 During a period of 2 million years , extrusion of highly fluid basic and silicic lava flows built up the gently sloping Late Miocene Rainbow Range . It is one of three large peralkaline shield volcanoes that lie on an east @-@ west trend in west @-@ central British Columbia . Alkaline and peralkaline lava flows from four volcanic episodes make up an 845 m ( 2 @,@ 772 ft ) composite section on the north flank of the shield volcano . Basal comenditic trachyte flows are unconformably overlain by flows and flow breccias of mugearite . A sequence of 40 – 60 m ( 130 – 200 ft ) thick columnar @-@ jointed comendite flows blankets the underlying units and gives the volcano its shield @-@ like form . Hawaiite dikes , plugs , and minor capping flows are scattered over the north flank . Comendite flows , which commonly have a glassy selvage at the base , account for 75 % of the lavas in the flank zone . The Rainbow shield was erupting until 6 @.@ 7 million years ago . 
