completely buried within one month.
Lava flow ignites and burns a building near Kalapana, Hawaii.
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