The 1975 eruption of Mauna Loa was a short-lived

Hawaiian eruption 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 basalti ...

that followed 25 years of quiescence at the

Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only state ...

an volcano

Mauna Loa Mauna Loa ( or ; Hawaiian: ; en, Long Mountain) is one of five volcanoes that form the Island of Hawaii in the U.S. state of Hawaii in the Pacific Ocean. The largest subaerial volcano (as opposed to subaqueous volcanoes) in both mass and ...

. The eruption began just before midnight on July 5 and involved

fissures A fissure is a long, narrow crack opening along the surface of Earth. The term is derived from the Latin word , which means 'cleft' or 'crack'. Fissures emerge in Earth's crust, on ice sheets and glaciers, and on volcanoes. Ground fissure ...

extending across the length of Moku‘āweoweo, Mauna Loa's summit

caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...

, and into the upper ends of the volcano's northeast and southwest

rift zone 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 t ...

s. After only 6 hours, activity in Moku‘āweoweo and on the southwest rift zone ended, but

lava fountain Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

ing continued along the northeast rift zone until 7:30 p.m. on July 6, when all activity ceased.

Precursors

No unusual activity was noted on the Mauna Loa

seismogram A seismogram is a graph output by a seismograph. It is a record of the ground motion at a measuring station as a function of time. Seismograms typically record motions in three cartesian axes (x, y, and z), with the z axis perpendicular to the ...

s during the early evening of July 5;

seismicity Seismicity is a measure encompassing earthquake occurrences, mechanisms, and magnitude at a given geographical location. As such, it summarizes a region's seismic activity. The term was coined by Beno Gutenberg and Charles Francis Richter in 19 ...

was at relatively low levels. At 23:18 HST, however, seismic alarms, signifying prolonged high-amplitude

volcanic tremor A harmonic tremor is a sustained release of seismic and infrasonic energy typically associated with the underground movement of magma, the venting of volcanic gases from magma, or both. It is a long-duration release of seismic energy, with distin ...

, were activated in homes of

Hawaiian Volcano Observatory The Hawaiian Volcano Observatory (HVO) is an agency of the U.S. Geological Survey (USGS) and one of five volcano observatories operating under the USGS Volcano Hazards Program. Based in Hilo, Hawaii, the observatory monitors six Hawaiian volcan ...

(HVO) staff. The tremor had been first recorded by three

seismometer A seismometer is an instrument that responds to ground noises and shaking such as caused by earthquakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The outpu ...

s near Moku‘āweoweo at 22:51 HST. Staff reached HVO at 23:30 HST and saw that harmonic tremor was being recorded on all Mauna Loa and

Kīlauea Kīlauea ( , ) is an active shield volcano in the Hawaiian Islands. Located along the southeastern shore of the Big Island of Hawaii, the volcano is between 210,000 and 280,000 years old and emerged above sea level about 100,000 years ago. His ...

seismographs.

Hawaiʻi Volcanoes National Park Hawaii Volcanoes National Park is an American national park located in the U.S. state of Hawaii on the island of Hawaii. The park encompasses two active volcanoes: Kīlauea, one of the world's most active volcanoes, and Mauna Loa, the world's mo ...

and civil defense authorities were alerted to the imminent probability of a Mauna Loa eruption.

Eruption

At 23:42 HST, a small glow was noted above the southwest end of Moku‘āweoweo. Within one minute the glow extended across the entire summit area and a fume cloud more than high was illuminated with a bright orange-red glow from the unseen lava fountains on the caldera floor. A light airplane with two HVO staff members aboard reached the summit area on July 6 at 01:48 HST; a line of lava fountains high crossed the entire floor of Moku‘āweoweo at this time and extended about down the southwest rift zone. Lava cascades high were pouring into three

pit crater A pit crater (also called a subsidence crater or collapse crater) is a depression formed by a sinking or collapse of the surface lying above a void or empty chamber, rather than by the eruption of a volcano or lava vent. Pit craters are found ...

s on the upper southwest rift zone. Lava flows were rapidly advancing to the west and southwest from the southwest rift zone vents. 1975 Mauna Loa lava cascades

The eruptive fissures extended rapidly northeastward, across North Pit, and into the upper northeast rift zone at 02:25 HST. At 02:45 HST lava from the North Pit vents began cascading into another summit pit crater, Lua Poholo. By 03:15 HST, the lava fountains on the southwest rift zone and within Moku‘āweoweo were waning, and the lava flows moving to the west and southwest had stagnated. Fountains continued to migrate eastward along the northeast rift zone, but by dawn eruptive activity was largely restricted to echelon vents near the northeast rift zone at elevation. A voluminous

a'a Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

flow moved about down the north flank of Mauna Loa and threatened to cut the paved access road and powerlines to the

Mauna Loa Observatory The Mauna Loa Observatory (MLO) is an atmospheric baseline station on Mauna Loa, on the island of Hawaii, located in the U.S. state of Hawaii. The observatory Since 1958, initially under the direction of Charles Keeling, followed by his s ...

. At about 07:15 HST, fountains feeding this a'a flow subsided and it soon stopped about from the observatory road, having traveled . Fountaining continued at greatly diminished levels throughout the day, but ceased by nightfall. Approximately of lava had been erupted and of the Mauna Loa summit area had been covered.

References

{{DEFAULTSORT:Mauna Loa, 1975 Mauna Loa 20th-century volcanic events 1975 in Hawaii July 1975 events in the United States VEI-0 eruptions Volcanic eruptions in the United States Hawaiian eruptions