The rhyolite of Gillies Hill forms a cluster of rounded hills between Beaver basin and Cove Fort basin in southwest-central Utah. These rocks were erupted as a series of viscous lava flows, volcanic domes, and minor pyroclastic rocks from centers localized along and near the main fault separating the volcanic rocks in the Marysvale volcanic field to the east from batholithic rocks in the Mineral Mountains to the west. Faulting began in middle Miocene time before eruption of the rhyolite of Gillies Hill and continued episodically into Pleistocene time. Potassium-argon dating indicates that the rhyolite of Gillies Hill formed from a rapid sequence of eruptions about 9.1 m.y. ago.
The rhyolite of Gillies Hill is made up of an older high-silica suite (SiO2 >75%) consisting of numerous short stubby flows and of a younger low-silica suite (SiO2 = 70%) represented by a single large volcanic dome. The high-silica suite is believed to have formed by eruptions from the top of a compositionally zoned magma chamber. The younger low-silica suite came either from a separate magma chamber or from a significantly different level within the same magma chamber. The proximity of sources for both suites suggests eruptions from different levels within a single source chamber, perhaps from different vertically stacked convection cells.