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. This enormous volcano covers half of the Big Island and, in terms of space, it spans 85 percent of the area of all the other Hawaiian Islands combined.
Mauna Loa is also one of the most active of the world's volcanoes. It has erupted 33 times since its first scientifically documented eruption in 1843. Its last major eruption was in 1984, when the lava flow came within 5 miles of Hilo, the largest populated area on the island. Vulcanologists certainly expect another big 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.