The Alaska Peninsula and the Aleutian Islands have about 80 major
volcanic centers that consist of one or more volcanoes. Recent violent
eruptions have demonstrated that volcanic hazards do exist in some areas
of Alaska, even though it is sparsely populated. Alaskan volcanoes have
produced one or two eruptions per year since 1900. At least 20
catastrophic caldera-forming eruptions have occurred in the past 10,000
years; the awesome eruption of 1912 at Novarupta in the Katmai National
Monument is the most recent. Scientists are particularly concerned about
the volcanoes whose eruptions can affect the Cook Inlet region, where 60
percent of Alaska’s population lives.
[Illustration: Alaskan volcanoes.]
EXPLANATION
Volcano active during past 2,000 years
• Population center 100,000 to 350,000
ALASKA
Wrangell
Hayes
•Anchorage
Spurr
Redoubt
Iliamna
Akutan
Novarupta
Augustine
_Cook Inlet_
Trident
Bogoslof
Ukinrek
Martin
Mageik
Kagamil
Chiginagak
Ugashik-Peulik
Carlisle
Emmons Lake
Yantami
Kiska
Cerberus
Fisher
Dutton
Aniakchak
Veniaminof
Little Sitkin
Kasatochi
Amukta
Pyre
Pavlof
Isanotski
Westdahl
Shishaldin
Makushin
Gareloi
Vsevidof
Okmok
Tanaga
Korovin
Cleveland
Kanaga
Great Sitkin
Yunaska
[Illustration: Redoubt Volcano, Alaska, erupting on December 16, 1989.
(_Photograph by National Park Service._)]
Redoubt Volcano.
Redoubt Volcano erupted for the fourth time this century on December 14,
1989. Following several days of strong explosive activity, a series of
lava domes grew in Redoubt’s summit crater during the next four months.
Most of the domes were destroyed by explosions or collapsed down the
volcano’s north flank. Some of these events triggered small pyroclastic
flows that melted snow and ice on the volcano to form lahars in Drift
River Valley, which empties into Cook Inlet 35 kilometers away.
Ash produced by the eruptions severely affected air traffic enroute to
Anchorage, Alaska’s largest city and a major hub of domestic and
international commercial air traffic. Many domestic carriers suspended
service to Alaska following major explosive events, and several
international carriers temporarily rerouted flights around Alaska. On
December 15, a jetliner enroute to Japan encountered an ash cloud while
descending into Anchorage. The plane quickly lost power in all four
engines and lost 4,000 meters in altitude before the pilots were able to
restart the engines. The aircraft landed safely in Anchorage, but it
sustained more than $80 million in damage.
[Illustration: Lava dome in the summit crater of Redoubt Volcano, which
grew between April 21 and June 1990. (_Photograph by David Wieprecht._)]
Lahars generated during the eruption threatened an oil-storage facility
located on the banks of Drift River. Oil is pumped from more than a
dozen wells in Cook Inlet to the facility and then loaded onto tankers,
which dock just offshore. A lahar on January 2 flooded part of the
facility with nearly a meter of water, forcing its shutdown until
workers could restore power. This and subsequent lahars prompted the
Cook Inlet Pipeline Company to temporarily halt oil production from some
oil wells and reduce the amount of oil stored at the facility between
tanker loadings.
[Illustration: Rock from the lava dome of Redoubt Volcano deposited in a
river valley by a lahar during an eruption on January 8, 1990. When
found 6 days later, the temperature of the rock was still 145°C.
(_Photograph by C. Dan Miller._)]
[Illustration: Steam plume rises from lava dome atop Augustine Volcano
on April 30, 1986. (_Photograph by M.E. Yount._)]
Augustine Volcano.
One of the most active volcanoes in Cook Inlet is Augustine, whose
symmetrical cone rises 1,254 meters above the sea. Since Captain James
Cook discovered and named it in 1778, Augustine has erupted in 1812,
1883, 1935, 1963-64, 1976, and 1986. Curiously, the quiet intervals
between these eruptions apparently have shortened from 70 to 10 years.
Augustine’s 1986 eruption was similar to the pattern of events observed
in 1976. After eight months of earthquake activity beneath the volcano,
a violent explosion began on March 26. Billowing ash plumes rose more
than 10 kilometers above the vent, pyroclastic flows sped down the
volcano’s flanks into the sea, and ash spread throughout Cook Inlet. A
second stage began April 23, when lava began erupting near the volcano’s
summit and added about 25 meters to the top of the existing lava dome.
Small pyroclastic flows accompanied growth of the dome.
Scientists were worried that this eruption might trigger a giant
landslide from Augustine’s steep upper cone, which could enter the sea
to create a _tsunami_ (powerful seismic sea wave). At least 12
landslides are known to have occurred at Augustine. The most recent
slide took place at the onset of the 1883 eruption when a part of the
volcano’s summit collapsed into the sea. Within one hour, a tsunami as
high as 9 meters crashed ashore on the coast of the Kenai Peninsula 80
kilometers away. No one was killed and property damage was only minor
because the tsunami hit at low tide. Subsequent eruptions have rebuilt a
steep cone of overlapping lava domes similar to the cone that existed
just before the 1883 landslide.
[Illustration: Pyroclastic flow descending the upper flanks of Augustine
Volcano. (_Photograph by M.E. Yount._)]
Novarupta, Katmai National Monument.
The largest eruption in the world this century occurred in 1912 at
Novarupta on the Alaska Peninsula. An estimated 15 cubic kilometers of
magma was explosively erupted during 60 hours beginning on June 6—about
30 times the volume erupted by Mount St. Helens in 1980! The expulsion
of such a large volume of magma excavated a funnel-shaped vent 2
kilometers wide and triggered the collapse of Mount Katmai volcano 10
kilometers away to form a summit caldera 600 meters deep and about 3
kilometers across. Extrusion of the lava dome, called Novarupta, near
the center of the 1912 vent marked the end of the eruption.
Little was known about the spectacular effects of this great eruption
until 1916, when a scientific expedition sponsored by the National
Geographic Society visited the area. To their amazement, they found a
broad valley northwest of Novarupta marked by a flat plain of loose,
“sandy” ash material from which thousands of jets of steam were hissing.
The eruption had produced pyroclastic flows that swept about 21
kilometers down the upper Ukak River valley. The thickness of the
resulting pumice and ash deposits in the upper valley is not known but
may be as great as 200 meters. In 1916, the deposits were still hot
enough to boil water and form countless steaming fumaroles; hence the
expedition named this part of the Ukak River the “Valley of Ten Thousand
Smokes.”
Trident Volcano.
Eruptions of andesitic lava flows between 1953 and 1960 built a new cone
on Trident’s southwest flank, adding yet another to the volcano’s older
complex of three overlapping cones (hence the name Trident). A huge
cloud of rising ash was seen on February 15, 1953, in the direction of
Katmai National Monument; 3 days later clear weather permitted U.S. Navy
pilots to spot a blocky lava flow emerging from Trident’s southwest
flank. Slow extrusion of lava during the next 4 months built an
irregularly shaped lava dome about 1.5 kilometers long and 600 meters
tall. Trident continued to erupt intermittently through 1960, generating
three lava flows as long as 4.5 kilometers from the same vent and
numerous ash-producing explosive eruptions.
[Illustration: Novarupta lava dome, Mt. Katmai, and Trident Volcano,
Alaska. The dome is 380 meters in diameter and occupies a small part of
the vent from which about 15 cubic kilometers of magma was erupted in
1912 (dashed line marks approximate outline of vent). (_Copyrighted
photograph reprinted with permission, Aero Map US Inc., August 21,
1987._)]
[Illustration: Small steam plume rises from a cinder cone within the
summit caldera of Mount Veniaminof, Alaska. The large pit in the ice
formed when lava (dark area) flowed beneath the ice and melted it.
(_Photograph by M.E. Yount._)]
Mount Veniaminof.
Mount Veniaminof is a massive composite volcano with a summit caldera
about 8 kilometers in diameter. Since its formation about 3,700 years
ago, the caldera has filled with ice to a depth of at least 60 meters.
Between June 1983 and January 1984, a series of small explosions, lava
fountains, and lava flows erupted from a small cinder cone within the
caldera. The explosions hurled molten lava from the cinder cone, and
lava flows melted a pit about 1.5 kilometers in diameter in the ice near
the base of the volcano. Water from the melting ice formed a temporary
lake.
Mount Spurr.
The summit cone of Mount Spurr consists of a large lava dome built in
the center of a horseshoe-shaped crater formed earlier by a large
landslide. At the southern edge of this ancient crater is a younger,
more active cone known as Crater Peak. Scientists have determined that
Crater Peak is the source for at least 35 ash layers found in the Cook
Inlet area, all of which were erupted in the past 6,000 years. Until
recently, a warm turquoise-colored lake partially filled its crater.
A series of explosive eruptions from Crater Peak on June 27, 1992,
generated ash plumes as high as about 14 kilometers, small pyroclastic
flows that swept down the south and east sides of the cone, and small
lahars. The reawakening of Crater Peak followed nearly a year of
increased earthquake activity, which escalated further on June 26, less
than 1 day before its first eruption. Not all of the explosive episodes
were preceded by a change in seismicity beneath the volcano, a condition
that required scientists to maintain a 24-hour watch for extended
periods of time in order to issue sudden reports and warnings of
eruptive activity. The west side of Cook Inlet received a light to
moderate ashfall during the largest explosive episode on August 18;
Anchorage was blanketed with about 3 millimeters of ash, causing the
Anchorage International Airport to close for a few hours. During an
eruption at night on September 17, a spectacular display of lightning
and incandescent ballistics and pyroclastic flows were witnessed by
hunters who camped about 18 kilometers to the southeast; a faint glow
above Crater Peak was also visible from as far away as Anchorage.
[Illustration: Steaming Crater Peak, a satellite vent on the south side
of Mount Spurr volcano, produced three explosive eruptions in 1992.
(_Photograph by Cynthia Gardner._)]
[Illustration: Aerial view of the Mono-Inyo Craters Volcanic Chain,
California. Several eruptions occurred along both chains as recently as
about 550 to 600 years ago. (_Photograph by C. Dan Miller._)]
Mono Craters
Wilson Butte
Northern part of Inyo Volcanic Chain
Obsidian Dome
Glass Creek Dome