Global Volcanism Program | Image GVP-04047
Thermal activity at the surface of a volcano is evidence of volcanic heat below. The fumarolic activity seen here produces vigorous gas-and-steam plumes along the sulfur-coated 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. The interaction of high-temperature volcanic fluids and gases with groundwater in hydrothermal fields can produce geysers, hot springs, and mud pools.
Photo by Kristal Dorion, 1994 (U.S. Geological Survey).
This image is made available as a Public Domain Work, but proper attribution is appreciated.