K

Kïlauea—an Explosive Volcano in Hawai‘i

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 2011–3064

July 2011

Kïlauea Volcano, on the Island of Hawai‘i, is best known for its frequent sustained eruptions 

of lava flows and lava fountains, which are gentle enough that they can be observed and 

enjoyed from close range. Occasionally, however, Kïlauea erupts explosively, hurling out 

rocks and plumes of fine pulverized material. Several small explosions occurred in 2008 

and 2011, like this one on October 12, 2008, but much larger explosive eruptions have occurred in past centuries, sometimes with deadly consequences. Inset shows a large block 

thrown out of Halema‘uma‘u Crater during an explosive eruption on March 19, 2008.

At 2:58 a.m. on March 19, 2008, a 

gaping, glowing hole, visible from the U.S. 

Geological Survey’s (USGS) Hawaiian 

Volcano Observatory (HVO) 1.8 km (1.1 mile) 

away, opened in the floor of Halema‘uma‘u 

Crater almost beneath the visitor overlook. A 

scientist working all night at HVO heard and 

felt nothing. Just after dawn, three geologists 

drove to investigate. Thick, sulfurous fume 

blowing across the road 500 m (yards) 

downwind from the hole obscured visibility. 

The driver, knowing the road was straight, 

tightly gripped the steering wheel and crept 

ahead. Crunch, bump—oh no, must have 

driven off the road! Then came the sudden 

realization that the car was still on the road 

but was lurching over rocks blown out of 

the crater. This was how the first explosive 

eruption at the summit of Kïlauea since 1924 

was discovered.

Later in 2008 and then in 2011 eight 

more similar events took place. All were 

 ïlauea Volcano on the Island 

 of Hawai‘i, though best 

known for its frequent quiet eruptions of lava flows, has erupted 

explosively many times in its 

history—most recently in 2011. 

At least six such eruptions in 

the past 1,500 years sent ash 

into the jet stream, at the cruising altitudes for today’s aircraft. 

The eruption of 1790 remains 

the most lethal eruption known 

from a U.S. volcano. However, the 

tendency of Kïlauea’s 2 million 

annual visitors is to forget this 

dangerous potential. Cooperative 

research by scientists of the U.S. 

Geological Survey, Smithsonian Institution, and University of 

Hawai‘i is improving our understanding of Kïlauea’s explosive 

past and its potential for future 

violent eruptions.

tiny by comparison to explosive eruptions in 

Kïlauea’s past, but they jarred the public into 

realizing that Kïlauea could be explosive. 

Long considered a “milquetoast” of a volcano, 

erupting only photogenic lava fountains and 

flows, Kïlauea has the capability of quickly 

turning violent. Its eruption of 1790 killed a 

large number of people (estimates, all made 

decades afterwards, range from about 80 to 

more than 5,000) within a few hundred meters 

of where the Jaggar Museum and the adjacent 

Hawaiian Volcano Observatory stand today. 

And, in 1924, a too-adventurous accountant 

was killed while photographing the largest in 

a 17-day series of explosive eruptions at the 

volcano's summit. 

Although they are not its usual style, 

Kïlauea produces explosive eruptions about as 

often as does Mount St. Helens in Washington 

State—several periods of multiple explosive 

events every millennium. Those at Kïlauea 

are generally smaller in volume and in area 

affected, but their risks are substantial. More 

than 5,000 people a day visit Kïlauea’s 

summit, where most of the explosive eruptions 

take place, and hundreds live nearby. A busy 

highway now passes through the area where 

so many died in 1790—a tragedy that, by 

itself, makes Kïlauea historically the most 

lethal of all volcanoes in the United States.

What Is the Nature of Explosive 

Eruptions at Kïlauea and How Large 

Can They Be?

Technically, even lava fountains are 

explosive eruptions. For the purpose of this 

fact sheet, however, an eruption at Kïlauea

is considered to be explosive if fragments of 

broken rock from the vent walls (called “wall 

rocks”) are ejected, generally but not always 

accompanied by fresh shreds and droplets 

of magma (molten rock). Lava fountains at 

Kïlauea eject only fresh magma particles and 

so are not considered explosive; in contrast, 

the explosive eruption of March 19, 2008, 

threw out only wall rocks.

Most explosive eruptions at Kïlauea are 

relatively small, but they can be hazardous 

to people and infrastructure. The pulverized 

wall rocks (and magma droplets) fall as 

volcanic ash around the vent and downwind. 

During heavy ash fall, breathing is difficult, 

and the weight of ash, especially when wet, 

can cause roofs of buildings to collapse. 

Such hazards are mainly of concern to people 

	 Kilauea...has not always been

	 	 the tame creature of today.

Hitchcock, C.H., 1909, Hawaii and its 

 Volcanoes, p. 167
In explosive eruptions, larger fragments of wall rock 

are thrown out 

shorter distances 

than smaller fragments, as shown 

here by samples of 

fragments ejected by 

the Kïlauea eruption 

of around 850-950 

C.E. Small dots are 

observation sites, 

larger dots are sites 

of photographed fragments. Inferred extent 

of ash fall from the 

eruption is shaded.

near the summit area, but more distant 

effects can also be substantial. In 1924, ash 

falling 40 km (24 miles) from the summit 

shut down a railroad, and a layer of ash 

amounting to 4.9-7.4 tonnes/hectare (2-3 

tons/acre) accumulated 30 km (18 miles) 

away, contaminating grass used for livestock 

feed. The 2008 explosive eruptions were 

much smaller than those of 1924, creating 

a hazard only directly adjacent to the vent. 

Such tiny eruptions could have happened 

many times in the past but their record may 

be overlooked today because they left such 

limited, transient deposits.

Much more powerful eruptions occurred 

in the more distant past. One large explosive 

eruption at Kïlauea’s summit 1,000-1,200 

years ago sent golf-ball-size dense wall rocks 

to the south coast of the island, 18 km (11 

miles) away. One block 24 cm (9.5 inches) 

across and weighing 4.43 kg (9.7 lbs) fell 

5 km (3 miles) from the vent. Several other 

large explosive eruptions, including the 

deadly one in 1790, produced volcanic ash 

that blew southeast, east, and northeast of the 

summit, as recognized from the distribution 

of the ash deposits. Dispersal of ash in those 

The deadly explosive eruption of Kïlauea in 1790 hurled numerous large blocks of 

rock—more than 1 meter (more than 3 feet) in diameter—out of the caldera, some as 

far as 2 kilometers (1.4 miles) beyond the rim.

National

Park

Headquarters

Hawaiian

Volcano

Observatory

directions could not have resulted from the 

normal trade wind, which blows from the 

northeast, or the less common south wind. 

Instead, such easterly dispersal implies that 

the eruption columns penetrated high into, or 

even above, the west winds of the subtropical 

jetstream, which in Hawai‘i blow year-round 

at altitudes of 4-17 km (13,000-56,000 feet). 

Volcanic ash drifting at these altitudes poses 

a clear hazard to modern aircraft—a chilling 

prospect never dreamed of a few years ago 

for Hawaiian volcanoes.

Explosive eruptions of Kïlauea between 

1500 and 1790 included the occurrence of at 

least a dozen fast-moving, ground-hugging 

clouds of ash, rock, and volcanic gas. Such 

“pyroclastic surges” are among the most 

dangerous of all volcanic phenomena—

speeding at hurricane velocities and having 

temperatures of several hundred degrees 

Celsius. Ongoing research indicates that 

a surge probably caused the deaths in 

1790. Surge deposits have been recognized 

as far as 3.5 km (2.1 miles) outside the 

summit caldera (the 4 km by 3 km volcanic 

depression within which lies the vent of 

Halema‘uma‘u Crater).

Although explosive eruptions at Kïlauea 

are infrequent in human terms, they are 

not rare geologically. Besides the eruption 

of 1924, significant explosive eruptions 

occurred repeatedly during a 300-year period 

between about 1500 and 1790, and also 

repeatedly between about 500 and 1000 C.E. 

Many deposits of older explosive eruptions 

are known but poorly understood. The 

eruptions appear to occur in clusters, during 

times when a deep caldera exists at Kïlauea’s 

summit. This is clear for the 1500-1790 and 

500-1000 periods. The 1924 events, however, 

don’t fit this picture easily.

Not all of Kïlauea’s explosive eruptions 

take place at the volcano’s summit. Several 

hundred years ago, blocks of wall rock were 

ejected from ‘Alae Crater (now buried by 

recent lava flows) and from Pu‘ulena, both 

on the east rift zone (a zone of fractures and 

eruptive vents trending east from the summit 

area). Even Pu‘u‘Ö‘ö, site of an ongoing 

eruption on the east rift zone since 1983, had 

small explosive eruptions in 1987, 1991, and 

Kïlauea

Caldera

5 Miles

0 5 Kilometers

0

00

820

INCM

Pacific Ocean
GROUNDWATER

WATER TABLE

ROCK

FALL

BLOCKAGE

LAVA

LAKE

Explosive eruptions at Kïlauea are thought 

to be caused when water comes into contact 

with hot or molten rock (magma) and flashes 

into steam. In 1924, this happened after the 

level of the lava lake in Halemauÿmaÿu Crater 

dropped below the water table. The walls of the 

crater then collapsed and blocked the opening 

down which the lava had drained, allowing 

steam pressure to build up and cause violent 

explosions.

WHAT CAUSES EXPLOSIVE ERUPTIONS AT KÏLAUEA?

Some large explosions from Kïlauea’s summit 

in the past have sent eruption clouds so 

high that they entered the west winds of the 

jetstream. As a result, their ash deposits were 

dispersed mainly in easterly directions. Shown 

here are the known distributions for explosive 

eruptions in 850–950 C.E., about 1650 C.E., 

in the mid-18th century (?), and in 1790 C.E. 

By contrast, the spectacular lava fountains of 

the 1959 eruption, the largest in the past 200 

years, only sent material some hundreds of 

feet into the air, where it was dispersed to the 

southwest by the prevailing low-level trade 

winds (shown by solid red oval). Although the 

on-land extent of the ash deposits is fairly 

well known, except for the mid-18th century 

deposit, all those ash layers extend offshore 

for unknown distances.

ASH DISPERSION BY THE JETSTREAM

1959

Pacific Ocean

850-950 C.E.

~1650 C.E.

Mid-18th century?

1790 C.E.

Puÿulena z o n e Puÿu ÿÖÿö

Mauna Loa

Kïlauea

12 Kilometers

N

12 Miles

Island of 

Hawai‘i Hilo

Area of

map

0

0

MAIN WIND DIRECTIONS

Kona winds blow

 north

Trade winds blow

 southwest

High-level westerlies

 (jetstream) blow

 into eastern sector

 of volcano

N

E a s t

Kïlauea

r i f t

ÿAlae Crater (site)

1996 and tiny bursts in 2008 and 2009; these 

eruptions were similar in size to those at the 

summit in 2008.

What Causes Kïlauea’s Explosive 

Eruptions?

Groundwater or surface water probably 

has played a major role in most of Kïlauea’s 

explosive eruptions. If water is heated under 

pressure and the pressure then relieved, 

it can flash into steam, rapidly expanding 

and providing the propellant for explosive 

eruptions. Scientists theorize that, when the 

caldera is deep enough to be at or below the 

water table (the level below which rocks are 

saturated with water), water can seep into the 

vent and explosive eruptions can take place. 

The water table is currently about 500 m 

(1,640 ft) below the floor of the caldera. If the 

caldera collapsed to about that level—as it did 

around 1500 C.E., probably remaining there 

until 1790—explosive eruptions could likely 

occur. In 1924, geologists noted that lava 

drained from Halema‘ma‘u Crater to a depth 

apparently below the water table; rock falls 

from the collapsing walls of the crater dammed 

rising steam, pressure built up, and explosive 

eruptions occurred. 

Not all of Kïlauea’s explosive eruptions 

may be related to groundwater, however. The 

summit explosive eruptions of 2008, and 

likely also those at Pu‘u‘Ö‘ö, were apparently 

propelled by gas carried by, and released 

from, magma independent of groundwater. 

The very powerful eruption around 800-1000 

C.E. carried up fragments of almost solidified 

magma that was apparently crystallizing at 

a depth of 5-6 km (3-3.5 miles) when the 

eruption began. At such depths, carbon dioxide 

(CO2) is bubbling from the magma, and 

perhaps a buildup in CO2 pressure triggered 

that explosive eruption.

Can Explosive Eruptions Be 

Foreseen?

Could we forecast future explosive 

eruptions at Kïlauea? Whenever the bottom 

drops out of the caldera, or of a crater 

(like Halema‘ma‘u) within it, explosive 

eruptions become a realistic possibility. 

But explosive eruptions driven mainly or 

exclusively by magmatic gas may be harder 

to anticipate. The explosive event in March 

2008, for example, was foreseen in only the 

most general way as a possible outcome 

of increased summit unrest (increased 

seismicity and gas output)–and everyone 

was surprised by the form it actually took. 

Deep-seated pressure buildup, such as might 
SIX EXPLOSIVE ERUPTIONS WITH JETSTREAM COLUMN HEIGHTS COMPARED 

WITH THE LARGEST 2008 ERUPTION (BOLD).

Date Dispersal

direction 

Material1

ejected

Geologic unit

of deposit

Nov 1790 ESE 0.02 Wall-rock lapilli and 

ash

Keanak ko‘i 

Tephra

Mid 18th

century(?)

ENE ? Wall-rock lapilli and 

ash

Keanak ko‘i

Tephra

Mid-late 17th

century

SE <0.01? Fine wall-rock ash Keanak ko‘i 

Tephra

~1650 SE 0.015 Scoria with wall rock Keanak ko‘i 

Tephra

850-950 C.E. SE 0.02 Scoria with wall rock Kulanaokuaiki 

Tephra

400-640 C.E. SE ? Scoria with wall rock Kulanaokuaiki 

Tephra

Sept. 2, 2008 SW 0.0000008 Wall-rock lapilli and 

ash

2008-present 

summit eruption

Volume,

cubic km

(cubic mi) 

(0.0000002)

(0.005)

(<0.0025?)

(0.003)

(0.005)

1

2

Footprint-bearing

ash bed 1790

~1650

~1500

Date (C.E.)

ASH DEPOSITS FROM KÏLAUEA'S EXPLOSIVE ERUPTIONS

1

The general term for fragmental volcanic material is tephra. Tephra particles less than 2 mm 

(0.08 in) in diameter are called ash; particles between 2 mm and 64 mm (2.52 in) are called 

lapilli; the term scoria refers to very crystal-rich tephra of ash and lapilli size.

Don Swanson1, Dick Fiske2, Tim Rose2, Bruce

Houghton3, and Larry Mastin4

1

U.S. Geological Survey, Hawaiian Volcano Observatory, 

Hawaii

2

Smithsonian Institution, Washington, D.C.

3

University of Hawai‘i, Honolulu, Hawaii

4

U.S. Geological Survey, Vancouver, Washington

Edited by Peter H. Stauffer

Graphic design by Stephen L. Scott

COOPERATING ORGANIZATIONS

Smithsonian Institution

University of Hawai‘i

National Park Service

For more information contact:

Don Swanson

Hawaiian Volcano Observatory

Hawaii National Park, HI 96718

donswan@usgs.gov

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

http://pubs.usgs.gov/fs/2011/3064/

Inclined ash beds in pyroclastic surge deposit probably dating from the deadly 1790 

eruption. Such surges are common explosive deposits at Kïlauea and would be deadly 

to anyone in their path. The knife rests on a massive fine ash bed, the surface of which 

is indented by thousands of human footprints west and southwest of the caldera. 

occur before a CO2-driven explosive eruption, 

would be very difficult to detect with current 

monitoring capabilities.

As volcanoes go, Kïlauea is a pretty safe 

place, generally producing lava fountains 

and lava flows quietly when it erupts. Once 

in a while, however, it will erupt explosively. 

That is when the volcano becomes the 

most hazardous and when lives could be 

lost without prompt, proper warnings and 

actions. USGS scientists working out of 

HVO, and their collaborators from the 

Smithsonian Institution, the University of 

Hawai‘i, and other organizations, continue 

to closely monitor Kïlauea and study how 

the volcano works. Improved understanding 

of its workings will enable better protection 

of the public from the hazards of Kïlauea’s 

occasional explosive episodes.

This exposure of layered ash 

beds on the south side of 

Kïlauea Caldera consists of 

more than 11 meters (36 feet) of 

deposits from explosive eruptions between 1500 and 1790 

C.E. White numbers identify two 

events whose eruption columns 

reached the jet stream: (1) eruption of about 1650; (2) eruption 

of 1790, the deposit of which 

lies on the ash bed that elsewhere bears human footprints.