Photograph by T.P. Miller in June 1979
Volcanic dome
Volcanic domes are rounded, steep-sided mounds built by very viscous
magma,
usually either dacite or
rhyolite. Such magmas are typically too
viscous (resistant to flow) to move far from the vent before cooling
and crystallizing. Domes may consist of one or more individual lava
flows. Volcanic domes are also referred to as lava domes.
Volcanic dome atop Novarupta vent, Valley of Ten Thousand Smokes,
Katmai National Park and Preserve, Alaska. The dome was erupted from the same
vent that expelled about 15 km3 of magma in an enormous
explosive eruption in 1912.
More about volcanic domes
Did you know?
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The longest historical dome-building eruption is still occurring at
Santiaguito Dome, which is erupting on the southeast flank of Santa
Maria volcano in Guatemala; the dome began erupting in 1922.
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Domes often erupt on the top and sides of stratovolcanoes. Of the 1,511 volcanoes known to have erupted in the past 10,000 years,
only 40 are classified as independent volcanic domes not associated with another
volcano.
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The volcanic dome in the Cascades is Lassen Peak in northern
California. Lassen Peak is part of a larger volcanic center, and its
most recent eruption between 1914 and 1917 added a new lava flow to its
summit.
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The Mono Lake-Inyo Craters volcanic chain in north-central California
consists of 13 domes; four erupted only about 600 years ago (Panum
Crater, South Deadman, Obsidian Flow, and Glass Creek domes).
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Three of the most active stratovolcanoes in the Cascades--Mount St.
Helens, Mount Shasta, and Glacier Peak--have erupted lava domes at their
summits. Before the 1980-1986 eruption of Mount St.Helens, its
symmetrical summit cone was capped by a lava dome erupted between the
1500's and late 1700's.
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Although volcanic domes are built by nonexplosive eruptions of viscous
lava, domes can generate deadly pyroclastic flows. The sides of a dome
or an erupting lava flow on a dome can collapse down a steep slope to
form a hot avalanche of hot lava fragments and gas (pyroclastic flow).
Recent eruptions of lava domes at Unzen volcano in Japan and
at Soufriere
Hills in Montserrat have forced thousands of people from
their homes.
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Some domes erupt obsidian, which is volcanic glass that may form in
rhyolite or dacite lava flows. Most obsidian is black, but red, green, and brown
obsidian is known. Obsidian forms when magma is cooled so quickly that
individual minerals cannot crystallize.