FAQs about volcanic eruptions
Can an eruption at one volcano trigger an eruption at another nearby volcano
(for example, within about 10 km)?
There are a few historic examples of simultaneous eruptions from volcanoes
or vents located within about 10 km of each other, but it's very difficult
to determine whether one might have caused the other. To the extent that
these erupting volcanoes or vents have common or overlapping magma reservoirs
and hydrothermal systems, magma rising to erupt from one volcano may effect
the other volcano's "plumbing" system and cause some form of
unrest, including eruptions. For example, the huge explosive eruption
of Novarupta vent in Alaska triggered the summit of nearby Mt. Katmai
volcano to collapse, thereby forming a new caldera (but no eruption!).
For a few of the historic examples of simultaneous eruptions from nearby
volcanoes, scientists actually consider the individual volcanoes or vents
to be part of a larger volcano complex consisting of overlapping stratovolcanoes,
cinder cones, fissures, vents, and even calderas. In such cases, the erupting
vents (or volcano) are actually part of the same volcano complex. For
example, Tavurvur and Vulcan cones that erupted at nearly the same time
in September 1994 are vents located within Rabaul Caldera in Papua New
Guinea. In such cases, one eruption does not really "trigger"
a nearby vent to erupt; instead, moving magma "leaks" to the
surface at multiple sites.
In contrast to these examples of simultaneous eruptions at volcanoes with
overlapping or related magma and hydrothermal systems, two of Earth's
most active volcanoes that are located close to each other -- Mauna Loa
and Kilauea in Hawaii -- have separate shallow magma reservoirs that don't
seem to affect each other. Even though Kilauea Volcano is located on the
southeastern flank of Mauna Loa (the summit calderas are only 33 km apart)
and magma rising into both volcanoes originates from the same mantle hot
spot, the chemistry of their magma is nevertheless distinct from each
other. Furthermore analysis of the timing of historic eruptions strongly
suggests that an eruption at one volcano does not cause or trigger an
eruption at the other volcano.
Is there evidence for a cause and effect relationship between eruptions
that occur at about the same time from volcanoes located hundreds to thousands
of km apart?
No. Since there are on average between 50 and 60 volcanoes that erupt
each year somewhere on Earth (about 1 every week), some of Earth's volcanoes
may actually erupt within a few days or hours of each other. Upon closer
inspection, however, the eruptions are almost always preceded by very
different build-up periods in terms of time (days to weeks to months to
years) and type of activity (earthquakes, ground deformation, gas emissions,
and small eruptions). The "trigger" of this precursory activity
is the key to understanding what causes an eventual eruption at any one
volcano, not the timing of significant eruptions hundreds to thousands
of km apart.
According to the theory of plate tectonics, the location and frequency
of volcanism on Earth is due primarily to the way in which our planet's
surface is divided into large sections or plates and how they move relative
to each other, and the formation of deep "thermal plumes" that
rise from the core-mantle boundary about 3,200 km below the surface. These
mechanisms and the fact that even nearby volcanoes erupt magma with different
and often unique chemical composition (evidence that each volcano has
a separate unique shallow magma reservoir) strongly suggests there is
unlikely to be any cause and effect relationship between volcanic eruptions
separated hundreds to thousands of km apart.
What volcanoes in the conterminous United States have erupted since the
Nation was founded?
Mount St. Helens, Washington
1980-1986: Large explosive eruption on May 18, 1980, followed by 21 smaller
eruptive episodes. The last 17 episodes built a lava dome in the volcano's
crater..
1800-1857: Large explosive eruption in 1800 was followed by extrusions
of lava that formed a lava flow on the volcano's northwest flank (Floating
Island lava flow) and a lava dome on the north flank (Goat Rocks lava
dome).
Late 1700's: Layers of volcanic rocks record a variety of activity related
to the growth of a lava dome at the volcano's summit, including pyroclastic
flows, lahars, and tephra fall.
Lassen Peak, California
1914-1917: A series of small explosions that began on May 30, 1914, was
followed 12 months later by extrusion of lava from the summit and a destructive
pyroclastic flow and lahars on May 21, 1915. Minor activity continued
through middle of 1917.
Mount Rainier, Washington
1894 & early 1800's: Several eyewitness accounts describe minor releases
of steam and ash-laden steam during November and December 1894. The most
recent eruption that formed a thin and discontinuous tephra layer, however,
occurred during the first half of the 19th century.
Mount Hood, Oregon
1856-1865 & late 1700's: According to eyewitnesses, small explosive
eruptions occurred from the summit area between 1856 and 1865. In the
latter half of the 18th century, however, a lava dome was erupted, which
was accompanied by pyroclastic flows, lahars, and tephra fall.
Mount Shasta, California
1786: An eruption cloud was observed above the volcano from a ship passing
by north coast California, and the activity included pyroclastic flows.
Glacier Peak, Washington
17th - 18th centuries: Between about 200 and 300 years ago, small eruptions
deposited pumice and ash east of the volcano, and may have been observed
by Native Americans.
Mount Baker, Washington
1840-1870: Historical literature refers to several episodes of small tephra-producing
events in the mid 1800's, and increased fumarolic activity began in Sherman
Crater near the summit in 1975 and remains elevated today.
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