Phreatic eruptions are known for their sudden onset, often with few or no precursors. However, some potential warning signs include[1] :

1. Gas leaks: The release of gases (mostly H2O, CO2, and SO2) from the magma into the atmosphere through cracks in the overlying rock.
2. Deformation of the volcano: This can indicate that a magma chamber at depth is swelling or becoming more pressurized.
3. Seismic activity: Many small earthquakes, indicating that magma is on the move, can be a sign of potential phreatic eruptions.
4. Steam eruptions: Sudden steam eruptions, also known as phreatic eruptions, can be a warning sign of potential volcanic activity.

It’s important to note that while these signs can be indicative of potential phreatic eruptions, the onset of such eruptions is often rapid and may not always be preceded by clear warning signals.