Hawaii’s Mauna Loa is the largest active volcano on Earth. The shield volcano rises gradually from sea level to 4,169 meters (13,678 feet). The flanks of the mountain extend another 5 kilometers (3 miles) below the surface, and the massive volcano pushes Earth’s crust down another 8 kilometers. Overall, Mauna Loa’s summit stands about 17,000 meters (56,000 feet) above its base. For comparison, Mount Everest stands 8,848 meters (29,029 feet) above sea level.

Though it has been quiet during recent decades, Mauna Loa has a long history of volcanic activity. Geologists estimate that the hotspot that feeds Mauna Loa first started to erupt about one million to 700,000 years ago. After underwater eruptions built up a seamount for hundreds of thousands of years, lava emerged above the Pacific surface about 400,000 years ago.

On average, Mauna Loa has erupted roughly every six years for the past 3,000 years. The most detailed eruption records are available for the past few centuries. Eruptions usually begin in the summit caldera with lava flowing down one of the volcano’s two rift zones: the Northeast Rift Zone and the Southwest Rift Zone. The rift zones are chains of craters and fissures that extend from the summit to the sea.

The Operational Land Imager (OLI) on Landsat 8 acquired this image of Mauna Loa on December 20, 2014. The elongated crater in the center is the summit caldera. Lava flows appear black; as they age, they fade and become gray. Very little vegetation appears on the summit of Mauna Loa because of the frequent lava flows and the high elevation. One of the largest flows in modern times occurred in 1950, when some 376 million cubic meters flowed out from the volcano and spread across 112 square kilometers (43 square miles) of the island.

Mauna Loa last erupted in 1984. In September 2015, volcanologists raised an alert after detecting an increase in the number of shallow earthquakes near the summit and the upper Southwest Rift Zone. However, Mauna Loa is not yet erupting. Rather, magma reservoirs beneath the summit and upper Southwest Rift Zone are filling.

NASA Earth Observatory images by Jesse Allen and Adam Voiland, using Landsat data from the U.S. Geological Survey. Caption by Adam Voiland.