The Trout Creek Mountains are composed mostly of basalt from a shield volcano that once stood where Steens Mountain is today. Crustal thinning and the Yellowstone hotspot, which was then beneath southeastern Oregon, induced eruptions from Steens and nearby vents about 17 million years ago, in the Miocene. The vents produced a series of lava flows that spread across the land now known as the Trout Creek Mountains. Eruptions from the Steens volcano lasted for about one million years, and at least 70 separate lava flows occurred. Under the resulting basalt rock lie much older metamorphic rocks that may be related to some of the Triassic age formations of the Blue Mountains in northeastern Oregon. Within these metamorphic rocks are diorite and granodiorite intrusive bodies which were presumably intruded during the Cretaceous Period.