Mount Washington is a deeply eroded volcano in the Cascade Range of Oregon. It lies within Deschutes and Linn counties and is surrounded by the Mount Washington Wilderness area. Like the rest of the Oregon Cascades, Mount Washington was produced by the subduction of the oceanic Juan de Fuca tectonic plate under the continental North American tectonic plate, forming during the late Pleistocene. Made mostly of mafic (rich in magnesium and iron) volcanic rock like subalkaline basalt and basaltic andesite, it has a volcanic plug occupying its summit cone and numerous dikes. It is surrounded by other volcanic features such as cinder cones and spatter cones. The volcano's last eruptions took place from spatter cones about 1,350 years ago, generating basaltic andesite lava deposits.