The Cascade Mountains are a major volcanic arc running north to south through the Pacific Northwest region of North America, stretching from southern British Columbia in Canada through Oregon and into northern California in the United States. Their formation is the result of the subduction of the Juan de Fuca Plate under the North American Plate. The convergence of these two tectonic plates has led to the formation of volcanoes and the extrusion of large volumes of magma, which over time formed much of the Cascade Range.

The rocks that make up the Cascade Mountains are predominantly igneous in origin, meaning they formed from magma that cooled and solidified. These rocks include lava flows, volcanic domes, ash-flow tuffs, cinder cones, and other types of volcanic deposits. The composition of the rocks varies depending on the type of volcanic activity that produced them. Some areas, such as the central part of the range, are dominated by silica-rich rocks, such as rhyolite, dacite, and andesite, which are light in color and have high viscosity. Other areas, particularly near the northern end of the range, are dominated by darker, more mafic lava flows and volcanic deposits composed of rocks like basalt and andesite.

Overall, the Cascade Mountains are a prime example of volcanic geology and showcase a wide range of igneous rocks that were formed by subduction-related volcanic processes over millions of years.