Hawaiian eruptions are the calmest of the eruption types. They are characterized by the effusive emission of highly fluid basalt lavas with low gas contents. The relative volume of ejected pyroclastic material is less than that of all other eruption types. The hallmark of Hawaiian eruptions is steady lava fountaining and the production of thin lava flows that eventually build up into large, broad shield volcanoes. Eruptions are also common in central vents near the summit of shield volcanoes, and along fissures radiating outward from the summit area. Lava advances downslope away from their source vents in lava channels and lava tubes.

Fissure eruptions are common occurrences on the "Big Island" of Hawaii. They often begin as a line of vents (curatin of fire) that gives way to eruptions concentrated at one or two cental vents lying along the fissure. The Pu'u O'o eruptive series, for example, has been erupting basaltic lava on the Kilauea shield volcano since 1983. These eruptions began on January 3 with a six-kilometer-long curtain of fire on the east rift system of Kilauea. Intermittent fissure eruptions soon gave way to a centralized eruption site on the east rift, about 15 km east of the Kilauea summit caldera, which generated a scoria-and-spatter cone, called the Pu'u O'o volcano. In 1986 the Kupaianaha volcano developed about 3 kilometers farther down rift. It erupted smoothed-surface pahoehoe lava until early 1992. Since that time the main eruption site has been centered at Pu'u O'o.

Fire Fountains

Central-vent Hawaiian eruptions are noted for their spectacular jet-like sprays of liquid lava called fire fountains. These incandescent jets ascend hundreds of meters into the air. They can occur in short spurts, or last for hours on end. One of the most spectacular fire fountaining events ever recorded on Kilauea produced a lava spray 580 m high at the Kilauea Iki vent in 1959. However, this is dwarfed by the 1600 m fire fountain generated by an Hawaiian eruption on the Japanese Island of Oshima in 1986. The top of fire fountains are often carried away downwind to produce an airborne curtain of glowing fragments that showers downward. The indivudual liquid-to-plastic fragments (clasts) generally cool quickly by radiating their heat into the atmosphere. Thus, they are chilled and solid by the time they hit the ground, where they accumulate as cindery fragments called scoria. However, during very high eruption rates, the fire fountains become so dense that the clasts can no longer radiate heat freely into the atmosphere. These clasts are kept hot by the heat of surrounding clasts. Under these conditions the molten clasts, spatter, may hit the ground and fuse together to form agglutinated spatter cones and spatter ramparts. If the eruption rates are high enough, spatter-fed flows (clastogenic lavas) may develop as hot spatter fragments blend together on the ground and flow away.

The smallest pyroclasts during fire-fountaining will be carried downwind from near the the top of the eruptive jet. They will chill quickly into small glassy black spheres, dumbells, or teardrop shapes called Pele's tears. During high winds, the teardrop shapes are sometimes drawn out as long filaments, the tails of which can break off to produce Pele's hair. During periods of high vesiculation, basalt foam can quench into the glassy rock recitulite, also known as thread-lace scoria, which has the lowest density of any known rock type.

Lava Lakes

The fluid basalt associated with Hawaiian eruptions sometimes ponds in vents, craters, or broad depressions to produce lava lakes. In some cases, lava may erupt from a vent located within a crater, or surface lava flow may pour into a crater or broad depression. The image shown here is a lava lake that occupied the Kupaianaha vent on the east rift system of Kilauea in 1986. Currently active lava lakes occur in only a few locations: Mt. Erebus in Antarctica, Erta' Ale in Ethiopia, and Nyiragongo in the Congo. The Kilauea volcano has had an active history of producing lava lakes in its numerous craters. Perhaps longest-lived lava lake in historic times was the near permanent lake that occupied the Halemaumau crater for most of the hundred-year period between 1823 to 1924. This lake was destroyed in 1924 by a massive hydrovolcanic eruption. As lava lakes cool, they produce a grey-silver crust that is usually only a few centimeters thick, as shown here in the image of the Kupaianaha lava lake. Active lava lakes contain young crust that is continually destroyed and regenereated. Convective motion of the underlying lava causes the crust to break into slabs and sink. This then exposes new lava at the surface that cools into a new crustal layer which will again break up into slabs and be recycled into the circulating lava beneath the crust.

© Dr. Vic Camp, How Volcanoes Work, http://www.geology.sdsu.edu/how_volcanoes_work/ 2004