Types and Processes Gallery

Magma meets Water
The interaction of magma with water near Earth's surface can intensify explosive eruptions. Steam-driven explosive eruptions, known as phreatic eruptions, can occur when an ascending magma body encounters groundwater. The ensuing eruptions often do not involve any ejection of new magma, but rather the fragmentation and explosive expulsion of pre-existing rock along the path of the volcanic conduit. Phreatic eruptions are gradational into magmatic eruptions, and many eruptions in fact involve the ejection of both old and new volcanic materials and are referred to as phreatomagmatic. Phreatic eruptions are sometimes accompanied by pyroclastic surges, dilute laterally moving clouds of gas, ash, and rock that sweep radially away from the vent.

A small phreatic eruption on February 29, 1980, produces a column of ash and steam above Ruapehu's Crater Lake. A darker central plug is surrounded by a white ring produced by pyroclastic surges traveling across the lake surface. This view is from the NW, with Mitre Peak at the upper left. A series of small phreatic explosions had begun December 5, 1979, and lasted until April 15 of the following year.

Photo by Peter Otway, 1980 (New Zealand Geological Survey).

The interaction of magma and water can produce strong phreatic (steam-driven) explosions, such as seen in this 1980 photo of New Zealand's Ruapehu volcano. Clouds of ash and steam trail from large ejected blocks in the eruption column. Laterally moving pyroclastic-surge clouds form a white basal ring above the surface of a crater lake. Phreatic or phreatomagmatic explosions are common at submarine volcanoes, crater lakes, and other places where hot magma (or associated gases) encounters surface water or groundwater.

Photo by Hollick, 1980, courtesy of Bruce Houghton (Wairakei Research Center).

Kelut volcano has been notorious for the repeated ejection of crater-lake water during eruptions, producing devastating lahars. A series of tunnels and shafts were constructed in the 1920's to lower the lake level and reduce the hazards of eruptions. The initial tunnels lowered the lake level 50 m, but the 1951 eruption deepened the crater by 70 m, leaving 50 million cu m of water. Following another devastating eruption in 1966, lower outlet tunnels were constructed, and prior to the 1990 eruption the lake contained only 1 million cu m of water.

Photo by John Dvorak, 1980 (U.S. Geological Survey).

The turquoise waters of Yu-gama, one of three craters at the summit of Japan's Kusatsu-Shirane volcano, are a popular tourist destination. Yellow rafts of sulfur float on the surface of the acidic lake, which prior to an eruption in 1882, was clear, with forested walls. Frequent phreatic explosions have occurred from Yu-gama and the two other summit craters during historical time. This 1981 photo was taken from the south crater rim.

Copyrighted photo by Dick Stoiber, 1981 (Dartmouth College).

A phreatomagmatic explosion on December 25, 1988, from Japan's Tokachi volcano ejects incandescent blocks and a dark ash cloud. The base of the ash column is the leading edge of a small pyroclastic surge that eventually traveled down the north flank to 1 km from the vent. The 1988 eruption began with a phreatic explosion on December 16. Intermittent explosive eruptions with small pyroclastic flows and surges began on December 19 and continued until March 5.

Photo courtesy of Japan Meteorological Agency, 1988.

A geyser-like ejection of steam and ash rises above the surface of the crater lake of Poás volcano in July 1977. The white ring at the base of the eruption column is a steam cloud that is traveling laterally away from the vent along the surface of the crater lake. Mild phreatic explosions such as this one were typical of an eruption that began in May 1977 and lasted at least until July. The crater walls rise about 250 m above the lake.

Photo by S. Racchini, 1977 (Universidad Nacional Costa Rica, courtesy of Jorge Barquero).

Soufriere St. Vincent
This December 1971 photo shows a lava dome rising above the surface of a crater lake on Soufrière St. Vincent volcano in the West Indies. The lake temperature rose to 80 degrees Centigrade during extrusion of the dome, but despite the extrusion of new magma in the crater no explosive eruptions occurred during the eruption.

Photo by Jack Frost, 1971.