McLoughlin is part of the High Cascades, which trend north-south. Formed towards the end of the Pleistocene Epoch, these mountains are underlain by more ancient volcanoes that subsided due to parallel north-south faulting in the surrounding region. Like other Cascade volcanoes, Mount McLoughlin was fed by magma chambers produced by the subduction of the Juan de Fuca tectonic plate under the western edge of the North American tectonic plate. Within Oregon, plutons, or bodies of intrusive igneous rock that crystallize from magma cooling below the surface of the Earth, lay between northwest of the major High Cascade axis. At McLoughlin, mean displacement rates for the past 16 million years have been each year.