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Links

 * https://www.science.org/content/article/world-s-biggest-volcano-barely-visible?cookieSet=1
 * https://www.sciencedirect.com/science/article/pii/S0012821X20302399
 * https://pubs.usgs.gov/gip/dynamic/Hawaiian.html

Setting
Like all Hawaiian volcanoes, Mauna Loa was created as the Pacific tectonic plate moved over the Hawaii hotspot in the Earth's underlying mantle. The Hawaii island volcanoes are the most recent evidence of this process that, over 70 million years, has created the 3700 mi-long Hawaiian–Emperor seamount chain. The prevailing view states that the hotspot has been largely stationary within the planet's mantle for much, if not all of the Cenozoic Era. However, while the Hawaiian mantle plume is well understood and extensively studied, the nature of hotspots themselves remains fairly enigmatic.

Mauna Loa is one of five subaerial volcanoes that make up the island of Hawaiʻi. The oldest volcano on the island, Kohala, is more than a million years old, and Kīlauea, the youngest, is believed to be between 210,000 and 280,000 years of age. Lōʻihi Seamount on the island's flank is even younger, but has yet to breach the surface of the Pacific Ocean. At 1 million to 600,000 years of age, Mauna Loa is the second youngest of the five volcanoes on the island, making it the third youngest volcano in the Hawaiian – Emperor seamount chain, a chain of shield volcanoes and seamounts extending from Hawaii to the Kuril–Kamchatka Trench in Russia.



Following the pattern of Hawaiian volcano formation, Mauna Loa would have started as a submarine volcano, gradually building itself up through underwater eruptions of alkali basalt before emerging from the sea through a series of surtseyan eruptions about 400,000 years ago. Since then, the volcano has remained active, with a history of effusive and explosive eruptions, including 33 eruptions since the first well-documented eruption in 1843. Although Mauna Loa has not erupted since 1984, the U.S. Geological Survey states that the volcano is certain to erupt again.

Structure
Mauna Loa is the largest active volcano and one of the largest volcanoes in the world. The subaerial portion of the volcano spans a maximum width of 120 km and, having a surface area of about 5100 km2, it makes up more than half the surface area of the island of Hawaiʻi. Estimates of the volcano's volume range from 74000 to 105000 km3; a 2013 analysis in Geosphere estimated the volcano's volume to be 83000 km3.

Combining the volcano's extensive submarine flanks (5000 m to the sea floor) and 4169 m subaerial height, Mauna Loa rises 9170 m from the seafloor to its summit. In addition, much of the mountain is invisible even underwater: its mass depresses the crust beneath it by another 8 km, in the shape of an inverse mountain, meaning that the total height of Mauna Loa from its lowest point to its summit is about 17000 m. This figure makes Mauna Loa the tallest mountain in the world when measured from base to summit, significantly taller than Mount Everest's 8848 m height from sea level to summit.

Mauna Loa is a typical shield volcano in form, taking the shape of a long, broad dome extending down to the ocean floor whose slopes are about 12° at their steepest, a consequence of its extremely fluid lava. The shield-stage lavas that built the enormous main mass of the mountain are tholeiitic basalts, like those of Mauna Kea, created through the mixing of primary magma and subducted oceanic crust. Mauna Loa's summit hosts three overlapping pit craters arranged northeast–southwest, the first and last roughly 1 km in diameter and the second an oblong 4.2 x feature; together these three craters make up the 6.2 by summit caldera Mokuʻāweoweo, so named for the Hawaiian ʻāweoweo fish (Priacanthus meeki), purportedly due to the resemblance of its eruptive fires to the coloration of the fish. Mokuʻāweoweo's caldera floor lies between 170 and 50 m beneath its rim and it is only the latest of several calderas that have formed and reformed over the volcano's life. It was created between 1,000 and 1,500 years ago by a large eruption from Mauna Loa's northeast rift zone, which emptied out a shallow magma chamber beneath the summit and collapsed it into its present form. Additionally, two smaller pit craters lie southwest of the caldera, named Lua Hou (New Pit) and Lua Hohonu (Deep Pit).

Mauna Loa's summit is also the focal point for its two prominent rift zones, marked on the surface by well-preserved, relatively recent lava flows (easily seen in satellite imagery) and linearly arranged fracture lines intersected by cinder and splatter cones. These rift zones are deeply set structures, driven by dike intrusions along a decollement fault that is believed to reach down all the way to the volcano's base, 12 to 14 km deep. The first is a 60 km rift trending southwest from the caldera to the sea and a further 40 km underwater, with a prominent 40° directional change along its length; this rift zone is historically active across most of its length. The second, northeastern rift zone extends towards Hilo and is historically active across only the first 20 km of its length, with a nearly straight and, in its latter sections, poorly defined trend. The northeastern rift zone takes the form of a succession of cinder cones, the most prominent of which the 60 m high Puu Ulaula, or Red Hill. There is also a less definite northward rift zone that extends towards the Humuula Saddle marking the intersection of Mauna Loa and Mauna Kea.

Simplified geophysical models of Mauna Loa's magma chamber have been constructed, using interferometric synthetic aperture radar measures of ground deformation due to the slow buildup of lava under the volcano's surface. These models predict a 1.1 km wide magma chamber located at a depth of about 4.7 km, 0.5 km below sea level, near the southeastern margin of Mokuʻāweoweo. This shallow magma chamber is significantly higher-placed than Mauna Loa's rift zones, suggesting magma intrusions into the deeper parts and occasional dike injections into the shallower parts of the rift zone drive rift activity; a similar mechanism has been proposed for neighboring Kīlauea. Earlier models based on Mauna Loa's two most recent eruptions made a similar prediction, placing the chamber at 3 km deep in roughly the same geographic position.

Mauna Loa has complex interactions with its neighbors, Hualālai to the northwest, Mauna Kea to the northeast, and particularly Kīlauea to the east. Lavas from Mauna Kea intersect with Mauna Loa's basal flows as a consequence of Kea's older age, and Mauna Kea's original rift zones were buried beneath post-shield volcanic rocks of Mauna Loa; additionally, Mauna Kea shares Mauna Loa's gravity well, depressing the ocean crust beneath it by 6 km. There are also a series of normal faults on Mauna Loa's northern and western slopes, between its two major rift zones, that are believed to be the result of combined circumferential tension from the two rift zones and from added pressure due to the westward growth of neighboring Kīlauea.

Because Kīlauea lacks a topographical prominence and appears as a bulge on the southeastern flank of Mauna Loa, it was historically interpreted by both native Hawaiians and early geologists to be an active satellite of Mauna Loa. However, analysis of the chemical composition of lavas from the two volcanoes show that they have separate magma chambers, and are thus distinct. Nonetheless, their proximity has led to a historical trend in which high activity at one volcano roughly coincides with low activity at the other. When Kīlauea lay dormant between 1934 and 1952, Mauna Loa became active, and when the latter remained quiet from 1952 to 1974, the reverse was true. This is not always the case; the 1984 eruption of Mauna Loa started during an eruption at Kīlauea, but had no discernible effect on the Kīlauea eruption, and the ongoing inflation of Mauna Loa's summit, indicative of a future eruption, began the same day as new lava flows at Kīlauea's Puʻu ʻŌʻō crater. Geologists have suggested that "pulses" of magma entering Mauna Loa's deeper magma system may have increased pressure inside Kīlauea and triggered the concurrent eruptions.

Mauna Loa is slumping eastward along its southwestern rift zone, leveraging its mass into Kīlauea and driving the latter eastward at a rate of about 10 cm per year; the interaction between the two volcanoes in this manner has generated a number of large earthquakes in the past, and has resulted in a significant area of debris off Kīlauea's seaward flank known as the Hilina Slump. A system of older faults exists on the southeastern side of Mauna Loa that likely formed before Kilauea became large enough to impede Mauna Loa's slump, the lowest and northernmost of which, the Kaoiki fault, remains an active earthquake center today. The west side of Mauna Loa, meanwhile, is unimpeded in movement, and indeed is believed to have undergone a massive slump collapse between 100,000 and 200,000 years ago, the residue from which, consisting of a scattering of debris up to several kilometers wide and up to 50 km distant, is still visible today. The damage was so extensive that the headwall of the damage likely intersected its southwestern rift zone. There is very little movement there today, a consequence of the volcano's geometry.

Mauna Loa is tall enough to have experienced glaciation during the last ice age, 25,000 to 15,000 years ago. Unlike Mauna Kea, on which extensive evidence of glaciation remains even today, Mauna Loa was at the time and has remained active, having grown an additional 150 to 300 m in height since then and covering any glacial deposits beneath new flows; strata of that age don't occur until at least 2000 m down from the volcano's summit, too low for glacial growth. Mauna Loa also lacks its neighbor's summit permafrost region, although sporadic ice persists in places. It is speculated that extensive phreatomagmatic activity occurred during this time, contributing extensively to ash deposits on the summit.

Notes to consider

 * See Reliable sources/Noticeboard/Archive 327 re: Mantleplumes.org as a reliable source
 * GIIP 117, re: evidence of explosive activity, paged 26, 34
 * Find USGS image of night view of lava flow from Hilo, 1984, by David Little (page 52)
 * Relationship with Kilauea (more) - PP 1806, page 208-214
 * Hawaii Volcano Watch: A Pictorial History, 1779-1991, Thomas L. Wright, Taeko Jane Takahashi, J.D. Griggs, University of Hawaii Press, 1992 (mostly for historical/eruptions)
 * Note to self - see PP1801, re: radial vents

Stuff

 * https://pubs.er.usgs.gov/search?q=mauna+loa
 * https://www.usgs.gov/volcanoes/mauna-loa
 * https://www.usgs.gov/news/volcano-watch-hvo-looks-past-better-understand-future-mauna-loa-eruptions
 * https://pubs.er.usgs.gov/publication/sir20195129 - The Lava Flow that Came to Hilo