The Hawaiian name "Mauna Loa" means "Long Mountain." This name is fitting, because the subaerial (above sea level) part of the volcano extends about 120 km (75 mi) from the southern tip of the Island of Hawai‘i across the volcano's summit to the eastern coastline near Hilo. Mauna Loa lava flows have also advanced to the north-northwest, reaching the ocean at Kīholo and PUAKU on the North Kona coast of the island.

Mauna Loa, the largest active volcano on Earth, rises about 4,170 m (13,680 ft) above the Pacific Ocean. But the submarine part of the volcano descends an additional 5,000 m (16,400 ft) to the sea floor, which is depressed by Mauna Loa's enormous mass another 8,000 m (26,250 ft). So, from its base to its summit, Mauna Loa is more than 17,000 m (56,000 ft) high.

Mauna Loa encompasses more than half the area of the Island of Hawai‘i. It is larger than all the rest of the Hawaiian Islands combined.

Additional information:

• How high is Mauna Loa above sea level?
• Mauna Loa is still the largest ACTIVE volcano on Earth

• Mauna Loa—History, Hazards, and Risk of Living with the World's Largest Volcano
• Mauna Loa is still the largest ACTIVE volcano on Earth

During the past 3,000 years, Mauna Loa has erupted lava flows, on average, every 6 years. Since 1843, Mauna Loa has erupted 33 times, averaging one eruption every 5 years. Dates, locations, durations, and other facts about Mauna Loa eruptions are compiled in "Geology & History."

Additional info about past Mauna Loa eruptions:

• The 1855-1856 Mauna Loa lava flow nearly devastated Hilo
  
• Hilo was again threatened by a Mauna Loa lava flow in 1881
  
• Mauna Loa's eruption of 1916: Great expectations
  
• There were technological leaps in volcano watching even 75 years ago (1933 eruption)
  
• A look back at Mauna Loa (1933 eruption)
  
• 1940 was a momentous year for Mauna Loa - and for Thomas A. Jaggar
  
• Sightseers jam roads and airspace during Mauna Loa's 1949 summit eruption
  
• Mauna Loa's most spectacular eruption(1950 eruption)
  
• Mauna Loa rumbled and stretched as it awakened from a 25-year slumber (1975 eruption)
  
• 1984 eruption

• Mauna Loa events in 1903 - separating the wheat from the chaff
  
• The Mauna Loa Eruption of 1926
  
• Shhh! Don't tell there is an eruption - Eruption of 1942
  

Eruptions on Mauna Loa typically occur within Moku‘āweoweo, a caldera (large oval depression) at the summit of the volcano, along one of its two rift zones (Northeast and Southwest), or from radial vents located on the north and west flanks, outside the caldera and rift zones.

The Mauna Loa Geology and History section includes additional information about the volcano's eruption History.

Mauna Loa erupted most recently in March-April 1984. Following more than two years of increased seismicity and summit inflation, the eruption began at 1:30 a.m., HST, on March 25, when a fissure opened in Moku‘āweoweo, the volcano's summit caldera. By 4:00 a.m., the eruption had migrated into Mauna Loa's upper Northeast Rift Zone, where active fissures eventually reached the 2,835-m (9,300-ft) elevation. Fast-moving ‘a‘ā flows advanced downslope, and, in a matter of days, lava was within 6 km (4 miles) of Hilo city limits. Fortunately, the eruption ended on April 15 before lava reached Hilo.

Volcanoes often show signs of unrest—increased seismicity (earthquakes), deformation (inflation) of the volcano's summit and flanks, and emission of volcanic gases—days to months in advance of an eruption. For example, Mauna Loa exhibited inflation and elevated seismicity prior to its two most recent eruptions, with increased rates of seismicity beginning one year before Mauna Loa erupted in 1975 and two years before it erupted in 1984.

USGS Hawaiian Volcano Observatory (HVO) scientists can detect and track these signs of volcanic unrest, but cannot forecast exactly when or exactly where lava will erupt on Hawaiian volcanoes until an eruption is about to happen. HVO has recently upgraded its monitoring networks to improve its ability to detect early unrest. Numerous seismic, GPS, and tilt stations across the flanks of Mauna Loa keep a vigilant eye on the mountain 24 hours a day.

Additional information:

• Restless activity beneath Mauna Loa continues
• Mauna Loa: Quiet for many years, but not to be forgotten Is Mauna Loa due to erupt soon?
• Volcano watching at Mauna Loa is a BIG job
• Volcanic gas measurements on Mauna Loa are an expanded challenge

The USGS Hawaiian Volcano Observatory has an extensive monitoring network of more than 100 field stations, each with multiple ground-based instruments to monitor volcanic activity in Hawaii. These instruments monitor for changes in:

• Number and size of earthquakes – Seismometers detect earthquakes and tremor.
• Deformation of the ground surface– Tiltmeters track changes in slope and GPS receivers track changes in elevation and position of the ground surface as the volcano deforms (in response to magma movement).
• Volcanic activity and eruption behavior – Webcams, thermal cameras, and time-lapse cameras record images of volcanic activity, often in remote locations.
• Subsurface processes – Gravimeters and other geophysical tools detect subsurface processes.

HVO scientists also use space-based technology to supplement ground-based techniques to monitor Hawaiian volcanoes. Satellites orbiting Earth provide data that can be used to identify and monitor thermal energy (heat) sources, discern and Global Positioning System (GPS), Interferometric Synthetic Aperture Radar (InSAR), and NASA Earth Observing 1 (EO-1) Satellite imagery. HVO is also exploring the use of infrasound sensors and other tools to help us evaluate changes in the volcanoes.

The evolution of HVO's monitoring tools and techniques is summarized in The Story of the Hawaiian Volcano Observatory—A Remarkable First 100 Years of Tracking Eruptions and Earthquakes.

Additional information:

• Mauna Loa Volcano monitoring map including earthquakes and instruments
• Mauna Loa Volcano: Current Monitoring Plots
• Mauna Loa Webcams
• An Analysis of Three New Infrasound Arrays around Kīlauea Volcano

Subscribe to the Volcano Notification Service and change your settings to receive status updates for Mauna Loa. All current reports are available on HVO website, and past reports can be searched on the Volcano Hazards Program website.

If the status of Mauna Loa changes, the USGS Hawaiian Volcano Observatory will elevate the Volcano Alert Level and Aviation Color Code for the volcano (see next question for more information about these levels and codes), in addition to issuing public notices through news media and HVO's website.

The USGS uses a standardized alert-notification system for characterizing the level of unrest and eruptive activity at U.S. volcanoes for people on the ground and in the air (aviation). The Volcano Alert Levels used by USGS volcano observatories are intended to inform people on the ground about a volcano's status. NORMAL indicates that a volcano is in a background, or non-eruptive state ADVISORY indicates that a volcano is exhibiting signs of unrest above known background levels but does not indicate an eruption is either likely or certain. WATCH indicates a volcano is showing heightened or escalating unrest with increased potential of eruption; it can also mean that an eruption is underway but poses only limited hazards. WARNING indicates a hazardous eruption is imminent, underway, or suspected to be occurring (when visual observations cannot verify an eruption in progress).

Aviation Color Codes are used in conjunction with the Volcano Alert Levels to provide information about volcanic-ash hazards in the atmosphere for the aviation sector (for example, airlines, dispatchers, air-traffic controllers, and pilots).

There are two main reasons why USGS scientists change the Volcano Alert Level from Normal to Advisory: (1) increased seismicity (earthquake activity) and (2) deformation of the volcano above typically quiet, or background levels. Such increases mean that changes are occurring within the volcano's magma storage system.

USGS Hawaiian Volcano Observatory scientists closely track Mauna Loa's seismicity, deformation activity, and gas emissions at the summit caldera through the HVO monitoring network, and they are prepared to deploy additional monitoring instruments on the volcano as needed. These might include additional seismometers, GPS receivers, webcams, and gas-measuring stations. HVO will also continue to provide information about Mauna Loa's status to Hawai‘i County Civil Defense, Hawai‘i Volcanoes National Park, and the public.

Additional information:

• Mauna Loa Volcano monitoring map including earthquakes and instruments
• Mauna Loa Volcano: Current Monitoring Plots
• Mauna Loa Webcams

At the ADVISORY level, an eruption of Mauna Loa is NOT considered likely or certain in the near future. When scientists declare an ADVISORY level for Mauna Loa, they will have determined that earthquake activity and changes in ground deformation (for example, ground tilt and spreading across the volcano's flanks) are occurring at rates above background levels. At the ADVISORY level, continued increases in earthquake and deformation activity are by no means certain—the level could return to NORMAL without an eruption.

Recent earthquakes beneath Mauna Loa have been smaller in magnitude and fewer in number than those that occurred prior to the volcano's two most recent eruptions in 1975 and 1984. Prior to Mauna Loa's 1975 and 1984 eruptions, shallow earthquakes significantly increased in number and frequency over a period of 1-2 years, clearly indicating that pressure was building within the volcano.

The locations of recent shallow earthquakes are similar to those prior to the 1975 and 1984 Mauna Loa eruptions, but other precursory signals are missing. These signals include stronger intermediate depth earthquakes and increasing rates of deformation (inflation) and seismicity.

The distinct clusters of intermediate depth earthquakes (red circles) located northwest of Moku‘āweoweo prior to the 1975 and 1984 eruptions have not been observed in recent years.

If seismicity and deformation rates increase above background levels, the Volcano Alert Level for Mauna Loa will be elevated from NORMAL to ADVISORY. However, an ADVISORY alert level does not mean that an eruption is likely to occur. See questions 11-14 for more information about Volcano Alert Levels.

Mauna Loa volcano Monitoring in near-real time is available in addition to data in various time frames, which include up-to-date deformation (GPS and tilt) and seismic data plots.

Mauna Loa tends to erupt more lava more quickly than Kīlauea—in other words, large volumes of lava erupt at higher effusion rates—so Mauna Loa produces voluminous, fast-moving lava flows. For comparison, during the 1984 eruption, Mauna Loa produced as much lava in 20 minutes as Kīlauea erupts in a day (at Kīlauea's eruption rate in 2015).

The slopes of Mauna Loa, especially on its southwest flank, are also quite steep compared to much of Kīlauea. These steep slopes also contribute to the speed of advancing Mauna Loa lava flows. Mauna Loa is similar to Kīlauea in that both volcanoes can produce ‘a‘ā lava or tube-fed pāhoehoe flows that can travel long distances from an active vent.

• voluminous, fast-moving lava flows
• potentially large and destructive earthquakes and ground motion due to vertical and horizontal movements of the volcano's flanks as it inflates
• volcanic gas emissions and dense vog (volcanic smog) that can be a nuisance or potential health hazard to people downwind of the vent
• possible explosive eruptions and associated ashfall that can impact air traffic

Additional information:

• Celebrating colossal Mauna Loa with a "Hi-yo, Silver!" (about vog)
• Mauna Loa's 1984 eruption taught us much about how lava flows work
• USGS Volcano Hazards Program – Types of Volcano Hazards
• Explosive Eruptions from Mauna Loa's Summit
• How fast does Hawaiian lava flow?

A Mauna Loa eruption could disrupt communication, traffic, and people's lives—disruptions that could be further complicated by the influx of large numbers of residents and visitors wanting to see lava flows. These spectators could add to the congestion of roadways and other infrastructure already impacted by the eruption.

It's important to note that these impacts will likely be far-reaching. In other words, the impacts will not necessarily be restricted to the immediate area of the eruption or only to the Island of Hawai‘i. For example, during the 1984 Mauna Loa eruption, vog (volcanic smog) blanketed much of the State of Hawaii.

Additional information:

Mauna Loa erupts lava at a much higher rate than any other Hawaiian volcano, even the highly active Kīlauea volcano. This results in fast-moving and long-travelled lava flows, which can require quick responses in order to protect life and property.

How long you have to respond to a Mauna Loa eruption depends on your proximity to the eruptive vent, how steep the slope is between you and the vent, and the rate at which lava is being erupted. If you are in close proximity to, or on a steep (more than 15 degrees) hillside downslope of, an erupting vent on Mauna Loa, you could have precious little time to respond or evacuate—perhaps only hours.

The Districts of Ka‘ū and South Kona on the Island of Hawai‘i are at significant risk during a Mauna Loa Southwest Rift Zone (SWRZ) eruption due to the steep slopes and proximity of developed areas relative to the rift zone. During most historic SWRZ eruptions (1868, 1887, 1919, 1950), lava flows reached the ocean in less than a day. In fact, during the 1950 Mauna Loa eruption, one lava flow traveled from the vent at an elevation of about 3,050 m (10,000 ft) to the sea in about 3 hours.

On the other hand, the District of Hilo is relatively far from the most active part of Mauna Loa's Northeast Rift Zone (NERZ) and the slopes above the town are fairly gentle. NERZ eruptions can certainly threaten Hilo (for example, the 1855-56 and 1881 lava flows covered land that is now within Hilo city limits), but the lead time for response and potential evacuation during a NERZ eruption is likely to be substantially longer (days to weeks) than for SWRZ eruptions (hours).

USGS Hawaiian Volcano Observatory scientists closely monitor Mauna Loa around the clock. If an eruption is likely, or is in progress, HVO will immediately notify Hawai‘i County Civil Defense, Hawai‘i Volcanoes National Park, and other emergency managers responsible for public safety. HVO will also issue public notifications through news media, and will post updates and status reports on the HVO website.

You can sign up to receive Volcano Activity Notices (updates, status reports, information statements, and alerts) via email for Mauna Loa and other Hawaiian volcanoes by signing up for the free USGS Volcano Notification Service.

HVO currently records a Mauna Loa update message that can be accessed by calling (808) 967-8866. This message is a summary of the Mauna Loa Status Report posted on the HVO website. The status report is updated as frequently as the status reports are released, which varies with alert level.

Preparing for an eruption can help you prepare for other emergencies, such as severe storms and earthquakes, so it's a good thing to do! Learn about the hazards that you might face during an eruption and how to evacuate from your home. Prepare an emergency kit and make a family plan. Know how to get information about the volcano should it become significantly restless or erupt.

Agencies that provide information on preparing for natural disasters and other emergencies include Hawai‘i Emergency Management Agency and the American Red Cross. FEMA offers a Ready.gov webpage that provides information specific to preparing for volcanic eruptions.