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Mauna Loa lava flows tend to be larger and move faster than at nearby Kilauea. HVO image from 1984, person for scale. (Edited and color balanced image from USGS HVO image library)

Mauna Loa erupted from NE rift zone vents high on the volcano in the early AM of 6 July 1975; lava flowed toward Mauna Kea. (image from USGS HVO website)

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Mauna Loa, or "Long Mountain" in Hawaiian, is located on the island of Hawaii. It is pictured above rising 13,680 ft. (4,170 m) above sea level (this photo was taken from over Loihi seamount, some 30 km or so to the south). Since 1832, Mauna Loa has erupted 39 times; its last eruption was in 1984. According to USGS estimates, the volcano has erupted an average of once every 6 years over the past 3000. The Hawaiian Volcano Observatory celebrated the 30th anniversary of Mauna Loa's last eruption (in 1984) on March 25, 2014, underscoring that it has been a realtively quite 3 decades at the world's largest active volcano.

How active? Mauna Loa is one of Earth's most active volcanoes. Since 1843 it has erupted 33 times producting lavas that have covered over 800 km ². The image at right shows Mauna Loa lava flows since 1832 (warm colors) overlain on a 3-d digital elevation model. The view is looking towards the west, with Hilo qt the bottom center, Kilauea at left, and Kona coast on the north. Learn all about the hazards associated with this eruption history at the HVO website

How Tall? Mauna Loa is a REALLY tall and big mountain. At 60 miles long and 30 miles wide, it makes up half of the entire island. When one considers that the flanks of Mauna Loa sit on sea floor that is about 16,400 ft (5,000 M) deep, the "height" of this volcano relative to neighboring land (the sea floor) is more like 30,080 ft (9,170 m)! Mauna Loa is the largest active volcano in the world. In fact, using this last measure of its height, it is one of the tallest mountain in the world (although many mountains, such as Mt. Everest in the Himalaya mountain range, sit higher relative to sea level). All large land masses (such as mountains) also push down upon the Earth's crust due to their enormous weight. So, directly beneath Mauna Loa, the sea floor on which it sits is depressed by and additional 26,000 ft (8000 m). Thus, if one wanted to say how thick (at its center) is the lava pile that makes up Mauna Loa, one would need to add its above sea level height, its sea floor to sea level height, and the thickness of its depression in the Pacific sea floor. These total 56,080 ft (17,170 m). For more details about estimation of the actual thickness of Mauna Loa volcano, see the write up on "How high is Mauna Loa volcano" at the HVO website.

Mauna Loa Awakening?
The summit of Mauna Loa began inflating slowly between 2002 and 2005 after a decade of slight deflation, after experiencing a brief swarm of deep long-period earthquakes. Inflation wasn't constant, with slowdowns in winter 2002-2003, Spring 2003, and since sept 2005. But, inflation was fairly steady in 2004 and the first half of 2005 following a more intense swarm of several thousand deep earthquakesin late 2004, all of which suggested that the magma reservoir within the volcano was swelling. Then inflation slowed again in 2006, ceased in late 2009, and resumed slowly in late 2010, continuing through mid-2015 (i.e., to date). Im late Aug 2015, HVO raised the alter level at Mauna Loa because of long-term increased seismicity rates and inflation of the summit (as measured by cross-caldera difference). This doesn't mean an eruption is imminent, but signs are consistent with magma entering the shallow part of the magmatic system at the volcano.
Earthquakes activity increased in early July 2004 and continued through Fall 2004, but then slowed significantly throughout 2005 and remained at a level of about 10 a week. Activity peaked in Sept. 2004 (for instance, there were more than 350 "long-period" earthquakes beneath Mauna Loa's summit and the upper southwest rift zone during the first week of that month, most which were deep (35 to 50 km) below the ground surface). That earthquake swarm was the greatest number of such earthquales since the beginning of the modern HVO earthquake catalog in the 1960s. HVO scientists continue to monitor the volcano closely. In an HVO press release on 26 Mar 2009, Frank Trusdell, an HVO geologist who has studied the volcano for nearly two decades, is quoted as saying that "Mauna Loa will erupt again, and there's a good chance that it will be during your lifetime." He and others are interesting in alerting Big Island of Hawaii residents about how to live safely and be aware of the potential hazards posed by Mauna Loa.
Views of some data that led to the Aug 2015 activity status upgrade:

The last decade of seismicity and deformation at Mauna Loa. Image redrafted from HVO website

Recent Mauna Loa earthquakes and before the last two historical Mauna Loa eruptions show a siumilar pattern. Image redrafted from HVO website

Get Mauna Loa activity updates from the HVO website

A peaceful Mauna Loa as viewed from the summit of Kilauea volcano in March of 1998

Mauna Loa is a "shield volcano", which means it is a gently sloping mountain produced from a large number of generally very fluid lava flows. The volcano has been erupting for at least 100,000 years (possibly more) from a primary volcanic center that is presently manifested by Moku'aweoweo crater at the mountain's summit. It has produced both a'a and pahoehoe flows, and lesser amounts of pyroclastic deposits. The volcano is constructed of a rock type known as tholeiitic basalt. Mauna Loa shares the Hawaiian hot spot with its smaller active siblings Kilauea and Loihi seamount. You can view a schematic representation of the geometry of this situation HERE. If you would like to learn more about how the Hawaiian islands formed from a single mantle hotspot, visit the Formation of the Hawaiian Islands web page at this site.

The earliest written account of a Mauna Loa eruption, based upon the observations of an elderly guide (Keaweehu) in the Wilkes exploration party to the summit in 1840-1841, says an eruption took place in 1780. This is shortly after Captain Cook's first visit to the islands. In 1868, and an eruption from Mauna Loa caused the largest recorded earthquake in Hawaii, a magnitude 8 by modern standards. The lava flow of 1855-1856 is said to be one of the greatest flows ever seen by modern observers. An eye witness account by the Rev. Titus Coan is available at this web site

The oldest remnants of subaerial Mauna Loa are to be found in an interesting rock formation known as the "Ninole Volcanic Series". It is a series of steep-sided hills, consisting of thin layers of pahoehoe and a'a lava. These hills form the so-called "Ninole Shield" which is thought to be either the remnants of a pre-Mauna Loa volcano, or uplifted blocks of old Mauna Loa from within the Honuapo-kaoiki fault system. Either way, they are some of the oldest exposed rocks on the southern part of the island.

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Mauna Loa lava flows tend to be larger and move faster than at nearby Kilauea. HVO image from 1984, person for scale. (Edited and color balanced image from USGS HVO image library) Mauna Loa erupted from NE rift zone vents high on the volcano in the early AM of 6 July 1975; lava flowed toward Mauna Kea.

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Currently under advisory, however it is not erupting.

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April 15, 1984
-no significant besides burying 16 miles of land owned by the state

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13,678′

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19.4721° N, 155.5922° W

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Located on the Island of Hawaii