Phreatic and phreatomagmatic eruptions Phreatic eruption: 1: water vapor cloud, 2: magma conduit, 3: layers of lava and ash, 4: stratum, 5: water table, 6: explosion, 7: magma chamber Phreatic eruptions occur when rising magma makes contact with ground or surface water.[11] The extreme temperature of the magma causes near-instantaneous evaporation, resulting in an explosion of steam, water, ash, rocks and volcanic bombs.[11] The temperature of the rock fragments can range from cold to incandescent.