Mauna Loa is a shield volcano, one produced by repeated, fluid lava flows building a very broad, subdued uplift that, in profile, resembles an enormous shield.
This and the other Hawaiian shield volcanoes derive from the Hawaiian hot spot, a stationary vent of magma over which the Pacific plate is moving. Active volcanoes form on the seafloor over the hot spot, and eventually go extinct as the tectonic action moves them away from the lava source.
This is the world's biggest active volcano. Mauna Loa rises 13,680 feet above the Pacific's surface, but its base on the seafloor, depressed by the massive peak, is actually 56,000 feet from the summit. The mountain is 60 miles long and 30 miles wide.
Mauna Loa is also one of the most active of the world's volcanoes. Its last major eruption was in 1984, and vulcanologists expect another blow. Around 98 percent of the peak is blanketed in lava flows under 10,000 years old.
The Long Mountain is one of five shield volcanoes comprising Hawaii. The others are Kohala, Hualalai, Mauna Kea, and Kilauea. Mauna Kea, while not as massive as Mauna Loa, is taller.