Lassen Peak(also known as
Mount
Lassen) is the southernmost active
volcano in the
Cascade
Range. It is part of the
Cascade
Volcanic Arc which is an
arc that
stretches from northern California to southwestern British
Columbia. Located in the
Shasta
Cascade region of
Northern
California, Lassen rises above the surrounding terrain and has
a volume of half a cubic mile, making it one of the largest
lava domes on Earth.
It was created on the
destroyed northeastern flank of now gone Mount Tehama
, a stratovolcano that
was at least higher than Lassen.
Lassen
Peak has the distinction of being the only volcano in the Cascades
other than Mount St.
Helens
to erupt during the 20th century. On May 22,
1915, an explosive eruption at Lassen Peak devastated nearby areas
and rained
volcanic ash as far away as
to the east.
This explosion was the most powerful in a
1914–17 series of eruptions that were the last to occur in the
Cascades before the 1980 eruption of Mount St.
Helens
in Washington
. Lassen Volcanic National Park
was created in Shasta County, California
to preserve the devastated area and nearby volcanic
geothermal features.
Unlike most lava domes, Lassen is topped by craters. A series of
these craters exist around Lassen's summit, although two are now
covered by solidified lava and sulfur deposits. Lassen is the
largest of a group of more than 30 volcanoes that have erupted over
the past 300,000 years in the Lassen Volcanic Center.
Climate
Lassen has
the highest known snowfall amounts in
California with an average annual snowfall of and some years
receiving over of snow at its base of at Lake
Helen
. Mount Lassen gets more precipitation (rain,
hail, melted snow) than anywhere in the
Cascades south of the Three Sisters
volcanoes.This heavy snowfall allows Lassen
to retain 14 permanent patches of snow despite Lassen's modest
elevation.
Lightning has been known to
frequently strike the summit of the volcano during summer
thunderstorms.
Geology

Vulcan's Eye on Lassen Peak from Lake
Helen
Lassen is
the southernmost in the chain of 18 large volcanic peaks that run
from southwestern British Columbia
to northern California. The peaks formed in
the past 35 million years as the
Juan
De Fuca plate and the tiny
Gorda
plate to its south have been pulled under the overriding
North American plate. As the
oceanic crust in the Juan de Fuca
plate melts under the pressure, it creates pools of
lava that drive up the Cascade Range and power periodic
eruptions.

Northeast side of Lassen Peak, showing
the area devastated by mudflows and a lateral blast in 1915.
Roughly 27,000 years ago, Lassen started to form as a mound-shaped
dacite lava dome pushed its way through
Tehama's destroyed north-eastern flank. As the lava dome grew it
shattered overlaying rock, which formed a blanket of angular
talus around the emerging steep-sided volcano.
It
probably looked much like the nearby 1,100-year-old Chaos Crags
. Lassen reached its present height in a
relatively short time, probably in as little as a few years.
From 25,000 to 18,000 years ago, during the last glacial period of
the current
ice age, Lassen’s shape was
significantly altered by
glacial
erosion. For example, the bowl-shaped depression on the
volcano’s northeastern flank, called a
cirque, was eroded by a glacier that extended out
from the dome.
Lassen's most recent eruptive period started in 1914 and lasted for
seven years (see below).
The most powerful of these eruptions was a
1915 episode that sent ash and steam in a tall mushroom cloud,
making it the largest recent eruption in the contiguous United
States until the 1980 eruption of Mount St.
Helens
. The region remains geologically active,
with mud pots, active fumaroles, and boiling water features,
several of which are getting hotter.
The area around Mount
Lassen and nearby Mount
Shasta
are considered the most likely volcanoes in the
Cascade Range to shift to active eruptions.
Human history
Before the arrival of white settlers, the areas surrounding Lassen
Peak, especially in the east and south, were the traditional home
of the northeastern
Maidu.
Lassen
Peak was named in honor of Danish
blacksmith
Peter Lassen, who guided immigrants
past the peak to Sacramento Valley
in the 1830s. Lassen's trail, however, never found general
long-term use because it was considered unsafe. It was replaced by
the
Nobles Emigrant Trail,
named after
William Nobles,
which linked the
Applegate Trail in
Nevada to northern Sacramento Valley.
In 1864 Helen Tanner Brodt became the first woman to reach the
summit of Lassen Peak.
A tarn on Lassen
was named Lake Helen
in her honor.
Starting in 1914 and ending in 1921 Lassen came alive with a series
of
phreatic eruptions (steam
explosions), dacite lava flows, and
lahars
(volcanic mud flows). There were 200 to 400
volcanic eruptions during this period of
activity. First-hand accounts of the time record at least one
fatality, in 1915.
Early 20th century eruptions

The "Great Explosion" eruption
column of May 22, 1915 was seen as far as away (Photograph by
R.E.

Mount Lassen Before 1914

Lassen Peak in June 1915 with
Loomis Hot Rock, one of the many large boulders set loose in the
eruption that were too hot to touch for days after, in the
foreground (photograph by B.F.
Initial rumblings (May 1914 to May 1915)
On May 30, 1914 Lassen became active again after 27,000 years
dormancy when it was shaken by a steam explosion. Such steam blasts
occur when molten rock (
magma) rises toward
the surface of a volcano and heats shallow
ground water. The hot water rises under
pressure through cracks and, on nearing the surface, vaporizes and
vents explosively. By mid-May 1915, more than 180 steam explosions
had blasted out a wide crater near the summit of Lassen Peak.
Then the character of the eruption changed dramatically.
On the
evening of May 14, 1915, incandescent blocks of lava could be seen bouncing down the flanks of Lassen
from as far away as the town of Manton, California
to the west. By the next morning, a growing
dome of
dacite lava (lava containing 63 to
68%
silica) had filled the volcano’s
crater.
Events of May 19–20, 1915
Late on the evening of May 19, a large steam explosion fragmented
the dacite dome, creating a new crater at the summit of Lassen
Peak. No new magma was ejected in this explosion, but glowing
blocks of hot lava from the dome fell on the summit and
snow-covered upper flanks of Mount Lassen. These falling blocks
launched a half mile (800 m) wide avalanche of snow and volcanic
rock that roared down the volcano’s steep northeast flank and over
a low ridge at Emigrant Pass into Hat Creek.
As the hot lava blocks broke into smaller fragments, the snow
melted, generating a mudflow of volcanic materials, called a
lahar. The bulk of this lahar was deflected
northwestward at Emigrant Pass and flowed 7 miles (11 km)
down Lost Creek. Even after coming to rest, both the avalanche and
lahar released huge volumes of water, flooding the lower
Hat Creek Valley during the early morning
hours of May 20. The lahar and flood destroyed six mostly
not-yet-occupied summer ranch houses. Fortunately, the few people
in these houses escaped with only minor injuries.
Also during the night of May 19–20, dacite lava somewhat more fluid
than that which erupted on the night of May 14–15 welled up into
and filled the new crater at Lassen’s summit, spilled over low
spots on its rim, and flowed down the steep west and northeast
flanks of the volcano.
Climactic eruption of May 22, 1915
Then at 4:30 p.m. on May 22, after two quiet days, Lassen exploded
in a powerful eruption (referred to as "the Great Explosion") that
blasted volcanic ash, rock fragments and pumice high into the air.
This created the larger and deeper of the two craters seen near the
summit of the volcano today.
A huge column of volcanic ash and gas rose
more than into the air and was visible from as far away as Eureka,
California
, to the west.
Pumice falling onto the northeastern slope of Lassen Peak generated
a high-speed avalanche of hot ash, pumice, rock fragments, and gas,
called a
pyroclastic flow, that
swept down the side of the volcano, devastating a area. The
pyroclastic flow rapidly incorporated and melted snow in its path.
The water from the melted snow transformed the flow into a highly
fluid
lahar that followed the path of the May
19–20 lahar and rushed nearly down Lost Creek to Old Station. This
new lahar released a large volume of water that flooded lower Hat
Creek Valley a second time.
The powerful climactic eruption of May 22 also swept away the
northeast lobe of the lava flow extruded two days earlier.
The
eruption produced smaller mudflows on all flanks of Lassen Peak,
deposited a layer of volcanic ash and pumice traceable for to the
northeast, and rained fine ash at least as far away as Winnemucca,
Nevada
, to the east. Together these events created
the Devastated Area which is still sparsely populated by trees due
to the low
nutrient and high porosity of
the
soil.
Later volcanic activity
For several years after the May 22, 1915, eruption, spring snowmelt
percolating down into Mount Lassen triggered steam explosions, an
indication that rocks beneath the volcano’s surface remained hot.
Particularly vigorous steam explosions in May 1917 blasted out the
second of the two craters now seen near the northwest corner of the
volcano’s summit. The two older craters are buried.
Steam vents could be found in the area of these craters into the
1950s but gradually waned and are difficult to locate today.
Since
then, the United States
Geological Survey in cooperation with the United States Park Service have
been monitoring Lassen and other volcanic and geothermal areas in the park
.
For more details, see
a map
of the deposits from the 1915 eruption.

Kings Creek with Lassen Peak on the
horizon.
See also
References
- Notes
- EO Newsroom
- Major works
- Geology of National Parks: Fifth Edition, Ann G.
Harris, Esther Tuttle, Sherwood D., Tuttle (Iowa, Kendall/Hunt
Publishing; 1997) ISBN 0-7872-5353-7
- EO Newsroom (NASA), New Images of Lassen
Volcanic National Park (adapted public domain text; Retrieved 18
September 2006)
- Eruptions of Lassen Peak, California, 1914 to 1917,
U.S. Geological Survey Fact Sheet 173-98 (adapted public domain
text; Retrieved 18 September 2006)
External links