Few places on Earth allow closer or more dramatic views of volcanic
activity than Mauna Loa and Kilauea volcanoes on the island of Hawaii.
Their frequent but usually non-explosive eruptions make them ideal for
scientific study. Kilauea’s eruptions are so intensely monitored that
scientists have assembled a detailed picture of the volcano’s magma
reservoir “plumbing” system and how it behaves before and during
eruptions. Studies of these volcanoes and the surrounding ocean floor
continue to improve our understanding of the geologic history of the
Hawaiian Island chain and the ability of scientists to determine
volcanic hazards that threaten island residents.
[Illustration: Hawaiian Volcanoes]
EXPLANATION
Volcano active during past 2,000 years
Potentially active volcano
Population centers
· 50,000 to 100,000
• 350,000 to 1,000,000
_PACIFIC OCEAN_
NIIHAU
KAUAI
OAHU
•Honolulu
MOLOKAI
MAUI
Haleakala
LANAI
KAHOOLAWE
HAWAII
Kohala
Mauna Kea
·Hilo
Hualalai
Mauna Loa
Kilauea
Lohi
Eruptions of Hawaiian volcanoes are typically non-explosive because of
the composition of the magma. Almost all of the magma erupted from
Hawaii’s volcanoes forms dark gray to black volcanic rock (called
_basalt_), generally in the form of lava flows and, less commonly, as
fragmented lava such as volcanic bombs, cinders, pumice, and ash. Basalt
magma is more fluid than the other types of magma (andesite, dacite, and
rhyolite). Consequently, expanding volcanic gases can escape from basalt
relatively easily and can propel lava high into the air, forming
brilliant fountains sometimes called “curtains of fire.”
Lava, whether erupted in high fountains or quietly pouring out, collects
to form flows that spread across the ground in thin broad sheets or in
narrow streams. The fluid nature of basalt magma allows it to travel
great distances from the _vent_ (the place where lava breaks ground) and
tends to build volcanoes in the shape of an inverted warrior shield,
with slopes less than about 10 degrees. Volcanoes with this kind of
profile are called _shield volcanoes_.
Hawaiian volcanoes erupt at their summit calderas and from their flanks
along linear rift zones that extend from the calderas. _Calderas_ are
large steep-walled depressions that form when a volcano’s summit region
collapses, usually after a large eruption empties or partly empties a
reservoir of magma beneath the volcano. _Rift zones_ are areas of
weakness within a volcano that extend from the surface to depths of
several kilometers. Magma that erupts from the flank of a volcano must
first flow underground through one of the volcano’s rift zones,
sometimes traveling more than 30 kilometers from the summit magma
reservoir before breaking the surface.
Mauna Loa.
Rising more than 9,000 meters from the seafloor, Mauna Loa is one of the
world’s largest active volcanoes; from its base below sea level to its
summit, Mauna Loa is taller than Mount Everest. It has erupted 15 times
since 1900, with eruptions lasting from less than 1 day to as many as
145 days.
The most recent eruption began before dawn on March 25, 1984. Brilliant
lava fountains lit the night-time sky as fissures opened across the
floor of the caldera. Within hours, the summit activity stopped and lava
began erupting from a series of vents along the northeast rift zone.
When the eruption stopped 3 weeks later, lava flows were only 6.5
kilometers from buildings in the city of Hilo. Mauna Loa erupts less
frequently than Kilauea, but it produces a much greater volume of lava
over a shorter period of time.
[Illustration: Lava fountains erupt from along Mauna Loa’s rift zone.
Fountains are about 25 meters high. (_Photograph by J.D. Griggs._)]
Kilauea Volcano.
Kilauea’s longest rift-zone eruption in historical time began on January
3, 1983. A row of lava fountains broke out from its east rift zone about
17 kilometers from the summit caldera; within a few months, the activity
settled down to a single vent. Powerful fountaining episodes hurled
molten rock 450 meters into the air and built a cone of lava fragments
that quickly became the tallest landmark on the rift zone.
The eruption changed style abruptly in July 1986 when lava broke out
through a new vent. Instead of regular episodes of high lava
fountaining, lava spilled continuously onto Kilauea’s surface. The
steady outpouring of lava formed a lake of molten rock that became
perched atop a small shield volcano. By June 1991, the shield was about
60 meters tall and 1,600 meters in diameter, and lava from the eruption
had covered 75 square kilometers of forest and grassland, added 120
hectares of new land to the island, and destroyed 179 homes.
[Illustration: Aerial view of Hawaii’s two most active volcanoes,
Kilauea and Mauna Loa. (_Photograph by J.D. Griggs._)]
Mauna Loa
Halemaumau Crater
Kilauea caldera
Although most of Kilauea’s historical rift eruptions were much briefer,
prolonged eruptive activity in the east rift zone from 1969 to 1974
formed a similar shield, Mauna Ulu (Hawaiian for “Growing Mountain”),
and an extensive lava field on the volcano’s south flank. The geologic
record shows that such large-volume eruptions from the rift zones and
the summit area, covering large parts of Kilauea’s surface, have
occurred many times in the recent past. In fact, about 90 percent of
Kilauea’s surface is covered with lava flows that are less than 1,100
years old.
[Illustration: The volcanic cone of Pu’u O’o, named after an extinct
Hawaiian bird, towers above an active lava lake (background).
(_Photograph by J.D. Griggs._)]
Most eruptions at Kilauea can be viewed at close range, but a few
historical eruptions were dangerously explosive. Fast-moving mixtures of
ash and gas, called _pyroclastic surges_, raced across the summit area
and into the southwest rift zone during an eruption in 1790. Footprints
preserved in a layer of ash 30 kilometers southwest of the summit
probably include those of a party of Hawaiian warriors and their
families who were crossing the volcano when the eruption struck. An
estimated 80 of the 250 people were killed by suffocating clouds
associated with the pyroclastic surges. A smaller explosive eruption in
1924 from Halemaumau Crater in Kilauea summit caldera, which killed a
photographer who was too close, hurled rocks weighing as much as 8 tons
as far as 1 kilometer.
[Illustration: Lava fountain erupting from Pu’u O’o cone. Forty-four
episodes of such fountaining between 1983 and 1986 built the cone 255
meters tall. (_Photograph by J.D. Griggs._)]