Under this system, the USGS ranks the level of activity at a U.S. volcano using the terms "normal", for typical volcanic activity in a non-eruptive phase; "advisory", for elevated unrest; "watch", for escalating unrest or an eruption underway that poses limited hazards; and, "warning", if a highly hazardous eruption is underway or imminent. These levels reflect conditions at a volcano and the expected or ongoing hazardous volcanic phenomena. When an alert level is assigned by an observatory, accompanying text will give a fuller explanation of the observed phenomena and clarify hazard implications to affected groups.