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Though the park celebrates its 100th anniversary on August 1, 2016, its main attractions-Kilauea and Mauna Loa volcanoes-have been adding to the land mass of Hawaii’s “Big Island” for at least 400,000 years.

USGS Hawaiian Volcano Observatory scientists are closely monitoring recent signs of unrest on Mauna Loa, the largest active volcano on Earth. In this 1985 aerial photo, Mauna Loa looms above Kīlauea Volcano's summit caldera (left center) and nearly obscures Hualālai in the far distance (upper right). USGS photo.

TOP: Mauna Loa weekly earthquake rates between 2010 and September 17, 2015. Blue bars indicate the number of earthquakes that were located by the USGS Hawaiian Volcano Observatory seismic network. Earthquakes of all magnitudes are plotted. Subtle increases in earthquake rates started in mid-2013, while more obvious changes in rates started in 2014.

After 25 years of slumber, Mauna Loa woke with a spectacular, but short-lived, eruption that began just before midnight on July 5. Lava fountains soon erupted from fissures extending across the length of Moku‘āweoweo, Mauna Loa's summit caldera, and into the upper ends of the volcano's northeast and southwest rift zones.

On June 1, 1950, a 2.4-km (1.5 mi) long fissure erupted high on Mauna Loa's Southwest Rift Zone at 9:04 p.m. Minutes later, the roar of lava fountains could be heard up to 24 km (15 mi) away.

As magma rises toward Earth's surface, gases dissolved in the molten rock bubble out and escape through surface vents called fumaroles. HVO established sensors atop Mauna Loa in late 2005 to continuously monitor the concentration of carbon dioxide and sulfur dioxide gases and fumarole temperature within Moku‘āweoweo, the summit caldera.

USGS Hawaiian Volcano Observatory geophysicist Ingrid Johanson measures gravity with a gravimeter on the slope of Mauna Loa with Mauna Kea visible in the background. USGS photo.

Earthquakes beneath Mauna Loa's summit prior to 1975 and 1984 eruptions. Green circles 0-5 km (0-3 mi) deep, red circles 5-10 km (3-10 mi) deep.

Earthquakes at Mauna Loa from June 2013 to June 2015. Green circles are 0-5 km (0-3 mi) deep and similar to ones prior to 1975 and 1984 eruptions, but cluster of 5-10 km (3-6 mi) deep quakes missing.

‘A‘ā lava flows erupt from the Northeast Rift Zone of Mauna Loa on March 25, 1984—the first day of the volcano's most recent eruption. USGS photo.

Map showing the lava flows that have erupted on Mauna Loa Volcano from 1843-1984.

A wide view of Kīlauea and Mauna Loa volcanoes, looking northwest. Pu‘u ‘Ō‘ō Crater, on Kīlauea's East Rift Zone, is the large fuming crater just to the left of the image center.

A sketch by Joseph Nāwahī showing the 1881 lava flow approaching Hilo. (Courtesy of National Park Service, Hawai‘i Volcanoes National Park, HAVO 394, Volcano House Guest Register 1873 to 1885, illustration by Joseph Nāwahī, February 21, 1881.)

Preview image for video: This Quicktime video shows a time-lapse sequence spanning from dawn to dusk on Tuesday, January 28, using images collected by our webcam near the summit of Mauna Loa Volcano (13,680 ft above sea level).

Preview image for video: This Quicktime video shows a time-lapse sequence spanning from dawn to dusk on Tuesday, January 28, using images collected by our webcam near the summit of Mauna Loa Volcano (13,680 ft above sea level).

During the 1926 Mauna Loa eruption, an ‘a‘ā flow about 457 m (1500 ft) wide and 9 m (30 ft) high headed straight for the village of Ho‘ōpūloa on April 18, as shown here. By the next day, the lava flow had destroyed a dozen houses, a church, and the wharf, and had nearly obliterated the bay. 

Erupting vents on Mauna Loa's Northeast Rift Zone near Pu‘u‘Ula‘ula (Red Hill) on Mar. 25, 1984—just hours after the eruption began—sent massive ‘A‘ā lava flows moving toward Hilo at 4 miles per hour. By the time the eruption ended on April 15, lava flows had reached to within four miles of Hilo city limits. USGS photo.

Island of Hawai‘i map, showing Mauna Loa and the other four volcanoes that make up the island. Mauna Loa structural features include summit caldera, rift zones, radial vents, and historical lava flows.

Thomas Jaggar and his wife, Isabel, hiking on Mauna Loa during the September 1919 Southwest Rift Zone eruption.

The summit of Mauna Loa (left) rises above the vog layer on the leeward side of the Island of Hawai‘i in November 2012. The flank of Hualālai is visible at right.

The Hawaiian Volcano Observatory and Jaggar Museum, shown in this photo, sit atop the summit of Kīlauea Volcano. The snow-capped summit of Mauna Loa, nearly 10,000 feet higher than Kīlauea, looms in the background.