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.
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.
A surtseyan eruption on May 8, 1971, from Crater Lake at the summit of Ruapehu volcano in New Zealand ejects a dark column of ash, mud, and steam. Individual ejected blocks can be seen at the margins of the cloud, trailing cockscomb sprays of ash and steam. This type of euption column is typical of explosions that involve water-magma interaction.
New Zealand Geological Survey volcanologist Brad Scott conducts theodolite measurements at the Crater Lake vent in 1988 at the summit of Ruapehu volcano. Measurements of the lake height, temperature and chemistry are made routinely, and along with seismic instrumentation, are used to help forecast future activity of the volcano. Intermittent steam explosions from beneath the lake have produced lahars, which have damaged ski facilities on the upper flanks and structures in river valleys below the volcano.