auna Loa on the Island of 

 Hawai‘i is the world’s largest 

volcano. People residing on its flanks 

face many hazards that come with 

living on or near an active volcano, 

including lava flows, explosive eruptions, volcanic smog, damaging 

earthquakes, and local tsunami (giant 

seawaves). The County of Hawai‘i 

(Island of Hawai‘i) is the fastest growing County in the State of Hawaii. Its 

expanding population and increasing development mean that risk from 

volcano hazards will continue to 

grow. U.S. Geological Survey (USGS) 

scientists at the Hawaiian Volcano 

Observatory (HVO) closely monitor 

and study Mauna Loa Volcano to 

enable timely warning of hazardous 

activity and help protect lives and 

property.

M

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Moku‘āweoweo

Caldera

Northeast Rift Zone

Southwest Rift Zone

Hilo

Kïlauea

Hualälai

Mauna Kea

Kohala

Mauna Loa

Legend

Mauna Loa boundary

#* Tilt sites

$+ GPS 

!( Seismic sites

Mauna Loa—History, Hazards, and Risk of 

Living With the World’s Largest Volcano

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 2012–3104

August 2012

Mauna Loa, the world’s largest volcano, encompasses more than half the Island of Hawai‘i 

and rises to 4,169 meters (13,679 feet) above 

the Pacific Ocean. A very active volcano, it has 

produced large, voluminous flows of basalt (very 

fluid, dark lava) that have reached the ocean eight 

times since 1868. It last erupted in 1984, when a 

lava flow came within 7.2 km (4.5 miles) of Hilo, 

the largest population center on the island. With 

such a propensity to produce large flows, Mauna 

Loa is considered a “high priority” for monitoring by the U.S. Geological Survey (USGS). 

In Hawai‘i, eruptions of basalt are, in general, placid outpourings of fluid lava flows, building the gentle slopes characteristic of “shield” 

volcanoes. The greatest direct hazards associated 

with shield volcanoes are the lava flows and 

emissions of volcanic gases. Although these seldom cause deaths, lava flows typically do extensive damage by covering, burning, and crushing 

everything in their path. Basalt flows can travel 

more than 50 km from their vent, making most 

communities on the Island of Hawai‘i susceptible 

to lava-flow inundation. And gas emissions can 

be carried by the wind and reach other islands in 

the State, producing human health concerns and 

damaging crops and livestock.

Mauna Loa has a summit caldera (a large 

oval volcanic depression) and two rift zones 

Mauna Loa, the world’s largest volcano, has erupted many times, producing large and 

long-traveled lava flows, and it will erupt again and pose a significant risk to inhabitants 

of the Island of Hawai‘i. During its most recent eruption in 1984, a lava flow approached 

the city of Hilo, as seen in this nighttime photograph (photo by David Little).

0

0 10 Kilometers

5 10 Miles

Ü Rift zone

Legend

Radial vents

Mauna Loa boundary

Historical lava flows

Moku‘āweoweo

Caldera

Northeast Rift Zone

Southwest Rift Zone

Hilo

Radial Vents

Kīlauea

Hualālai

Mauna Kea

Kohala

Mauna Loa

South Kona

Waikoloa

Ka‘u

Hawaiian 

Ocean 

View 

Estates

Map of the Island 

of Hawai‘i, showing Mauna Loa 

and the other four 

volcanoes that 

make up the island. 

Structural features 

shown on Mauna 

Loa include the 

summit caldera, rift 

zones, radial vents, 

and historical lava 

flows.
extending northeast and southwest from the caldera. Most of Mauna Loa’s eruptions begin with 

lava fountains from a series of fissure vents in the 

summit region and then often migrate to vents 

down either rift zone. 

Mauna Loa eruptions produce lava at a 

much higher rate than those of any other Hawaiian volcano, even the highly active neighboring 

Kïlauea volcano, resulting in fast-moving and 

long-travelled lava flows. Such high eruption 

rates require quick responses in order to protect 

life and property. 

Eruptive History of Mauna Loa

Mauna Loa erupts frequently—over the 

past 3,000 years it has erupted lava flows, on 

average, every 6 years. Since 1843, Mauna Loa 

has erupted 33 times, averaging one eruption 

every 5 years. All historically recorded Mauna 

Loa eruptions started in the summit area, and 

approximately half have stayed in the summit. 

Seventeen continued from vents in the volcano’s 

rift zones, producing lava flows that covered 

broad areas on the lower slopes of the volcano. 

Seven flows from the southwest rift zone reached 

the ocean along the west coast of Hawai‘i within 

a matter of hours. In 1855–56 and 1880–81, lava 

from Mauna Loa’s northeast rift zone did not 

reach the ocean but covered land now within the 

city limits of Hilo. Geologic mapping of Mauna 

Loa shows that, in the past 150 years, 14 percent 

(714 km2

) of the volcano’s surface was covered 

by new lava flows. As much as 35 percent to 40 

percent (1,800 to 2,060 km2

)—an area equal to 

or greater than O‘ahu or Maui—was covered in 

the past 1,000 years.

Seismic station near north rim of 

Mokuÿäweoweo Caldera.

Seismic station at 10,000 feet 

elevation on the Southwest Rift 

Zone.

Seismic and GPS stations on Mauna 

Loa’s eastern flank (person at left gives 

scale).

Seismic station high on the Southwest Rift 

Zone.

Map of the Island of Hawai‘i, showing the numerous monitoring stations across the island in 2011. Both Mauna Loa 

and Kïlauea Volcanoes are intensively monitored by the 

Hawaiian Volcano Observatory. Inset photographs show a 

few of the stations on Mauna Loa.
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Moku‘āweoweo

Caldera

Northeast Rift Zone

one

Hilo

Kïlauea

Mauna Kea

Mauna Loa

Legend

Mauna Loa boundary

#* Tilt sites

$+ GPS 

!( Seismic sites

Aerial photograph looking north-northeast across the summit area of Mauna Loa (elevation 4,169 meters or 13,679 feet). At the summit is Moku’äweoweo Caldera, a volcanic 

depression 3 miles (5 kilometers) long.The pit craters in the foreground are part of the 

upper southwest rift zone. Mauna Kea Volcano rises on the skyline.

Risk Posed to Residents of the 

Island of Hawai‘i

According to the 2010 census, the Island of 

Hawaii is the fastest growing region in the State 

of Hawai‘i, with more than 100,000 residents 

and a population that grew by 24 percent in the 

past decade. High levels of interest and affordable land lead to developmental pressures, which 

means that more and more construction will 

occur on the flanks of the island’s volcanoes. 

This rapid growth includes several 

multimillion-dollar developments built or proposed on the slopes of Mauna Loa in Waikoloa, 

Hilo, and Hawaiian Ocean View Estates—the 

last is the Nation’s largest development in terms 

of area. Between 1984, the most recent eruption 

of Mauna Loa, and 2002, approximately $2.3 

billion were invested in new construction on the 

flanks of the volcano. 

Monitoring and Emergency Planning Are Ongoing

Volcanoes often show signs of unrest—

such as increased seismicity (earthquakes), 

deformation (inflation) of the volcano’s flanks, 

and emission of volcanic gases—days to 

months in advance of an eruption. The USGS’s 

Hawaiian Volcano Observatory (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. When unrest is 

detected, scientists notify emergency-management officials and increase monitoring efforts. 

Volcano Hazards from Mauna Loa

Lava-flow hazard maps prepared by the 

USGS for the Island of Hawai‘i divide the island into nine hazard zones, with Zone 1 being 

the area of greatest hazard. The criteria used to 

establish these zones are (1) proximity to the 

summits and rift zones of the active volcanoes, 

(2) frequency of lava coverage during historical time (since about 1800), (3) area covered 

within the past 750 years, and (4) topography. 

For Mauna Loa, Zone 1 includes the 

summit and rift zones and Zone 2 includes 

the regions downslope and adjacent to Zone 

1. Zone 3 includes regions that appear less 

hazardous because of greater distance from 

the summit and rift zones. These three zones 

cover most of Mauna Loa, although portions 

of the east and southeast flanks are protected 

from lava flows by topography and consequently are in a lower hazard designation of 

Zone 6. 

Although the threat posed by eruptions 

from Mauna Loa’s summit and rift zones is 

clear, the threat posed by vents outside these 

areas was not fully appreciated until recently. 

Such “radial vents” occur over a wide area 

on Mauna Loa’s north and west flanks, and 

new geologic mapping, additional age dating, 

and bathymetric surveys have resulted in the 

discovery of additional ones. Radial vents are 

located at all elevations from the summit of the 

volcano, at more than 4,000 m (>13,000 ft), 

to below sea level on the submarine flanks. Of 

the more than 45 known eruptions from radial 

vents, 3 have occurred in the past 150 years 

and 2 of those reached the sea.

0

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Ü

Legend

Radial vents

Mauna Loa boundary

Historical Flows

Mauna Loa Hazard zones

1

2

3

6

Roads

Moku‘āweoweo

Caldera

Northeast Rift Zone

Southwest Rift Zone

Hilo

Kīlauea

Hualālai

Mauna Kea

Kohala

Mauna Loa

3

3

3

3

6

6

2

2

1

1

Increasing severity

j

 of hazard

South Kona

Ka‘u

Waikoloa

Hawaiian

Ocean View

Estates

Map of the Island of 

Hawai‘i, showing the 

lava-flow hazard zones 

only for Mauna Loa. 

Lower numbers indicate 

greater lava-flow hazards; for example, the 

rift zones and summit 

area of the volcano, 

the source regions for 

31 out of 33 historical 

eruptions, are in zone 

1. Also shown are some 

places mentioned in 

the text. (Modified after 

Wright and others, 

1992).
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Moku‘āweoweo

Caldera

Northeast Rift Zone

 Rift Zone

Hilo

Kïlauea

Mauna Kea

Mauna Loa

Legend

Mauna Loa boundary

#* Tilt sites

$+ GPS 

!( Seismic sites

Characterizing the hazards posed by radial 

vents is a challenge. Radial vents are scattered 

over a broad area, unlike the narrow, well-defined 

rift zones of Mauna Loa, which more easily allow 

forecasting of future eruption locations. By using 

the same criteria that define lava-flow hazard 

zones on the current Lava Flow Hazards map with 

the new data on Mauna Loa’s radial vents, Zone 

2 should now extend down the northwest flank 20 

km toward the coast, a distance comparable to the 

width of Zone 2 on the west side of the southwest 

rift zone.

If a Mauna Loa eruption occurs along the 

southwest rift zone, slopes steeper than 15 degrees, 

a history of high effusion rates, and the close 

proximity of developed areas put South Kona and 

Kaÿü at significant risk. Precious little time may be 

available for response or evacuation efforts. On the 

other hand, the Hilo District is relatively far from 

the most active part of Mauna Loa’s rift zones, and 

the slopes above that city are fairly gentle. Thus, 

while northeast rift zone eruptions can certainly 

threaten Hilo (parts of which are built on the 1881 

flow), the lead time for response and evacuation 

is likely to be substantially longer (days to weeks) 

than for eruptions on the southwest rift zone 

(hours). 

Other hazards posed by Mauna Loa include 

earthquakes and ground motion due to volcanic 

activity and to vertical and horizontal movements 

of the volcano’s flank that can cause significant 

damage. As magma enters and inflates Mauna 

Loa, the volcano becomes unstable, setting the 

stage for the flank to slip and generate large 

earthquakes. Extremely destructive earthquakes 

have occurred beneath the Island of Hawai‘i 

during times of inflation—notably the magnitude 

8 earthquake of April 2, 1868, on Mauna Loa 

and, more recently, the magnitude 7.2 earthquake 

of November 29, 1975, on Kïlauea. Rapid flank 

adjustments that accompany large earthquakes 

can affect hundreds of square kilometers and can 

generate tsunamis that pose great risks to coastal 

communities. Although earthquakes cannot 

be predicted, people can be prepared for them 

through sound construction of buildings and 

protection of valuables. 

Plumes of volcanic gases associated with 

eruptions, otherwise known as “vog” or volcanic 

smog, are also a concern—a nuisance and potential health hazard to people downwind and a threat 

to agricultural crops, livestock, and native biota. 

During the 1984 eruption of Mauna Loa, vog 

blanketed much of the State of Hawaii. 

Explosive eruptions at the summit of Mauna 

Loa are a new discovery, and ash deposits on 

the southeast flank of the volcano are evidence 

of a potentially explosive past. Tiny chilled lava 

particles from high lava fountains, as well as ash 

from more powerful explosive eruptions, can drift 

into the airline approach corridors for Hilo and 

Kona airports.

Eruptions are an inevitable part of Mauna 

Loa’s future, and risk will continue to grow as 

Frank A. Trusdell

Edited by

Peter H. Stauffer

Graphic design by

Stephen L. Scott

population and development increase. But the hazards associated with this majestic volcano can be 

anticipated, and the risks diminished substantially, 

by vigilant monitoring of the volcano’s behavior, 

careful documenting of its past behavior as a guide 

to the future, and educating residents, visitors, and 

local leaders. 

What You Can Do if You Live On or Near 

Mauna Loa

• Learn about the volcano hazards that 

could affect your community. 

• Develop an emergency plan with your 

family so that you are prepared for 

natural hazards and emergencies.

• Help your community be prepared. A 

few moments spent in preparation now 

could keep you, your family, and your 

community safe when Mauna Loa next 

erupts. 

The primary mission of HVO is to issue 

timely and accurate warnings of hazardous 

volcanic activity, and one of its goals is to assess 

volcano hazards and promote public awareness 

through education. HVO is part of the USGS 

Volcano Hazard Program’s ongoing efforts to 

~3 hrs

14.5 hrs

18 hrs

~24hrs

~4 days

29 hrs

3.5 hrs

8 days

280 days

12 Mm3/d

6 Mm3

/d

3 Mm3

/d

0

0 10 Kilometers

Ü5 10 Miles

Legend

Mauna Loa boundary

Historical flows

Slope

High 

Low

South Kona

Ka‘ü

Hilo

Waikoloa

Slope map of Mauna Loa. Warmer colors indicate steeper slopes. Large, 

bold numbers record the average rates of effusion (outpouring of lava) for the 

different parts of the volcano in millions of cubic meters per day (Mm3

/d). By 

comparison, Kïlauea erupts 0.5 Mm3

/d. Historical flows are shown in gray, 

and the numbers along the coastline indicate the approximate time that lava 

flows took to reach the sea after the vent(s) opened high on the volcano.

help protect people’s lives and property in all of 

the volcanic regions of the United States, including Hawai‘i, California, the Pacific Northwest, 

Wyoming, Alaska, and the Commonwealth of the 

Northern Marianas.

For more information contact:

U.S. Geological Survey

Hawaiian Volcano Observatory

P.O. Box 51

Hawai‘i National park, HI 96718

http://hvo.wr.usgs.gov/

or

USGS Volcano Hazards Program

http://volcanoes.usgs.gov/

See also The National Volcano Early Warning System 

(NVEWS)

 (USGS Fact Sheet 2006–3142) 

This Fact Sheet and any updates to it are available online at

http://pubs.usgs.gov/fs/2012/3104/