Volcanoes do not announce a change through one universal signal. Observatories combine different measurements because each instrument observes a different part of the system. This guide explains what common signals can contribute—and why a single chart, sensor, or webcam image cannot tell the whole story.
INTERACTIVE FIELD SKETCH
One volcano, several observation windows
EarthquakesSeismometers record shaking and tremor at instrument locations. Events can have volcanic, tectonic, or other causes.
Conceptual illustration only. Instruments measure different things; no single sketch or signal predicts an eruption.
What each signal can—and cannot—tell you
| Evidence | What it can contribute | Important limitation |
|---|---|---|
| Earthquakes and tremor | Changes in where and how often earthquakes occur, or sustained vibration, can help observatories track changes in a volcanic system. | Earthquakes also have tectonic and other causes. A count or isolated event does not, by itself, identify an eruption or its timing. |
| Ground deformation | GNSS, tiltmeters, radar, and other methods measure changes in the ground surface that can reflect changes below the volcano. | Ground movement has more than one possible cause, and instruments sample particular locations. The pattern must be interpreted with other observations. |
| Volcanic gases | Gas composition and emission rates can provide evidence about magma and changing pathways through a volcanic system. | Wind, weather, access, and measurement method affect observations; gas data need expert interpretation and may not be continuous. |
| Visible and thermal cameras | Images can document visible changes at a particular place and time. Thermal cameras measure emitted heat and can reveal some hot features in low light. | A camera sees only its field of view. Clouds, darkness, distance, blocked views, outages, and image delays can hide activity; ordinary cameras do not see underground. |
Why observatories compare measurements
A change is more informative when scientists can compare its timing, location, size, and pattern with other observations and with that volcano’s usual behavior. For example, an earthquake sequence and measured ground movement are different observations; together, they may help an observatory assess unrest. The meaning depends on the volcano and the full record. The public dashboard should not turn a sensor change into a simple eruption prediction.
Monitoring networks also differ. Some volcanoes have dense instrument coverage; others are remote or have fewer continuous measurements. A blank chart or absent feed can mean that a source is unavailable, not that the volcano has no activity.
A careful way to use a webcam
- Find the camera’s source, location, viewing direction, and capture or refresh time.
- Check whether it is a visible-light image, a thermal image, a live stream, or a refreshed still image.
- Consider weather, darkness, distance, and anything outside the camera’s field of view.
- Compare an interesting image with the observatory’s latest notice and other monitoring evidence.
A quiet-looking view cannot confirm that a volcano is quiet, and an unusual-looking image cannot confirm an eruption. If a feed appears stale or unavailable, open the original provider’s page. Never use a public webcam as an evacuation, travel, or safety signal.
How this relates to MagmaAlert
MagmaAlert brings selected public notices, camera feeds, maps, and alert history into one interface. Camera availability and refresh timing depend on the provider. We link to source organizations so you can check the original context; MagmaAlert does not analyze these signals to issue independent forecasts.
Primary sources and further reading
Written by the MagmaAlert team for general education. This guide summarizes public USGS material; it is not produced or endorsed by the USGS and is not a scientific interpretation of live data. Send a correction if you find an error.