Sierra Blanca is a massive complex of volcanic rocks, including pyroclastic materials, lava flows, and intrusions. An ancient and heavily eroded volcanic pile, it is the largest mid-Tertiary volcanic complex east of the Rio Grande, with an estimated volume of erupted products of . Eruptions began about 38 million years ago, and extended over a twelve-million-year period. Most of the eruptions produced voluminous lava flows and breccias, with numerous intrusive dikes emplaced throughout the complex. The final activity produced the intrusions which form the present-day Sierra Blanca Peak. Following the volcanic period, the range's topography was modified by Pleistocene glaciation, block faulting, and erosion.