Hawaiian volcanoes can erupt either at their summits or on their flanks. Young Hawaiian volcanoes, such as Kilauea and Mauna Loa, have summit calderas. A caldera is a crater several miles in diameter that forms as the result of collapse when magma drains from beneath the summit. (Magma is the term used for molten rock that is still beneath the earth's surface; it is called lava when it reaches the surface.) Summit eruptions of Kilauea and Mauna Loa occur within or near their calderas. Flank eruptions usually take place along rift zones, which are highly fractured zones of weakness within the volcano. Rift zones typically extend from the summit of a volcano toward the coastline and may continue for many miles under the sea.
Lava flows are the most common of the direct hazards created by Hawaiian
eruptions and pose the greatest threat to property. Other hazards include
airborne particles of ash, cinder, and fragile strands of volcanic glass called
Pele's hair, and corrosive volcanic gases. Explosive eruptions occur much less
often than non-explosive eruptions at Hawaiian volcanoes but have been witnessed
in historic time. The greatest danger associated with explosive eruptions is
their potential to produce pyroclastic surges. These surges are highly
destructive, turbulent gas clouds that flow rapidly along the ground carrying
hot ash and rock fragments. A lesser volcanic hazard is created by ground
movement, which may result in large cracks across roads and other property or
cause uneven settling of foundations. Generally, only areas near an active or
recently active volcanic vent are affected by large-scale ground cracks and
Lava flows are the most common of the direct volcanic hazards in
Hawaii. Flows may endanger people's property, livelihood, and peace of mind, but
seldom their lives. The fronts of Hawaiian lava flows generally move more slowly
than the speed at which people walk, although the lava in the channel behind the
front may be flowing much faster. On steep slopes a large flow could travel
rapidly enough to endanger persons in its path. During the 1950 eruption of
Mauna Loa, a flow front advanced at an average speed of almost 6 mph for over 2
The speed of a lava flow is determined not only by the steepness of the terrain, but also by the volume of lava that is erupted, because large flows tend to advance more rapidly than do small flows. The distance that a flow travels ultimately depends both on the eruption rate and on the duration of the eruption.
The chemical composition of lava will also affect how rapidly a flow travels. Most Hawaiian lavas are classified as basalts, but within this category there are many types. Some basalts are more fluid and will flow at greater speeds than others. The eruption of Hualalai in 1800-1801 produced lava flows that appear to have been more fluid than flows from similar eruptions on Kilauea and Mauna Loa.
|An active lava pond overlies the Kupaianaha vent. Lave enters the lava tube system at the far end of the long neck on the pond.(Photograph by E.W. Wolfe, USGS)|
The continuing eruption on Kilauea's east rift zone, which began in 1983, provides a good example of two common, but very different, types of eruptive behavior: rapidly-moving flows produced during brief, high-volume eruptions, and slow-moving flows created by a prolonged low-volume eruption. The Pu'u 'O'o-Kupaianaha eruption on Kilauea's east rift zone, which began in 1983, provides a good example of two common, but very different, types of eruptive behavior: rapidly-moving flows produced during brief, high-volume eruptions, and slow-moving flows created by a prolonged low-volume eruption. The episodic eruptions at the Pu'u 'O'o vent, which was active from June 1983 through June 1986, produced a large volume of lava within a few hours' time. These outbursts were characterized by spectacular lava fountains and lava flows that moved rapidly down the volcano's south flank. The flows entered the Royal Gardens subdivision during 7 episodes and destroyed 16 homes. Each flow was short-lived, however, and stagnated soon after the lava fountains died. None of these flows reached the coastline.
In July 1986, the site of the eruption shifted to the Kupaianaha vent, 1.8 miles to the northeast of Pu'u 'O'o. Kupaianaha erupted almost continuously for over 5 years but at a much lower rate than Pu'u 'O'o. During the first few months of activity at Kupaianaha, the lava flows did not advance more than a mile beyond the vent. But after months of continuous eruption, a lava tube system formed as channeled lava flows gradually formed roofs, enclosing the rivers of lava within. Lava tubes are significant in terms of hazard, since the tubes insulate the lava and allow it to flow much farther before cooling and stopping.
|A Kalapana house is ignited by a lava flow. By 1991, this community lay buried beneath 50-75 feet of lava. (Photograph by J.B. Stokes, USGS)|
The hazards posed by a prolonged low-volume eruption soon became apparent as lava tubes from Kupaianaha extended toward the Kalapana coast. From November 1986 to October 1991, tube-fed flows repeatedly engulfed residential areas on the coastal plain, destroying 165 houses. Although these flows buried many acres within a single day, there was ample time to evacuate residents. Warnings issued by the Hawaii County Civil Defense allowed people enough time to remove most of their belongings and, in some cases, even to dismantle and move their homes. In 1992, the threat to inhabited areas eased when the eruption shifted to new vents on the southwest flank of the Pu'u 'O'o cone, inside Hawaii Volcanoes National Park.
The chief threat of lava flows to property owners is that the flows may burn structures and bury land. There are other effects, however, that may be almost as disruptive, as the Kalapana community discovered during the repeated inundations of the area by lava. In addition to destroying homes, the flows covered almost 2 miles of the coastal highway. Some residents were forced to move when the highway closure increased their daily commute by nearly 100 miles. Many more residents of the Kalapana area were faced with financial losses as land values dropped and insurance companies refused to issue new homeowners policies.
Even houses that are spared by the lava, however, may be rendered uninhabitable
when the roads and utility lines leading to them are destroyed. By 1996, lava
flows from Kilauea's eruption had covered 8 miles of the coastal highway,
isolating the few structures that remained within the area.
Airborne Lava Fragments
Most volcanic eruptions produce fragments of lava that
are airborne for at least a short time before being deposited on the ground.
These fragments are called "tephra," and include ash, cinders, and Pele's hair.
In Hawaii, tephra is usually ejected by lava fountains and poses a serious
hazard only in the immediate vicinity of an erupting vent. Windborne tephra,
however, can be disruptive at greater distances. The combination of high lava
fountains and strong winds may result in tephra being carried many miles
downwind of the eruption site. During lava fountaining episodes at Pu'u 'O'o
from 1984 to 1986, the prevailing trade winds deposited most of the tephra in
remote areas of Hawaii Volcanoes National Park, but small particles reached the
town of Naalehu 39 miles away. During the same episodes, Kona winds (from the
southwest) occasionally carried tephra to Hilo, 22 miles from the vent.
small amount of tephra that fell on inhabited areas was not harmful to most
people, but it was a source of irritation to those with respiratory problems and
an inconvenience to the many residents with rain-water-catchment systems.
Following at least three high-fountaining episodes, Hawaii County Civil Defense
recommended that people disconnect and clean their rain-water catchment systems
to prevent the particles from washing into their water supply.
|Each episode of high fountaining from Pu'u 'O'o released a tremendous plume of volcanic gas into the atmosphere. (Photograph by J.D. Griggs, USGS)|
Volcanic gases are emitted during all types of eruptions. Gases also can be released during repose periods by inactive eruptive vents and by fumaroles, vents that may never have produced any lava. The gas plume rising from an active vent on Kilauea consists of about 80 percent water vapor with lesser amounts of sulfur dioxide, carbon dioxide, and hydrogen. Small quantities (typically less than 1 percent by volume) of carbon monoxide, hydrogen sulfide, and hydrogen fluoride are also present. Extremely small amounts of mercury and other metals have been detected in gases emitted from vents along the east rift zone of Kilauea, but none have been found in concentrations large enough to create a direct health hazard.
Any hazard posed by volcanic gases is greatest immediately downwind from active vents; the concentration of the gases quickly diminishes as the gases mix with air and are carried by winds away from the source. Brief exposure to gases near vents generally does not harm healthy people, but it can endanger those with heart and respiratory ailments, such as chronic asthma.
A common gas produced during Hawaiian eruptions that is potentially harmful to human health is sulfur dioxide. Even small concentrations of sulfur dioxide can combine with water to form sulfuric acid, which can attack skin, cloth, metal, and other materials. When a volcanic plume mixes with atmospheric moisture, acid rain results. Acid rain can significantly retard the growth of cultivated or natural plant life downwind of a vent that degasses over a long period of time.
The sulfur dioxide emitted from Kilauea's summit during typical non-eruptive periods affects a relatively small area downwind of the summit. Similarly, the gases produced during short-lived eruptions affect only a limited area, although their odor may be detected many miles from the vent. The continuous emission of volcanic fumes during Kilauea's Pu'u 'O'o-Kupaianaha eruption, however, resulted in persistent volcanic haze and acid-rain conditions in the South Kona district on the leeward side of the island.
|Broccoli plants damaged by volcanic fumes from Pu'u 'O'o vent, 12 miles away. (Photograph by J.D. Griggs, USGS)|
In late 1987, studies conducted on private water-catchment systems in the South Kona area revealed higher than average acidity in several water samples. Drinking the acidic water does not pose a health hazard, but such water can leach lead from the lead roof flashings, lead-headed nails, and solder connections found in many plumbing systems, resulting in unsafe levels of lead in the drinking water. Extensive testing in 1988 determined that many water-catchment systems on the island, particularly those in the districts adjacent to or downwind of the active vent, contained elevated levels of lead. Residents with rain-catchment systems should contact the Hawaii State Department of Health for information on how to avoid lead contamination of their drinking water.
Volcanic fumes can also
damage agricultural crops. During the 1969-74 eruption of Kilauea's Mauna Ulu
vent, the South Kona district experienced prolonged periods of eruption-related
smog. A study conducted in 1972 by the University of Hawaii's Agricultural
Experiment Station at Hilo concluded that the acid rain resulting from the fume
was responsible for severe damage to the Kona tomato crop. The Pu'u
'O'o-Kupaianaha eruption of Kilauea caused similar problems for vegetable and
flower growers in both the Kona and Puna districts, who reported
light-to-moderate crop damage during periods when winds blew the gases over
The rare explosive eruptions in Hawaii generally are caused
by the interaction of magma and ground water. The magnitude of the resulting
steam explosion varies from harmless to catastrophic. Small steam-blast
explosions occurred during the 1960 Kapoho eruption when the magma beneath the
vents, which were near sea level, encountered saltwater trapped in the
surrounding rocks. These steam blasts ejected black clouds of pulverized rock
fragments but were of little hazard except to scientists working close to the
|The 1924 explosive eruption of Kilaeua mainly affected the immediate vivinity of Halemaumau crater. (Photograph courtesy of the Bishop Museum)|
A much larger steam-blast eruption occurred at the summit of Kilauea in 1924, when ground water apparently flowed into the heated rocks beneath the Halemaumau vent, which had been erupting nearly continuously for over a century. The explosions continued at intervals for 2 weeks, carpeting the area around Halemaumau crater with large rocks and a thin layer of ash. Boulders weighing several tons were thrown as far as 3,000 feet from the crater. The greatest hazard posed by this type of activity is that it may start abruptly and endanger unwary onlookers. The 1924 eruption claimed one fatality--a man who ventured too close to. the vent between explosions to take photographs and was struck by a rock when the activity suddenly resumed.
The largest explosive eruption on Hawaii within historical time occurred in 1790. This eruption produced pyroclastic surges (turbulent clouds of hot gas and rock fragments) that originated at Kilauea's summit and flowed several miles to the southwest. Pyroclastic surges are extremely dangerous because they move at speeds of 30 to 200 mph, and humans and animals caught in their path are killed by either asphyxiation or heat. A band of Hawaiian warriors traveling from Hilo to the Ka'u district to battle with Chief Kamehameha were overtaken by one of the 1790 pyroclastic surges, and about 80 of them were killed. The 1790 eruption left deposits of rock fragments and ash up to 30 feet thick on the rim of Kilauea's summit caldera.
The thick deposits of ash exposed at many sites on the island
indicate that even larger explosive eruptions occurred in prehistoric times and
probably originated from Mauna Kea as well as from Kilauea. Explosive eruptions
of any size take place infrequently in Hawaii, but the possibility of one
occurring in our lifetime should not be totally discounted. Such eruptions are
unlikely to begin without some warning. The most widespread hazard from an
explosive eruption would be windborne ash, which could damage structures,
machinery, and agricultural crops.
Ground Cracks and Settling
Ground cracks and settling are commonly associated
with volcanic activity; both generally occur near active or recently active
volcanic vents as the result of shallow underground movement of magma. The
beginning of an eruption at a new site is preceded by cracking of the ground as
magma is forcefully injected into the area. The cracks may be as much as 6 feet
wide and over a mile long; typically they form within a period of hours. The
Kapoho area on Kilauea's lower east rift zone experienced such ground breakage
prior to eruptions in 1924, 1955, and 1960.
Ground settling may occur near a vent at the end of an eruption as magma drains away from beneath the vent area. This process produces both small depressions and large collapse features, such as the pit craters and summit calderas of Kilauea and Mauna Loa. In either case, the subsidence may be gradual or abrupt.
The hazard presented by ground cracks
and settling associated with eruptions is usually limited to areas near the
active vent and thus is overshadowed by the hazard posed by lava flows. Man-made
structures that escape other damage from an eruption, however, can be damaged or
destroyed by cracking, tilting, or settling of the ground beneath them. Ground
cracks will remain after the eruption is over and can pose a threat to unwary
people and animals if the cracks are obscured by heavy vegetation.
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