Character of volcanism, volcanic hazards, and risk, northern end of the Cascade magmatic arc, British Columbia and Washington State

Abstract

Listen

Translate article

A band of Oligocene (30 Ma) to Recent volcanoes and associated intrusive rocks extend from latitude 44°N in northern California to latitude 52°N in southwestern British Columbia. These centres make up the Cascade magmatic arc. The arc formed in response to subduction of the Farallon Plate (the present day Juan de Fuca Plate). Volcanism from 5 Ma to the present is restricted to a linear belt of volcanoes broken into six segments. The northernmost segment extends from Glacier Peak in Washington State to the Bridge River cones in British Columbia. The most recent significant volcanic event in this segment of the arc is a Plinian to Peléan eruption from Mount Meager, (2400 BP). This event blocked the Lillooet River and spread ash across southern British Columbia into Alberta. Activity at Mount Baker occurred as recently as 1975. The geological record of lava flows and volcanic deposits suggests that both basaltic and andesitic eruptions, and infrequent violent dacitic explosive eruptions, may occur. Of the numerous hazards associated with volcanic eruptions, the most likely to affect large numbers of people are airfall tephras and debris flows. A continuing hazard is posed by the extreme relief of many vent areas and the unstable nature of volcanic deposits. At least 44 large postglacial debris flows are known at Mount Baker and landslides from Mount Cayley and the "Barrier" (near Mount Garibaldi) have blocked the Squamish and Cheakamus Rivers. The loose unconsolidated nature of some volcanic deposits leads to increased sediment loads in surrounding drainages and leaching of soluble elements into the groundwater.