The mountain also forms part of a group of more than 30 large shield volcanoes and stratovolcanoes that form a segment of Pleistocene-to-Holocene-epoch volcanic vents that produced mafic lava (rich in magnesium and iron). Three Fingered Jack includes several, overlapping cinder cones and composite cones over underlying lava flows from shield volcano activity. These volcanic edifices and their lava flow deposits cover an area of . Between Three Fingered Jack and the North Sister volcano and among recently active basalt lava fields, there is an array of cinder cones encompassing an area of .