Some volcanoes have thermal features such as steaming vents, geysers, hot springs, lava flows, or lava domes. Surface temperature changes at these thermal features sometimes occur before a volcanic eruption. Recognizing these "thermal anomalies" can be helpful in forecasting changes in activity. To identify thermal anomalies, continuous monitoring is needed so that we understand the normal/background thermal characteristics.
The USGS Volcano Hazards Program monitors three types of volcanic thermal features.
- Steam, or vapor-dominated features such as gas vents, fumaroles, and mud pots range in temperature from boiling up to several hundred degrees (about 400 °C or 750 °F).
- Water-dominated features include geysers, hot springs/pools, crater lakes, elevated sea surface temperature (e.g., from underwater or island volcanic activity), and even melting ice (e.g., sub-glacial volcanoes). These range in temperature from freezing to boiling.
- Lava-dominated features include lava lakes, lava flows, lava domes, and pyroclastic flows, and can reach temperatures up to about 1200 °C (2200 °F).
Steam and water-dominated thermal features are formed as cooling magma beneath the surface heats the rocks and ground water above it, which causes the heated fluids to circulate via convection in the overlying rocks.
Temperature measurements from the ground must be taken cautiously.
Directly measuring the temperature of hot spring basins, fumarole fields, or active lava flows can be dangerous. For very precise measurements, scientists place devices called thermocouples directly onto the hot surface of a thermal feature. Such measurements are most often made at volcanoes that are easily accessible, and scientists only conduct the measurements if the areas are safe to be in for several minutes. Although direct measurements are very precise, they are limiting because they can only cover a small or specific area.
Temperature measurements can also be made from a distance with cameras or instruments that measure thermal infrared (TIR) radiation, which is emitted by the heated surface. However, TIR radiation can be partially or completely obscured by thick clouds or ash plumes, so these types of measurements are best made during favorable atmospheric conditions. A common instrument used by scientists in the Volcano Hazards Program is a forward-looking infrared (thermal) camera, which produces an image using a color spectrum that correlates to detected temperature.
When volcanoes release heat, satellite sensors can detect it from space.
Infrared satellite sensors can detect volcanic thermal features. This is an important tool for monitoring remote volcanoes with sparse ground-based equipment. Some satellite sensors make daily or semi-daily observations above U.S. volcanoes, but these sensors record images of large areas with low resolution, which limits the fine-scale details that can be seen of the thermal features. More detailed, higher resolution, TIR images from satellite and airborne sensors are possible, but observations are usually less frequent. Therefore, USGS remote sensing scientists use a combination of sensor imagery to monitor volcanic thermal features and look for anomalies.