Bruneau-Jarbidge Eruptive Center
Owyhee Mountains
The Owyhee Mountains are underlain by Cretaceous granitic rock and Tertiary volcanic rocks ranging from Eocene to Miocene age. These Cretaceous granitic rocks are bordered by Cretaceous hornblende gabbro and overlain by pre-Cretaceous metasediments and are thought to be a southward extension of the Idaho Batholith. The Eocene-age silicic volcanics correlate with the Challis volcanics. They are up to 33,000 feet thick and are dated at 44.7 million years. Olivine basalts and andesitic lavas of Oligocene age reach up to 3,800 feet thick and are dated at 30.6 million years. Miocene volcanic rocks of basalt and latite are dated at 17 million years and are 3,300 feet thick. The Miocene rocks are intruded by rhyolites. Olivine basalts and interbedded sedimentary rocks are dated at 8 to 10.5 million years.

Bruneau-Jarbidge Eruptive Center
Volcanic flows, exposed by the canyon of the Bruneau River, were erupted from the Bruneau-Jarbidge eruptive center. This eruptive center is about 5 9 miles long and lies southeast of the scenic Bruneau Canyon. Numerous ash flows, lava flows and basalt flows were erupted during Miocene and Pliocene time. The Banbury Basalt was erupted from shield volcanoes and filled in the low areas. The Idavada volcanics, known as the Cougar Point Tuff, consist of densely-welded, ash-flow tuffs and younger rhyolite flows about 10 to 12 million years old. Altogether there are nine or more of these welded tuff cooling units which filled a large basin. The eruptive center was named by Bonnichsen after the Bruneau and Jarbidge Rivers which have cut canyons in the area. The canyons expose both silicic and basalt flows.

Ash-Flow Tuffs (Cougar Point Tuffs)
The Cougar Point ash-flow tuffs were formed by a succession of pyroclastic eruptions that discharged hot, vesiculated magma particles into the air. The ash was buoyed by air and transported long distances over a smooth flat surface. The hot ash consisted of shards with minor pumice and lithic fragments. The eruptive material flowed away from the source on a cushion of hot air. Once the gases were dissipated, the hot mass collapsed, compacted and cooled. The emplacement temperature is estimated to be in the vicinity of 900 to 1,000 degrees centigrade. The ash flows coalesced to a viscous liquid as indicated by flow marks and elongate vesicles and folds developed during the final stages of movement.