Structure
Mauna Loa is the world's largest shield volcano in terms of area covered. Mauna Loa is shaped like a shield, because its lava is extremely fluid (it has low viscosity), and its slopes are not steep. Eruptions are rarely violent, and the most common form is in the Hawaiian style, which involves lava fountains feeding lava flows. Typically, at the start of an eruption, a rift up to several kilometers long opens, with lava fountains occurring along its length in a so-called "curtain of fire." After a few days, activity normally becomes concentrated at one vent.
Eruptions generally occur in three regions on the mountain: at the summit and in two rift zones extending northeast and southwest of the summit. About 38% of eruptions in the last two hundred years have occurred at the summit, 31% in the northeast rift zone, and 25% in the southwest rift zone. The remaining 6% have occurred from vents to the northwest of the summit, away from the rift zones. Its summit caldera is called Mokuʻāweoweo; it is 1.75 to 3 miles (3–5 km) in diameter. The caldera probably formed 1,000–1,500 years ago when a very large eruption from the northeast rift zone emptied out the shallow magma chamber beneath the summit, which then collapsed.
Seismic data can reveal the locations of the magma chambers beneath the volcano which feed activity. Some types of seismic waves, known as "S-waves," cannot travel through liquid rock, so magma chambers cast 'shadows' in seismic data. Seismic shadows reveal a magma chamber about 1.75 miles (3 km) beneath the summit and smaller magma bodies beneath the rift zones.
Trade winds blow from east to west across the Hawaiian islands, and the presence of Mauna Loa strongly affects the local climate. At low elevations, the eastern (windward) side of the volcano receives heavy rain, and the city of Hilo is the wettest in the United States. The rainfall supports extensive forestation. The western (leeward) side has a much drier climate. At higher elevations, the amount of precipitation decreases, and skies are very often clear. Very low temperatures mean that precipitation often occurs in the form of snow, and the summit of Mauna Loa is described as a periglacial region, where freezing and thawing play a significant role in shaping the landscape.
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