Quaternary extrusion rates of the Cascade Range, northwestern United States and southern British Columbia

Quaternary (2–0 Ma) extrusion rates change significantly along the Cascade Range volcanic arc. The extrusion rate north of Mount Rainier is about 0.21 km³ km⁻¹ m.y.⁻¹; the rate in southern Washington and northern Oregon south to Mount Hood is about 1.6 km³ km⁻¹ m.y.⁻¹; in central Oregon the rate is 3–6 km³ km⁻¹ m.y.⁻¹; and in northern California, the rate is 3.2 km³ km⁻¹ m.y.⁻¹. Eruption style also changes along the arc but at latitudes different from rate changes. At the ends of the arc, volcanism is focused at isolated intermediate to silicic composite volcanoes. The composite volcanoes represent ∼30% of the total volume of the arc. Mafic volcanic fields partly ring some composite volcanoes, especially in the south. In contrast, volcanism is diffused in the middle of the arc, where numerous overlapping mafic shields and a few composite volcanoes have built a broad ridge. Contrasting eruption style may signify diffuse versus focused heat sources or may reflect changes in permeability to ascending magma along the arc.