Kīlauea in the context of History of Hawaii

A caldera (/kɔːlˈdɛrə, kæl-/ kawl-DERR-ə, kal-) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcanic eruption. The ejection of large volumes of magma in a short time can upset the integrity of a magma chamber's structure by in effect removing much of the chamber's filling material. The walls and ceiling of a chamber may now not be able to support its own weight and any substrate or rock resting above. The ground surface then collapses into the emptied or partially emptied magma chamber, leaving a large depression at the surface that may have a diameter of dozens of kilometers. Although sometimes described as a crater, the feature is actually a type of sinkhole, as it is formed through subsidence and collapse rather than an explosion or impact. Compared to the thousands of volcanic eruptions that occur over the course of a century, the formation of a caldera is a rare event, occurring only a few times within a given window of 100 years. Only nine caldera-forming collapses are known to have occurred between 1911 and 2022, with the caldera collapses at Kīlauea, Hawaii, in 2018 and Hunga Tonga–Hunga Haʻapai in 2022 being the most recent. Volcanoes that have formed a caldera are sometimes described as "caldera volcanoes".

A fissure vent, also known as a volcanic fissure, eruption fissure or simply a fissure, is a linear volcanic vent through which lava erupts, usually without any explosive activity. The vent is often a few metres wide and may be many kilometres long. Fissure vents can cause large flood basalts which run first in lava channels and later in lava tubes. After some time, the eruption tends to become focused at one or more spatter cones. Volcanic cones and their craters that are aligned along a fissure form a crater row. Small fissure vents may not be easily discernible from the air, but the crater rows (see Laki) or the canyons (see Eldgjá) built up by some of them are.

The dikes that feed fissures reach the surface from depths of a few kilometers and connect them to deeper magma reservoirs, often under volcanic centers. Fissures are usually found in or along rifts and rift zones, such as Iceland and the East African Rift. Fissure vents are often part of the structure of shield volcanoes.

A Hawaiian eruption is a type of volcanic eruption where lava flows from the vent in a relatively gentle, low level eruption; it is so named because it is characteristic of Hawaiian volcanoes. Typically they are effusive eruptions, with basaltic magmas of low viscosity, low content of gases, and high temperature at the vent. Very small amounts of volcanic ash are produced. This type of eruption occurs most often at hotspot volcanoes such as Kīlauea on Hawaii's big island and in Iceland, though it can occur near subduction zones (e.g. Medicine Lake Volcano in California) and rift zones. Hawaiian eruptions may occur along fissure vents, such as during the eruption of Mauna Loa in 1950, or at a central vent, such as during the 1959 eruption in Kīlauea Iki Crater, which created a lava fountain 580 meters (1,900 ft) high and formed a 38-meter cone named Puʻu Puaʻi. In fissure-type eruptions, lava spurts from a fissure on the volcano's rift zone and feeds lava streams that flow downslope. In central-vent eruptions, a fountain of lava can spurt to a height of 300 meters or more (heights of 1600 meters were reported for the 1986 eruption of Mount Mihara on Izu Ōshima, Japan).

Hawaiian eruptions usually start from an increase earthquake activity in a localized region followed with the formation of cracks in the ground from which a curtain of incandescent lava or several closely spaced lava fountains appear. The lava can overflow the fissure and form ʻaʻā or pāhoehoe style of flows. When such an eruption from a central cone is protracted, it can form lightly sloped shield volcanoes, for example Mauna Loa or Skjaldbreiður in Iceland. Geologists can predict where new eruptions will take place by tracking the earthquakes that precede the eruptions.

A volcanic bomb or lava bomb is a mass of partially molten rock (tephra) larger than 64 mm (2.5 inches) in diameter, formed when a volcano ejects viscous fragments of lava during an eruption. Because volcanic bombs cool after they leave the volcano, they are extrusive igneous rocks. Volcanic bombs can be thrown many kilometres from an erupting vent, and often acquire aerodynamic shapes during their flight. Bombs can be extremely large; the 1935 eruption of Mount Asama in Japan expelled bombs measuring 5–6 m (16-20 ft) in diameter up to 600 m (2,000 ft) from the vent. Volcanic bombs are a significant volcanic hazard, and can cause severe injuries and death to people in an eruption zone. One such incident occurred at Galeras volcano in Colombia in 1993; six people near the summit were killed and several seriously injured by lava bombs when the volcano erupted unexpectedly. On July 16, 2018, 23 people were injured on a tour boat near the Kilauea volcano as a result of a basketball-sized lava bomb from the 2018 lower Puna eruption.

Volcanic bombs are known to occasionally explode from internal gas pressure as they cool, but in most cases, most of the damage they cause is from impact, or subsequent fire damage. Bomb explosions are most often observed in "bread-crust" type bombs.

A rift zone is a feature of some volcanoes, especially shield volcanoes, in which a set of linear cracks (or rifts) develops in a volcanic edifice, typically forming into two or three well-defined regions along the flanks of the vent. Believed to be primarily caused by internal and gravitational stresses generated by magma emplacement within and across various regions of the volcano, rift zones allow the intrusion of magmatic dykes into the slopes of the volcano itself. The addition of these magmatic materials usually contributes to the further rifting of the slope, in addition to generating fissure eruptions from those dykes that reach the surface. It is the grouping of these fissures, and the dykes that feed them, that serves to delineate where and whether a rift zone is to be defined. The accumulated lava of repeated eruptions from rift zones along with the endogenous growth created by magma intrusions causes these volcanoes to have an elongated shape. Perhaps the best example of this is Mauna Loa, which in Hawaiian means "long mountain", and which features two very well defined rift zones extending tens of kilometers outward from the central vent.

An active volcano is a volcano that is currently erupting, or has the potential to erupt in the future. Conventionally it is applied to any that have erupted during the Holocene (the current geologic epoch that began approximately 11,700 years ago). A volcano that is not currently erupting but could erupt in the future is also known as a dormant volcano. Volcanoes that will not erupt again, or is thought to never erupt again, are known as extinct volcanoes.

Lava lakes are large volumes of molten lava, usually basaltic, contained in a volcanic vent, crater, or broad depression. The term is used to describe both lava lakes that are wholly or partly molten and those that are solidified (sometimes referred to as frozen lava lakes).

Kīlauea Iki ("Little Kilauea") is a volcanic crater next to the main summit caldera of Kīlauea on the island of Hawaiʻi in the Hawaiian Islands. Shaped like a pit, the crater existed prior to its most recent eruption, which occurred from 14 November to 19 December 1959. This eruption was notable for several unique phenomena, including the formation of a large lava lake, lava rapids, waves, a lava whirlpool, and a lava fountain whose height set a record in Hawaii. Observers also recorded a two-way circulation of lava between the magma chamber and the surface. Today, Kīlauea Iki is a popular hiking destination within Hawaii Volcanoes National Park.

The 2018 lower Puna eruption was a volcanic event on the island of Hawaiʻi, on Kīlauea volcano's East Rift Zone that began on May 3, 2018. It is related to the larger eruption of Kīlauea that began on January 3, 1983, though some volcanologists and USGS scientists have discussed whether to classify it as a new eruption. Outbreaks of lava fountains up to 300 feet (90 m) high, lava flows, and volcanic gas in the Leilani Estates subdivision were preceded by earthquakes and ground deformation that created cracks in the roads.

On May 4, a 6.9 magnitude earthquake hit Puna. By May 27, 2018, 24 fissures had erupted lava in and near the Leilani Estates and Lanipuna Gardens subdivisions. The eruption forced the evacuation of approximately two thousand residents. The Puna Geothermal Venture, which provided one-quarter of the island's electricity, was forced to shut down and was later damaged by lava. The fissures had sent lava rivers that buried part of Hawaii Route 137 on May 19, and began flowing into the ocean.

A volcanic bomb or lava bomb is a mass of partially molten rock (tephra) larger than 64 mm (2.5 inches) in diameter, formed when a volcano ejects viscous fragments of lava during an eruption. Because volcanic bombs cool after they leave the volcano, they are extrusive igneous rocks. Volcanic bombs can be thrown many kilometres from an erupting vent, and often acquire aerodynamic shapes during their flight. Volcanic bombs can be extremely large; the 1935 eruption of Mount Asama in Japan expelled bombs measuring 5–6 m (16-20 ft) in diameter up to 600 m (2,000 ft) from the vent. Volcanic bombs are a significant volcanic hazard, and can cause severe injuries and death to people in an eruption zone. One such incident occurred at Galeras volcano in Colombia in 1993; six people near the summit were killed and several seriously injured by lava bombs when the volcano erupted unexpectedly. On July 16, 2018, 23 people were injured on a tour boat near the Kilauea volcano as a result of a basketball-sized lava bomb from the 2018 lower Puna eruption.

Volcanic bombs are known to occasionally explode from internal gas pressure as they cool, but in most cases, most of the damage they cause is from impact, or subsequent fire damage. Bomb explosions are most often observed in "bread-crust" type bombs.