Goat Rocks is a stratovolcano with a somewhat complicated eruptive history. It first became active approximately 3.2 million years ago during the Pliocene epoch, undergoing explosive eruptions that ejected silicic lava with highly felsic rocks like rhyolite. One of these events produced of tuff that remains, exposed, on the east flank of the existing mountain. Three million years ago Goat Rocks shifted to mafic volcanism, erupting olivine and basalt. As eruptions continued into the Pleistocene epoch, the lava flows became increasingly andesitic, containing mostly pyroxene with phenocrysts as well as hornblende minerals. These andesitic flows formed the volcano's major edifice between roughly 2.5 and 0.5 million years ago, which may have resembled contemporary Mount Rainier. At this point in time, Goat Rocks's eruptions had very high volumes and extended far from the volcano, possibly because multiple satellite vents contributed to eruptions. Approximately 1 million years ago, Goat Rocks generated an andesitic lava flow extending down the Naches and Tieton Rivers, which has been recognized as the longest andesite flow on Earth. Many of the flows immediately surrounding the volcano have since been cut by intrusive dikes, which form a radial arrangement around the volcano's core. As erosion occurred on a large scale, the volcano remained active, producing more hornblende andesite lava flows. Eruptive activity continued at Old Snowy Mountain into the middle to late Pleistocene, yielding hornblende andesite lava flows that were subsequently glaciated in the Cispus River valley; whether Old Snowy Mountain is a vent of Goat Rocks or an independent volcano remains unclear. Hornblende andesite can also be found on top of the highest point in Goat Rocks, Gilbert Peak.