DESCRIPTION:
Lava Domes, Volcanic Domes, Composite Domes

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Aerial view, Novarupta Dome, Katmai Vicinity, Alaska.
-- USGS Photo by Gene Iwatsubo, July 29, 1987

Volcanic domes are mounds that form when viscous lava is erupted slowly and piles up over the vent, rather than moving away as a lava flow. The sides of most domes are very steep and typically are mantled with unstable rock debris formed during or shortly after dome emplacement. Most domes are composed of silica-rich lava which may contain enough pressurized gas to cause explosions during dome extrusion.

The direct effects of dome eruption include burial or disruption of the preexisting ground surface by the dome itself and burial of adjacent areas by rock debris shed from the dome. Because of their high temperatures, domes may start fires if they are erupted in forested areas. Domes are extruded so slowly that they can be avoided by people, but they may endanger man-made structures that cannot be moved. The principal hazard associated with domes is from pyroclastic flows produced by explosions or collapses. Such pyroclastic flows can occur without warning during active dome growth and can move very rapidly, endangering life and property up to 20 kilometers from their sources. Such pyroclastic flows can also cause lahars if they are erupted onto snow and ice or incorporate water during movement.

Dome: A steep-sided mass of viscous (doughy) lava extruded from a volcanic vent, often circular in plan view and spiny, rounded, or flat on top. Its surface is often rough and blocky as a result of fragmentation of the cooler, outer crust during growth of the dome.

Volcanic domes are masses of solid rock that are formed when viscous lava is erupted slowly from a vent. If the lava is viscous enough, it will pile up above the vent to form a dome rather than move away as a lava flow. The sides of most domes are very steep and typically are mantled with unstable rock debris formed during or shortly after dome emplacement. Most domes are composed of silica-rich lavas that have a lower gas content than do the lavas erupted earlier in the same eruptive sequence; nevertheless, some dome lavas still contain enough gas to cause explosions within a dome as it is being formed.

Volcanic or lava domes are formed by relatively small, bulbous masses of lava too viscous to flow any great distance; consequently, on extrusion, the lava piles over and around its vent. A dome grows largely by expansion from within. As it grows its outer surface cools and hardens, then shatters, spilling loose fragments down its sides. Some domes form craggy knobs of spines over the volcanic vent, whereas others form short, steep-sided lava flows known as "coulees." Volcanic domes commonly occur within the craters of on the flanks of large composite volcanoes.

Molten rock (magma) that pours or oozes onto the Earth's surface is called lava and forms lava flows. The higher a lava's content of silica (silicon dioxide, SiO2), the less easily it flows. For example, low-silica basalt lava can form fast-moving (10 to 30 miles per hour) streams or can spread out in broad thin sheets up to several miles wide. Since 1983, Kilauea Volcano on the Island of Hawaii has erupted basalt lava flows that have destroyed more than 200 houses and severed the nearby coastal highway. In contrast, flows of higher-silica andesite and dacite lava tend to be thick and sluggish, traveling only short distances from a vent. Dacite and rhyolite lavas often squeeze out of a vent to form irregular mounds called lava domes. Between 1980 and 1986, a dacite lava dome at Mount St. Helens grew to about 1,000 feet high and 3,500 feet across.

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Aerial view of the south flank of South Sister composite volcano showing numerous blocky lava flows erupted about 2,000 years ago. Early eruptions formed Rock Mesa (just above center, far left), a broad flat flow emplaced on nearly level ground. Subsequent eruptions formed a line of lava domes and flows that extend from Sparks Lake meadow (lower right) to Green Lakes (just above center, far right). Several small lava domes were also formed on the northeast flank, out of view. Cratered cone in lower left is Talapus Butte, a basaltic scoria cone.
USGS Photograph taken in September 1977 by C.Dan Miller.
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Domes ranging in composition from dacite to rhyolite have been erupted repeatedly during late Pleistocene and Holocene time in the Cascade Range. Domes at Mount Shasta, Mount St. Helens , Glacier Peak , Mount Hood , and near Lassen Peak have collapsed or exploded to produce hot pyroclastic flows, some extending as far as 20 kilometers from their sources. Lines of domes erupted at Medicine Lake and South Sister volcanoes within the last several thousand years appear to have formed over short intervals of time when vertical dikelike magma bodies reached the surface. Dome emplacement typically follows more explosive eruptions.

Cascade Range Volcanoes and Volcanics Menu

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Since December 1980, eruptions of Mount St. Helens have added material to a dacitic lava dome with the crater, as seen here in this 1984 view from the north-northwest.
USGS Photograph taken on September 13, 1984, by Lyn Topinka.
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The dome at Mount St. Helens is termed a composite dome by scientists, because it represents the net result of many eruptive events, not just one event. The dome-building process may be pictured as the periodic squeezing of an upward-pointing tube of toothpaste or caulking compound. The process is dynamic, involving the upward movement of new material, cracking and pushing aside of old material, sloughing of material from steep surfaces of the dome, and occasional, small but violent explosions that blast out pieces of the dome.

At the start of 1990, the composite dome was about 3,480 feet by 2,820 feet in diameter and rose about 1,150 feet above the low point on the adjacent crater floor. It has a volume of about 97 million cubic yards, less than 3 percent of the volume of the volcano (about 3.5 billion cubic yards) removed during the landslide and lateral blast on May 18, 1980. If the dome resumes growth at its average rate of the 1980s (about 17 million cubic yards per year), it would take nearly a century to fill in the summit crater and more than 200 years to rebuild Mount St. Helens to its pre-1980 size.

Mount St. Helens Lava Dome Menu

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Lassen Peak, California, and Kings Creek Meadows, Lassen Volcanic National Park.
USGS Photograph taken in October 1982 by Lyn Topinka
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Domes ranging in composition from dacite to rhyolite have erupted on numerous occasions during Holocene time in the Mount Shasta, Medicine Lake, Lassen Peak, and Mono Lake-Long Valley areas. ... Domes at Mount Shasta and Lassen Peak have collapsed or exploded on several occasions to produce hot pyroclastic flows some extending as far as 18 kilometers from their sources. Lines of domes, erupted at Medicine Lake volcano and at the Inyo volcanoes about 1,200 years ago and between about 800 and 600 years ago, appear to have been formed over short intervals of time when vertical tabular dikelike magma bodies approached the surface. ...

Lassen Peak is the largest of a group of more than 30 volcanic domes erupted over the past 300,000 years in Lassen Volcanic National Park in northern California. These mound-shaped accumulations of volcanic rock, called lava domes, were created by eruptions of lava too viscous to readily flow away from its source. Eruptions about 27,000 years ago formed Lassen Peak, probably within only a few years. With a height of 2,000 feet and a volume of half a cubic mile, it is one of the largest lava domes on Earth.

When Lassen Peak formed, it looked much like the nearby 1,100-year-old Chaos Crags Domes, with steep sides covered with angular rock talus. However, from 25,000 to 18,000 years ago, during the last 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 circue, was eroded by a glacier that extended out 7 miles from the dome.

California Volcanoes and Volcanics Menu

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Aerial view, Novarupta Dome, Katmai Vicinity, Alaska.
-- USGS Photo by Gene Iwatsubo, July 29, 1987

The nearly circular Novarupta Dome that formed during the 1912 eruption of Katmai Volcano, Alaska, measures 800 feet across and 200 feet high. The internal structure of this dome -- defined by layering of lava fanning upward and outward from the center -- indicates that it grew largely by expansion from within.

Mount Pelee in Martinique, West Indies, and Lassen Peak and Mono domes in California are examples of lava domes.

An extremely destructive eruption accompanied the growth of a dome at Mount Pelee in 1902. The coastal town of St. Pierre, about 4 miles downslope to the south, was demolished and nearly 30,000 inhabitants were killed by an incandescent, high-velocity ash flow and associated hot gases and volcanic dust. Only two men survived; one because he was in a poorly ventilated, dungeon-like jail cell and the other who somehow made his way safely through the burning city.

How a Dome Grows

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