How did lava and volcanic ash help form the Sierra Madre Occidental?

The Sierra Madre Occidental was formed through a complex geological history involving both ancient basement rocks and significant volcanic activity during the Cenozoic Era. Contrary to the assumption that the range consists solely of simple, horizontal lava layers, it is characterized by deformed underlying strata that were shaped by folding and faulting over several periods.1

During the Cenozoic Era, large quantities of lava and ash were deposited across earlier surfaces, creating a volcanic mantle that contributes to the range's current structure. While the range was once thought to be a structurally simple feature composed of these nearly horizontal lavas covering Mesozoic sediments and older crystalline rocks, it is now understood to have a more intricate history involving numerous igneous intrusions of varying sizes. Over time, the margins of the plateau block—which was uplifted during the Cenozoic—were deeply dissected, resulting in the rugged, angular landscape and formidable escarpments that define the Sierra Madre Occidental today.12