Lava eruptions, from a plume of magma known as the Hawaiian hot spot, began the formation of Hawaii's Mauna Loa 600,000 to 1 million years ago. This plume of magma is responsible for the formation of the entire chain of Hawaiian islands. Continuously monitored as a potential threat to lives and property, Mauna Loa's last eruption was in 1984, when the lava flow came within four miles of the city of Hilo.
Mauna Loa is an active shield volcano, with lava consisting primarily of highly fluid basalt. To attain its present size, the volcano went through several cycles of eruptive activity at its summit, followed by activity in the lower rift zones. Over the last 100,000 years, eruptions have slowed. However, since the first recorded eruptive event in 1843, numerous eruptions have followed, some of which have threatened and destroyed human habitation.
The sides of Mauna Loa are very shallow, but its dome is 75 miles wide and encompasses a land area of 2,035 square miles. Its height above sea level is 13,677 feet, and it descends 16,400 feet more to the ocean floor where it depresses the Earth's crust for a further 5 miles. From its true base to its summit, it is about 56,000 feet.