Three Fingered Jack is estimated to be between 500,000 and 250,000 years old. It underwent more explosive eruptions as its eruptive history progressed, generating large amounts of tephra, which created a loose, unconsolidated summit and upper cone. Lava flows near the summit have an average thickness of . Secondary volcanic craters produced lava flows and pyroclastic material, which traveled both north and south of the volcano. Cone-building eruptions ceased before the Pleistocene epoch's glacial period, allowing the expanded glacial cover to remove most of the cone, especially on the eastern and northeastern sides.