Mauna Loa
The largest active volcano on Earth, which ended 38 quiet years with a 12-day eruption in 2022
Mauna Loa does not look like a classic volcano. Its slopes rise so gently from the sea that the mountain seems more like a long, dark dome, and that is what its Hawaiian name means: Long Mountain. Yet it is the largest active volcano on Earth, covering about half of the Island of Hawaiʻi and rising about 4,169 meters above sea level and some 9 kilometers above its base on the ocean floor. It has erupted 34 times since 1843, sending lava toward Hilo and the Kona coast. After 38 quiet years it erupted again from November 27 to December 10, 2022, and thousands of people came to watch its glowing lava from the road it threatened.
The volcano is a shield built from countless thin flows of fluid basalt. Its summit caldera, Mokuʻāweoweo, is about 5 by 3 kilometers, and two rift zones run from it to the northeast and southwest, guiding most eruptions. Its volume is estimated at about 75,000 cubic kilometers. Lava from its flanks has reached the sea many times, and in 1880–81 a flow advanced into what are now the city limits of Hilo. The 2022 eruption began in the summit caldera and moved to the Northeast Rift Zone, where fissures fed flows toward the Daniel K. Inouye Highway, the main road across the island, before they stalled a few kilometers short of it. The lava cut the power line to NOAA's Mauna Loa Observatory, which has kept the longest continuous record of atmospheric carbon dioxide since 1958.
Mauna Loa is one of five volcanoes that built the Island of Hawaiʻi above the Hawaiian hotspot. Its historical eruptions have been effusive, producing lava flows rather than explosive blasts, so its main hazard is fast-moving lava that can reach populated coasts within hours or days. In 1950 lava reached the sea on the west flank in about three hours, and in 1984 a flow stopped about 6 kilometers from the outskirts of Hilo. The USGS Hawaiian Volcano Observatory, founded in 1912, monitors earthquakes, ground deformation and gas. In its 2026 updates it kept Mauna Loa at alert level Normal, noting slow inflation of the summit as the magma reservoir refills after 2022 and no signs of an imminent eruption. Much of the volcano lies in Hawaiʻi Volcanoes National Park, a UNESCO World Heritage Site.