 Over a period of 2 million years , extrusion of highly fluid basic and silicic lava flows built up the gently sloping Late Miocene @-@ to @-@ Early Pliocene Ilgachuz Range . It is the second youngest of the three large peralkine shield volcanoes that lie on an east @-@ west trend in west @-@ central British Columbia with a diameter of 25 km ( 16 mi ) . The Ilgachuz shield was created by two chemically separate magmatic periods ; an early complex series of trachyte and rhyolite eruptions and late extrusion of a sequence of basaltic lava flows . The surface of the outer shield , cut by deeply incised radial valleys , rises to a complexly dissected summit area where remnants of a small central caldera and numerous felsic domes and basaltic feeders are exposed . The outer flanks of the shield merge with flat @-@ lying basalt of the Chilcotin Group , which is thought to have been formed by a back @-@ arc basin behind the Cascadia subduction zone . The volcano was spewing out ash , cinder , pumice , rhyolite , trachyte and basaltic lava up until 4 million years ago . 
