Phreatic eruptions are steam-driven explosions that occur when magma heats groundwater or surface water. Unlike magmatic eruptions, phreatic eruptions do not involve the direct expulsion of new magma. Instead, they are characterized by the rapid conversion of water to steam, which then expands explosively, shattering surrounding rock and ejecting a mixture of steam, water, ash, and rock fragments. These eruptions are often precursors to larger magmatic eruptions, as they can clear pathways for magma to ascend.

The consequences of phreatic eruptions can be significant, despite their non-magmatic nature. The primary hazards include:

1. Ballistic Projectiles: Large rock fragments, sometimes weighing several tons, can be ejected at high velocities, traveling several kilometers from the vent. These projectiles pose a direct threat to life and infrastructure.
2. Ashfall: While typically less voluminous than magmatic ash, phreatic ash can still cause respiratory problems, contaminate water sources, damage crops, and disrupt transportation and communication systems.
3. Steam and Gas Emissions: The sudden release of steam can be accompanied by volcanic gases like sulfur dioxide (SO2) and hydrogen sulfide (H2S), which can be hazardous to health, especially in confined areas.
4. Lahars: If the eruption occurs in an area with significant snow or ice, or if it mobilizes loose volcanic debris and water, it can trigger destructive lahars (volcanic mudflows) that flow down valleys, burying everything in their path.
5. Landslides and Rockfalls: The explosive force can destabilize slopes, leading to landslides and rockfalls, particularly on the flanks of the volcano.
6. Precursor to Larger Eruptions: Perhaps one of the most critical consequences is their role as indicators. Phreatic eruptions often signal increased volcanic activity and can precede more dangerous magmatic eruptions, providing a warning but also creating uncertainty about the volcano's future behavior.