Mauna Loa is a shield volcano on the Island of Hawaiʻi and the largest active volcano on Earth by mass and volume. Its summit caldera, Mokuʻāweoweo, stands 4,169 m (13,679 ft) above sea level; measured from the sea floor it rises about 9,170 m (30,085 ft), more than Everest’s height above sea level. It covers 5,271 km² (2,035 sq mi), more than half the island, and holds an estimated 65,000 to 80,000 km³ of rock. It has erupted 34 times since the first well-documented eruption in 1843, most recently from 27 November to mid-December 2022. The mountain is so large that it makes its own weather: Hilo, on its wet windward foot, averages about 3,058 mm (120.4 in) of rain a year, while a NOAA weather station high on the north flank near the observatory gets about 363 mm (14.3 in), and at the summit much of the precipitation falls as snow. Tracking the mountain through one year of rain, snow and drought shows how a single volcano holds so many climates.
November to March: Kona Lows, Summit Snow and Winter Eruptions
The stormiest part of Mauna Loa’s year runs from October to April, when Kona lows arrive. These cold-core cyclones typically reach Hawaii two or three times a year in that window, and some winters bring four or five. They reverse the usual trade winds, bringing heavy rain, hail, flash floods and high-elevation snow. On Mauna Loa the result is a white cap on Mokuʻāweoweo. European visitors noticed it at once. In 1779 John Ledyard of James Cook’s expedition set out for a peak “generally covered with snow”, and in February 1794 the botanist Archibald Menzies reached the summit by the ʻAinapō Trail and was surprised to find heavy snow and a morning temperature of −3 °C (27 °F). He correctly guessed that Mauna Kea was the taller mountain because its snow cap was larger.
The United States Exploring Expedition under Charles Wilkes learned the same lesson in December 1840. A snowstorm hit its camp on 23 December and the weight of snow collapsed the canvas roof; Christmas Day was spent building rock walls against wind and blowing snow on the caldera rim. Despite the snow, water was the expedition’s scarcest supply, and porters walked 16 km (10 mi) back down to a lava tube on the ʻAinapō Trail that held a known store. Long before either visit, Hawaiians had climbed the same route. The ʻAinapō Trail rose more than 3,400 m (11,155 ft) over about 56 km (35 mi) from the village of Kapāpala to Mokuʻāweoweo, and camps along it supplied water and food; pilgrims probably made the journey during eruptions to leave offerings for Pele. The coldest reading at the slope observatory, −8 °C (18 °F), was taken on 20 February 1962.
Winter is also when two of the volcano’s best-known eruptions came. In late 1935 lava headed for Hilo, and on 27 December US Army aircraft, in an operation planned by Lieutenant Colonel George S. Patton, bombed the flow; it stopped by 2 January 1936, though volcanologists still dispute whether the bombs helped. On 27 November 2022, after 38 years of quiet, lava broke out in Mokuʻāweoweo and migrated to the Northeast Rift Zone the next morning, with fountains up to 61 m (200 ft) high.
The flows cut the access road to the Mauna Loa Observatory on 28 November and stalled about 2.7 km (1.7 mi) from Saddle Road, the island’s cross-island highway, before the eruption ended in mid-December. No one was hurt.
Hilo’s 3,058 Millimetres: Rain on the Windward Flank
For most of the year the north-east trade winds push moist Pacific air against Mauna Loa’s eastern slopes. Forced upward, the air cools and drops its water at low elevations, which is why Hilo is the wettest city in Hawaii and the fourth-wettest in the United States, behind three cities in south-east Alaska. The rain feeds the forests on the windward flank. Even Hilo’s driest month, May, gets nearly 180 mm, and November is the wettest at about 366 mm.
Hilo itself is partly built on the volcano’s late 19th-century lava. Flows from the eruption of 1880–81 came within today’s city limits, although the town was then a smaller shoreside village farther downslope and escaped. The rain that makes the windward side green therefore falls on ground that Mauna Loa repaved less than 150 years ago.
March to May: The 1942 and 1984 Flows Toward Hilo’s Water
Two of the eruptions that most alarmed the windward side broke out in spring. In 1942, four months after the attack on Pearl Harbor, lava from the north-east flank threatened the ʻOlaʻa flume, the main water supply for the community of Mountain View. With the island under a wartime blackout and the press under a gag order, the US Army Air Force dropped sixteen bombs of 136 to 272 kg (300 to 600 lb) to divert the flow, with little effect, before the eruption stopped on its own.
The 1984 eruption ran from 25 March to 15 April. It began at the summit and then fed a narrow channel of ʻaʻā lava down the Northeast Rift Zone that came within about 6 km (4 mi) of Hilo, close enough to light the city at night. Two natural levees upslope then broke and diverted the active flows, and the front never advanced farther.
May to September: Dry Air and the Keeling Curve at 3,394 Metres
High on the north flank the year looks completely different. Above the trade-wind inversion, the air is dry and skies are usually clear, and the NOAA station near the observatory receives only 15 mm (0.6 in) in May and 11 mm (0.43 in) in June. That dryness, the absence of vegetation and the distance from any continent are why the Mauna Loa Observatory was founded here on 28 June 1956, at 3,394 m (11,134 ft), after attempts in 1951–1954 to run a station at 4,100 m on the summit could not be sustained.
Continuous carbon dioxide measurements began there in March 1958 under Charles David Keeling, producing the Keeling Curve, adjusted to remove gas leaking from the volcano itself. The record rose from 313 parts per million in March 1958, and on 9 May 2013 the daily mean passed 400 ppm. Each year the curve also swings by about 6 ppm in step with the northern seasons: it peaks in May, falls through spring and summer as plants grow, and bottoms out in September. August is that station’s wettest month, at 60 mm (2.38 in), and the highest temperature recorded at the slope observatory, 26 °C (78 °F), came on 26 September 1990. Even so, the whole year brings the station less rain than Hilo receives in a single November.
Kona’s Rain Shadow and the 1950 Flow to the Sea
The western, leeward side of Mauna Loa lies in the mountain’s rain shadow and stays much drier through every season. Mauna Loa’s fluid lava builds flanks that slope at only about 12° at their steepest, and on this side lava has repeatedly run all the way to the coast. The 1859 eruption, the largest of three centred on the north-west flank, sent a long flow to the west coast north of Kīholo Bay. In 1877 a brief eruption even broke out underwater in Kealakekua Bay, less than a mile offshore, churning the water and floating up blocks of lava.
The fastest event came in 1950, the largest rift eruption in modern records. Over 23 days the Southwest Rift Zone poured out 376 million m³ of lava, and flows covered the 24 km (15 mi) to the ocean within about three hours. They overran the village of Hoʻokena-mauka in South Kona and crossed Hawaii Route 11; nobody died, but the village was never rebuilt. In 1926 an earlier flow had already destroyed 12 houses, a church and a small harbour near Hoʻōpūloa on the same coast.
Ice Older Than the Calendar: Mauna Loa’s Glacial Past
One kind of water on Mauna Loa does not follow the yearly cycle at all. The summit is classed as a periglacial region, where repeated freezing and thawing shape the ground, and sporadic ice persists in places, although the volcano lacks the summit permafrost found on neighbouring Mauna Kea. During the last ice age, between about 25,000 and 15,000 years ago, Mauna Loa was tall enough to carry glaciers. Unlike Mauna Kea, it kept erupting, adding 150 to 300 m of height and burying the glacial deposits; rocks of that age now lie at least 2,000 m below the summit.
The ice may still have left a mark. The Pāhala ash, laid down roughly 13,000 to 30,000 years ago, coincides with that glaciation, and geologists consider it possible that the ash was produced when magma met the long-vanished glaciers. Combined with the Hilo downpours, the dry observatory air and the snow of Kona-low winters, it shows a single volcano spanning nearly every kind of water climate found in the tropics.
Mauna Loa shares its southern flank with a smaller, more restless neighbour, described in our guide to Kīlauea volcano; its broad, gentle profile is the textbook case in what is a shield volcano; and the question of how its height from the sea floor compares with Everest is settled in Mauna Kea vs Everest.