Types and Processes Gallery

Pyroclastic Fall
Pyroclastic-fall deposits are produced by the gravitational fall and accumulation of fragmental material ejected by explosive eruptions. Tephra is a generic term referring to fragmental material of all size classes. Explosive eruptions are one of Earth's most dramatic geological processes and have been the subject of the brush and pen for thousands of years. Eruptions can involve new magma (referred to as juvenile material) or the ejection of pre-existing rock and debris from around the vent. They can range from minor, relatively passive gas-and-ash emissions reaching only a hundred meters or so above the vent to powerful magmatic explosions that can eject large volumes of material and produce eruption columns that reach tens of kilometers above the volcano. Smaller eruptions eject material that accumulates near the vent, but large explosions inject ash and gases into Earth's stratosphere, where it can circle the planet and have major impact on global climate. Pyroclastic-fall deposits vary widely in particle size, but typically have a narrow range of particle sizes at a given location and become finer-grained away from the vent. They provide valuable time layers useful in unraveling the eruptive history of a volcano and a volcanic region.

White Island (New Zealand)
This 4-m-wide, water-filled impact crater was formed when the block in background, with volcanologist Ian Nairn providing scale, was ejected during a strombolian eruption from White Island in New Zealand in late March 1977. The block, composed of pre-existing wall rock of the crater, bounced, forming the impact crater, and then slid to its present location, 250 m from the source vent.

Photo by Bruce Houghton, 1977 (Wairakei Research Center).

A vulcanian explosion from Ngauruhoe volcano in New Zealand on February 19, 1975, ejects a dark, ash-laden cloud. Large, meter-scale ejected blocks trailing streamers of ash can be seen in the eruption column. Blocks up to 20 m across were projected hundreds of meters above the vent.

Photo by Ian Nairn, 1975 (New Zealand Geological Survey).

A vigorous eruption column rises above Indonesia's Makian volcano in this July 31, 1988, view from neighboring Moti Island. The six-day eruption began on July 29, producing eruption columns that reached 8-10 km altitude. Pyroclastic flows on the 30th reached the coast of the island, whose 15,000 residents had been evacuated. A flat-topped lava dome was extruded in the summit crater at the conclusion of the eruption.

Photo by Willem Rohi, 1988 (Volcanological Survey of Indonesia).

This classic outcrop at Oshima volcano, in the Izu Islands, south of Tokyo, shows more than 100 individual layers of pyroclastic-fall deposits. They were produced by eruptions of Oshima volcano at fairly regular intervals over a period of about 10,000 years. The dips of the tephra layers do not result from folding or faulting. The lower layers drape the sides of an old valley. A prominent unconformity in the center of the outcrop is an erosional surface that truncated deposits of the earlier eruptions. The upper layers mantled this uneven surface.

Photo by Richard Fiske, 1961 (Smithsonian Institution).

A plinian eruption column rises above Komaga-take volcano in northern Japan on June 17, the opening day of a major eruption in 1929. The column rose to a maximum height of 13 km and produced thick, pumice-rich pyroclastic-fall deposits around the volcano. Pyroclastic flows from the 1929 eruption, one of the largest in historical time from Komaga-take, also swept down the flanks of the volcano. This photo is from the shores of Onuma lake, south of Komaga-take.

Photo courtesy of the Komaga-take Disaster Prevention Council, 1929.

Volcanologists from the Institute of Volcanic Geology and Geochemistry in Petropavlovsk and the New Mexico Institute of Mining and Technology excavate a section through layered pyroclastic-fall deposits from Ilyinsky volcano in southern Kamchatka. Detailed study of the products of individual eruptions are used to determine the timing, frequency, and magnitude of those eruptions. The sequence of tephra layers shown here was deposited by explosive eruptions from Ilyinsky during the last 5000 years.

Photo by Phil Kyle, New Mexico Institute of Mining and Technology, 1996 (courtesy of Vera Ponomareva, IUGG, Petropavlovsk).

An ash plume ejected from the summit crater of Kamchatka's Kliuchevskoi volcano on February 9, 1987 is blown to the west. Ashfall from earlier eruptions darkens the southern flank of the volcano. Explosive eruptions from the summit crater during 1986-1990 were accompanied by lava flows from both summit and flank vents.

Photo by Alexander Belousov, 1987 (Institute of Volcanology, Petropavlovsk).

This spectacular nighttime time-exposure of México's Parícutin volcano in 1948 shows strombolian ejection of incandescent blocks and their trails as they roll down the slopes of the cone. Parícutin is renowned as the volcano that was born in a cornfield in 1943. It grew to a height of more than 150 m within the first week of its appearance, and remained active until 1952.

Photo by Carl Fries, 1948 (U.S. Geological Survey).

Strombolian eruptions at Pacaya volcano in Guatemala produce a colorful nighttime display. This November 1988 time exposure traces the incandescent parabolic arcs of individual volcanic bombs explosively ejected from the vent. Larger bombs remain incandescent after they hit the surface of the cone and roll down its flank. The orange line at the lower right is a lava flow that issued from a fissure on the upper NW flank of MacKenney cone.

Photo by Lee Siebert, 1988 (Smithsonian Institution).

Negro, Cerro
Vigorous strombolian eruptions from Nicaragua's Cerro Negro volcano in 1968 produce an ash-rich column above the vent. Strong incandescent lava fountaining can be seen at the base of the column. Steam rises from fumaroles on the righthand side of the cinder cone. Ash and cinders fall from the eruption column at the left.

Photo by William Melson, 1968 (Smithsonian Institution).

Negro, Cerro
A time exposure captures a nighttime view of a strombolian explosion in November 1968 from Cerro Negro volcano in Nicaragua. The trajectory of individual incandescent volcanic bombs can be seen radiating from the vent. Still-hot bombs continue to glow after landing on the outer flanks of the cinder cone. The 1968 eruption was one of many from Cerro Negro, Central America's youngest volcano.

Photo by Robert Citron, 1968 (Smithsonian Institution).

An erosional unconformity cutting diagonally across the center of the photo due to a glacial advance about 20,000-18,000 years ago separates two sequences of late-Pleistocene tephra layers from Ecuador's Chimborazo volcano. A less prominent unconformity below the light-colored tephra layer at the top of the sequence marks a 16,000-14,000 year old glacial advance. This ~12-m-thick exposure lies on the SW flank of Chimborazo, Ecuador's highest volcano.

Photo by Lee Siebert, 2006 (Smithsonian Institution).

Four days after the start of an eruption from Lonquimay volcano on December 25, 1988, a vigorous ash column rises above a vent on the NE flank. Winds distribute the ash column to the SE. Heavy ashfall from the eruption, which lasted over a year, caused severe economic disruption. This east-looking view shows the ice-filled summit crater of Lonquimay at the lower left. The dark area extending to the left from the eruptive vent, which formed along a fissure system trending NE from the summit, is a lava flow that originated on December 27.

Photo by Jeffrey Post, 1988 (Smithsonian Institution).

Deception Island
Ashfall darkens the sky above Antarctica's Deception Island in December 1967. Falling ash can be seen trailing from the margins of the eruption cloud. Winds distributed the ashcloud to the NE and deposited 30 cm of ash 2 km across Port Foster caldera bay onto the Chilean Antarctic research station, where this photo was taken.

Photo by Bernardo Blass, 1967 (published in González-Ferrán, 1995).