 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 . 
