The Idaho Rift System is a 62-mi-long, north-to northwest-trending fissure zone that arcs across the eastern Snake River Plain. It consists of four rift sets, which are, from north to south: (1) Great Rift set, 34 mi long and trending N. 35° W., mostly in Craters of the Moon National Monument; (2) Open-Crack rift set, 13 mi, N. 30° W., from which practically no volcanic products have issued; (3) King's Bowl rift set, 6.8 mi, N. 10° W., which has been descended to a depth of 800 ft; (4) Wapi rift set, 11 mi, north-south, presumed to be present beneath the Wapi lava field. Although the rift sets differ in strike, they are part of a single system. A radiocarbon date of 2130 + 130 yrs for the King's Bowl rift is presumed to represent the age of opening of the rift system. A persistent northeast-trending older rift direction is also prominent in this part of the plain. The tectonic significance of the Idaho Rift System is difficult to evaluate, but it may be related to crustal plate movements or to fault-block ranges flanking the plain.