Mauna Loa is the largest active volcano on Earth. Mauna Loa is in the shield-building stage of Hawaiian volcanism, a period when the volcanoes grow most rapidly, adding as much as 95 percent of their ultimate volume.
Earth's Largest Active Volcano
Mauna Loa dominates the Island of Hawai‘i, covering just over half the island. It has a surface area of about 5,100 km2 (1900 mi2) and a submarine area that is even more massive. Scientists calculate Mauna Loa's volume to be at least 75,000 km3 (18,000 mi3). The volcano's earliest lava flows erupted onto the seafloor and submarine flanks of adjacent Hualālai or Mauna Kea volcanoes between about 0.6 and 1 million years ago. It likely emerged above sea level about 300,000 years ago, and it has grown rapidly upward since then.
When describing the location of eruptions and lava flows on Mauna Loa, scientists refer to five broad areas on the volcano. The summit area is that part of the volcano above the 3,660-m (12,000 ft) elevation, includes Moku‘āweoweo Caldera and the uppermost parts of the two rift zones. Below that elevation are the northeast and southwest rift zones, and the southeast and north and west flanks. Geologists have mapped at least 33 radial vents on the north and west sectors of the volcano, which signifies lava can erupt from these sectors of the volcano in addition to the rift zones and summit area.
Map of Mauna Loa's activity over the past ~200 years
Recent lava flows spread alternately from the summit area and rift zones.
Detailed geologic mapping and dating of lava flows above sea level have shown that about 90 percent of Mauna Loa's surface is covered with flows erupted within the past 4,000 years. Hundreds of lava flows were erupted during this time, but they did not cover the volcano evenly. By tracing the flows back to their vents and knowing their ages based on radiocarbon dating, geologists have recognized a general pattern in the frequency of lava flows spreading from the summit area and the rift zones during the past few thousand years.
Vent locations for these recent flows have generally alternated twice between the summit area and the rift zones, with each period lasting many centuries. Since 1200 CE, roughly the past 700 years, eruptions occurred primarily from vents located on Mauna Loa's rift zones. Before about 700 years ago back to 200 BCE, lava was erupted primarily from the summit area onto the west, north, and east flanks of the volcano with flows reaching the sea. Summit derived activity lasted for 10 centuries! The decline in summit-derived flows and increase in rift zone activity was likely related to the formation of Moku‘āweoweo Caldera due to collapse of the summit—lava flows erupted within the caldera were trapped, unable to overflow the caldera rim.
The cause of the transition from summit-dominated lava flows to prolonged eruption of flows from the rift zones is not known. It was probably related to significant change in the volcano's magma supply or reservoir plumbing system, the advent of explosive activity, and/or flank instability.
Large landslides dissected Mauna Loa during its rapid growth
The submarine flanks of Mauna Loa and adjacent seafloor are mantled by several enormous landslide deposits that significantly altered the shape of the volcano. Two of the youngest and largest landslides occurred from a source area on the west flank of the volcano that includes Kealakekua Bay—the ‘Ālika 1 and 2 landslides. The ‘Ālika 1 slide moved directly west about 80 km (50 mi) down the steep submarine slope and produced a broad hummocky deposit on the seafloor. The younger ‘Ālika 2 slide also moved west for a short distance, then turned northwest because it was diverted by the ‘Ālika 1 deposit; the second landslide also traveled a distance of about 100 km (60 mi). The ‘Ālika 2 landslide may have produced a giant tsunami that swept Lāna‘i about 105,000 years ago. Upslope of Kealakekua Bay, a zone of unusually steep slopes is interpreted by geologists as buried faults, the head land of one or both landslides that subsequently were covered by lava flows.
Landslides have also occurred from the volcano's southwestern flank. The South Kona landslide occurred after about 250,000 years ago, which corresponds to the approximate time when Mauna Loa emerged above sea level. Landslides and faulting helped shape the southern embayment of the coast and steep west-facing scarp of the submerged Southwest Rift Zone.
Nīnole Hills – old flows mark the site of abandoned rift zone
The Nīnole Hills are steep-sided and heavily vegetated flat-topped ridges located on the southeastern flank of Mauna Loa. Age dating and chemical analysis of lava flows exposed in Nīnole Hills indicates they were erupted about 125,000 years ago from Mauna Loa. In 2013, a detailed gravity survey of the area identified an elongate gravity high, which suggests a concentration of intrusive rocks associated with a rift zone. Geologists now interpret the Nīnole Hills as part of the volcano's original southern rift zone, which was later abandoned as a new rift zone formed to the west. The "hills" subsequently formed as erosion carved deep canyons and valleys into the old flows. Geologists infer that the large landslides from Mauna Loa's western and southern flanks caused the rift to migrate westward to the present location of the Southwest Rift Zone.
Mauna Loa has an explosive history
Mauna Loa is not known to have produced an explosive eruption since 1843, but there is geologic evidence for some explosive activity in the past 1,000 to 300 years. Geologists have identified at least 4 debris fans comprised of fragmented rock deposits on top of pāhoehoe lava flows that spread from the summit. The largest blocks found in these deposits are as large as 2.2 m (7.2 ft) in diameter and weigh more than 17,000 kg (38,000 lbs). The fine-grained fragments typical of explosive deposits are hard to find, and were probably removed by storms, rain and strong winds that frequently sweep across the summit. Geologists estimate that the 4 sites represent three separate explosive eruptions. This evidence suggests that future explosive eruptions in the summit area are possible.