DESCRIPTION:
Volcanic Eruptions, Blasts, Plumes, Ballistics, etc.

During an episode of activity, a volcano commonly displays a distinctive pattern of behavior. Some mild eruptions merely discharge steam and other gases, whereas other eruptions quietly extrude quantities of lava. The most spectacular eruptions consist of violent explosions that blast great clouds of gas-laden debris into the atmosphere.

The type of volcanic eruption is often labeled with the name of a well-known volcano where characteristic behavior is similar -- hence the use of such terms as "Strombolian", "Vulcanisn", "Vesuvian", "Pelean", "Hawaiian", "Phreatic", and others. Some volcanoes may exhibit only one characteristic type of eruption during an interval of activity -- others may display an entire sequence of types.

Strombolian

In a "Strombolian"-type eruption observed during the 1965 activity of Irazu Volcano in Costa Rica, huge clots of molten lava burst form the summit crater to form luminous arcs through the sky. Collecting on the flanks of the cone, lava clots combined to stream down the slopes in fiery rivulets.

Vulcanian

In contrast, the eruptive activity of Paricutin Volcano in 1947 demonstrated a "Vulcanian"-type eruption, in which a dense cloud of ash-laden gas explodes from the crater and rises high above the peak. Steaming ash forms a whitish cloud near the upper level of the cone.

Vesuvian

In a "Vesuvian" eruption, as typified by the eruption of Mount Vesuvius in Italy in A.D.79, great quantities of ash-laden gas are violently discharged to form a cauliflower-shaped cloud high above the volcano.

Pélean

In a "Pélean" or "Nuee Ardent" (glowing cloud) eruption, such as occurred on the Mayan Volcano in the Philippines in 1968, a large quantity of gas, dust, ash, and incandescent lava fragments are blown out of a central crater, fall back, and form tongue-like, glowing avalanches that move down-slope at velocities as great as 100 miles per hour. Such eruptive activity can cause great destruction and loss of life if it occurs in populated areas, as demonstrated by the devastation of St. Pierre during the 1902 eruption of Mount Pelee on Martinique, West Indies.

Hawaiian

"Hawaiian" eruptions may occur along fissures or fractures that serve as linear vents, such as during the eruption of Mauna Loa Volcano in Hawaii in 1950, or they may occur at a central vent such as during the 1959 eruption in Kilauea Iki Crater of Kilauea Volcano, Hawaii. In fissure-type eruptions, molten, incandescent lava spurts from a fissure on the volcano's rift zone and feeds lava streams that flow downslope. In central-vent eruptions, a fountain of fiery lava spurts to a height of several hundred feet or more. Such lava may collect in old pit craters to form lava lakes, or form cones, or feed radiating flows.

Phreatic

The eruption of Taal Volcano in the Philippine Islands in 1965 typifies "Phreatic" (or steam-blast) behavior. Here, a great column of steam, dust, ash, and cinders is blasted to a height of several thousand feet. This type of violent eruption is believed to occur when a large quantity of ground or surface water comes in contact with hot rock or magma in a volcanic vent and is instantly and explosively flashed to steam.

During volcanic eruptions, discrete explosions eject clouds of ash, lapilli, blocks and bombs, volcanic gas, and, in some circumstances, external water. Within seconds after ejection, the cloud front generally evolves from a more-or-less spherical form into one consisting of finger-like projections ("finger jets"); at the apex of each finger is a single ballistic block or a collection of larger fragments. Behind the finger front is a cloud of smaller tephra fragments and gas that ride within the slipstream of the larger blocks. Seconds after the front of ballistic fingers becomes perceptible, blocks may completely separate from the trailing cloud and travel on their own through the ambient atmosphere.

MSH80_eruption_mount_st_helens_05-18-80.jpg
On May 18, 1980, at 8:32 a.m. Pacific Daylight Time, a magnitude 5.1 earthquake shook Mount St. Helens. The bulge and surrounding area slid away in a gigantic rockslide and debris avalanche, releasing pressure, and triggering a major pumice and ash eruption of the volcano. Thirteen-hundred feet (400 meters) of the peak collapsed or blew outwards. As a result, 24 square miles (62 square kilometers) of valley was filled by a debris avalanche, 250 square miles (650 square kilometers) of recreation, timber, and private lands were damaged by a lateral blast, and an estimated 200 million cubic yards (150 million cubic meters) of material was deposited directly by lahars (volcanic mudflows) into the river channels. Fifty-seven people were killed or are still missing.
USGS Photograph taken on May 18, 1980, by Austin Post
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An explosive eruption blasts molten and solid rock fragments (tephra) into the air with tremendous force. The largest fragments (bombs) fall back to the ground near the vent, usually within 2 miles. The smallest rock fragments (ash) continue rising into the air, forming a huge, billowing eruption column. ... Eruption columns can be enormous in size and grow rapidly, reaching more than 12 miles above a volcano in less than 30 minutes. Once in the air, the volcanic ash and gas form an eruption cloud. ... Large eruption clouds can travel hundreds of miles downwind from a volcano, resulting in ash fall over enormous areas. ...

MSH80_aerial_harrys_ridge_09-30-80.jpg
Harrys Ridge, five miles north of Mount St. Helens' crater was within the blast zone. The view is from the southeast.
USGS Photograph taken on September 30, 1980, by Lyn Topinka.
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The May 18, 1980 eruption of Mount St. Helens generated a horizontally directed series of explosions that formed a lateral blast. This blast destroyed an area of 230 square miles. Trees 6 feet in diameter were mowed down like blades of grass as far as 15 miles from the volcano. The blast exhibited characteristics of both pyroclastic flows and surges.

Volcanic blasts are explosions which may be directed vertically or at some lower angle. Vertically directed explosions may produce mixtures of rock debris and gases that flow, motivated chiefly by gravity, down one or more sides of a volcano. ...

A volcanic explosion that has a significant low-angle component and is principally directed toward a sector of no more than 180 degrees is referred to as a lateral blast. ...

Lateral blasts may affect only narrow sectors or spread out from a volcano to cover a sector as broad as 180 degrees, and they can reach distances of several tens of kilometers from a vent ... Because of they carry rock debris at high speeds, lateral blasts can devastate areas of tens to hundreds of square kilometers within a few minutes, and can destroy manmade structures and kill all living things by abrasion, impact, burial, and heat. ...

Volcanic blasts are most likely at steep-sided stratovolcanoes and may occur when viscous gas-rich magma is emplaced at a shallow level within the volcano.

Directed blast: A hot, low-density mixture of rock debris, ash, and gases that moves at high speed along the ground surface. Directed blasts are generated by explosions.

The near-supersonic lateral blast -- (May 18, 1980 at Mount St. Helens) -- , loaded with volcanic debris, caused widespread devastation as far as 19 miles from the volcano. The area affected by the blast can be subdivided into three roughly concentric zones:

1. Direct blast zone, the innermost zone, averaged about 8 miles in radius, an area in which virtually everything, natural or manmade, was obliterated or carried away. For this reason, this zone also has been called the "tree-removal zone". The flow of the material carried by the blast was not deflected by topographic features in this zone.

2. Channelized blast zone, an intermediate zone, extended out to distances as far as 19 miles from the volcano, an area in which the flow flattened everything in its path and was channeled to some extent by topography. In this zone, the force and direction of the blast are strikingly demonstrated by the parallel alignment of toppled large trees, broke off at the base of the trunk as if they were blades of grass mown by a scythe. This zone was also known as the "tree-down zone."

3. Seared zone, also called the "standing dead" zone, the outermost fringe of the impacted area, a zone in which trees remained standing but were singed brown by the hot gases of the blast.

MSH82_st_helens_plume_from_harrys_ridge_05-19-82.jpg
Plumes of steam, gas, and ash often occured at Mount St. Helens in the early 1980s. On clear days they could be seen from Portland, Oregon, 50 miles (81 kilometers) to the south. The plume photographed here rose nearly 3,000 feet (1,000 meters) above the volcano's rim. The view is from Harrys Ridge, five miles (8 kilometers) north of the mountain.
USGS Photograph taken on May 19, 1982, by Lyn Topinka.
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Phreatic eruption (explosion): An explosive volcanic eruption caused when water and heated volcanic rocks interact to produce a violent expulsion of steam and pulverized rocks. Magma is not involved.

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