A large steam column rises from a fumarole at about 1600 m altitude on the north flank of Chiginagak volcano on the Alaska Peninsula in May 1994. Strong steam emission is a common occurrence at this volcano and can be mistaken for eruptive activity.
Brilliantly colored deposits of elemental sulfur surround fumarolic vents on the NW side of the central cone of Mendeleev volcano, in the southern Kuril Islands. Fumarolic areas on Mendeleev are associated with lateral craters at this location and at several areas from the NE to SE flanks, where the central cone meets the inner caldera wall of Mendeleev. Hot springs occur on the NE flank and along the NE coast, where the Goriachi-Pliazh geothermal field is located.
Vigorous mud eruptions occur at Las Hornillos thermal area on the west flank of Costa Rica's Miravalles volcano. A geothermal project in the 15 x 20 km Guayabo caldera (inside which Miravalles volcano was constructed) provides a major component of the electrical power needs of Costa Rica.
Vigorous steam plumes rise from craters near the summit of On-take volcano on November 9, 1979. The first historical eruption from On-take, in central Honshu, Japan, began on October 28 and produced ashfall to the NE from a 1.5-km-high eruption plume. Intense vapor emission (with minor ash that dusted the summit region) continued for several months.
Thermal activity at the surface of a volcano is evidence of volcanic heat below. The fumarolic activity seen here produces vigorous steam plumes along the sulfur-encrusted wall of the summit crater at El Salvador's Santa Ana volcano. Thermal activity is common during non-eruptive periods at many volcanoes and may persist for many thousands of years. In addition to the fumarolic activity in this photo, the interaction of high-temperature volcanic fluids and gases with groundwater in hydrothermal fields can produce geysers, hot-spring pools, and mudpots.
Mud pots boil in this 1978 view of the San Jacinto thermal area at El Salvador's Telica volcanic complex. The alignment of fumaroles and mud pots suggests an underlying fault running NNE. Magmatic steam and gases have been emitted from the thermal area for many years at a fairly constant rate.
A small mudpot vent in Pocket Basin at the north end of Yellowstone's Lower Geyser Basin produced this intriguing feature that mimics a spatter cone that issued a lava flow. Mudpots form in areas of intense, clay-rich hydrothermal alteration where the thermal system is dominated by the gas phase.
Riverside Geyser in the Upper Geyser Basin is the most regular of Yellowstone's geysers. Every six hours it erupts a 25-m-high inclined jet from a small vent hole on the east bank of the Firehole River 2 km downstream from Old Faithful.