The Hawaiian Islands are volcanic in origin. Each island is made up of at least one primary volcano, although many islands are composites of more than one. The Big Island, for instance, is constructed of 5 major volcanoes: Kilauea, Mauna Loa, Mauna Kea, Hualālai and Kohala. Mauna Loa is the largest active volcano on Earth.
Kilauea is presently one of the most productive volcanoes on Earth (in terms of how much lava it erupts each year). The primary volcanoes on each of the islands are known as a shield volcanoes, which are gently sloping mountains produced from a large number of generally very fluid lava flows. (SOEST)
To the casual observer, Kilauea appears to be part of the larger volcano Mauna Loa, but geological data indicates that it is a separate volcano with its own vent and conduit system. (livescience) When Kilauea began to form is not known, but various estimates are 300,000-600,000-years ago. (USGS)
Kilauea is a broad shield volcano built against the southeastern slope of Mauna Loa. The summit presently has a caldera that is roughly 2.5-miles by 2-miles wide, and walls nearly 400-feet. Another feature, known as Halemaʻumaʻu crater, lies within the main caldera (on the southwestern side.)
For the past century, Halemaʻumaʻu has been the principal site of activity at Kilauea’s summit. There has also been frequent activity along the Southeast rift zone (such as the presently active Puʻu ʻOʻo eruption, which started in 1983.)
While at nearby Mauna Loa eruptions tend to occur in pairs (i.e., a summit eruption followed by one on the flank), Kilauea’s pattern of summit versus flank activity appear to be more random. However, evidence of numerous flank eruptions occurring after a summit eruption can be found. (SOEST)
Sometimes, volcano summits form calderas. When an erupting volcano empties a shallow-level magma chamber, the edifice of the volcano may collapse into the voided reservoir, thus forming a steep, bowl-shaped depression called a caldera (Spanish for kettle or cauldron.)
The summit regions of many active shield volcanoes are marked by calderas. Hawaiian examples include the Mokuaweoweo caldera on Mauna Loa and the Kilauea caldera on Kilauea. (sdsu-edu)
So, when and how was the Kīlauea Caldera formed?
The modern caldera at Kilauea collapsed in about 1470-1500 AD, as recognized by geologic field work and C-14 radiocarbon dating. Two lines of geologic evidence indicate it was very deep soon after it formed, as is also suggested by Hawaiian oral tradition.
Scientists suggest at least a couple possibilities exist for the collapse. One is that the magma reservoir emptied rapidly without erupting; the other possibility is that magma withdrawal was prolonged, not rapid, in response to slow eruption of the ʻAilāʻau lava flow (the largest in Hawaiʻi in more than 1000 years.)
The flow erupted on the east side of Kilauea’s summit during a ca. 60-yr period ending in about 1470. It has typical tube-fed formation, consistent with slow eruption. The eruption just predates the caldera – its shield is cut by the outermost caldera fault – and in fact could have ended when the caldera formed. (Swanson)
Geologic evidence indicates that the caldera appeared in about AD 1500, give or take several decades. It developed at the end of, or soon after, a 60-year-long eruption from the ʻAilāʻau shield just east of Kilauea Iki, flooding much of Puna with lava flows. The caldera formed at the start of, or shortly before, a series of explosions that radiocarbon ages date to about AD 1500. (USGS)
However, most geologic models assume very rapid removal of material to form a caldera. The argument is that a large void cannot support itself underground, so an empty volume of a few cubic miles could not exist for long. If so, a huge volume of magma must have almost instantaneously left the reservoir under Kilauea’s summit, leaving a void into which the summit fell.
Both geologic and cultural evidence should exist for such a major volcanic event. Indeed, the events are likely chronicled in the Pele-Hiʻiaka chants. But neither cultural nor geologic evidence tells us where the rocks disappeared when the top of the volcano sunk to form the caldera.
How much material was removed? The volume is uncertain but very large. If the entire caldera formed at one time, scientists estimate a volume of 1.4 cubic miles, equivalent to 780-million loads for a 10-cubic-yard dump truck. That estimate is based on the caldera’s dimensions when first surveyed in the 1820s and on guesses as to how much deeper it might have been. (USGS)
Historical eruptions at Kilauea volcano have occurred from both the summit caldera and from vents along the East Rift Zone. The historical record began with a very explosive eruption in 1790.
After this, the volcano was almost continuously active, mostly showing gentle effusion from a lava lake at the summit until 1924, when it again erupted explosively. The period 1924 to 1955 saw mostly short-duration summit eruptions.
From 1955 to the present Kilauea has seen mostly East Rift Zone activity interspersed with small summit eruptions. Two notable rift eruptions were Mauna Ulu (1969-1974) and Puʻu ʻOʻo (1983-present). Kilauea’s Eruption rate diminished steadily over the first half of the historic period but has been increasing again since 1924. (SOEST)
Since the Puʻu ʻOʻo eruption started on January 3, 1983, the southern part of the caldera has been sinking 2.5-4 inches/year. (USGS)
Currently there are 3 Hawaiian volcanoes that we can be classified as active: Kilauea, actively erupting since 1983; Mauna Loa, which last erupted in 1984 and is building for a new eruption in the next few years; and Loihi, which erupted in 1996.
All three of these active Hawaiian volcanoes share the Hawaiian hot spot, but retain unique volcanic histories and compositions.
The most likely dormant volcanoes are: Hualālai, which last erupted in 1801; Haleakalā, which last erupted in about 1790; and Mauna Kea, which last erupted about 4,000-years ago. (SOEST)
Due to the ongoing eruption, portions of the Hawaiʻi Volcanoes National Park are periodically closed, due the hazardous and unsafe conditions; check conditions and area closures before venturing.