By late Pliocene time the Farallon Plate had been greatly reduced in size and its northern portion ultimately broke off between five and seven million years ago to form a new plate boundary called the Nootka Fault. This rupture created the two small Juan de Fuca and Explorer plates that lie off the west coast of Vancouver Island. The four-million-year-old Garibaldi Volcanic Belt, a north-south trending zone of volcanoes and volcanic rock in the southern Coast Mountains of southwestern British Columbia, can be grouped into at least three enechelon segments, referred to as the northern, central, and southern segments. The northern segment overlaps the older Pemberton Volcanic Belt at a low angle near the Mount Meager massif where Garibaldi Belt lavas rest on uplifted and deeply eroded remnants of Pemberton Belt subvolcanic intrusions and combines to form a single belt. A few isolated volcanoes northwest of the Mount Meager massif, such as Silverthrone Caldera and Franklin Glacier Complex, are also grouped as part of the Garibaldi Volcanic Belt. However, their tectonic origins are largely unexplained and are a matter of going research. When the Farallon Plate ruptured to create the Nootka Fault between five and seven million years ago, there were some apparent changes along the Cascadia subduction zone. At issue is the current plate configuration and rate of subduction but based on rock composition is for Silverthrone Caldera and Franklin Glacier Complex to be subduction related. The roughly circular, wide, deeply dissected Silverthrone Caldera in the northern segment of the Garibaldi Volcanic Belt, was formed one million years ago during the Early Pleistocene period. The bulk of the volcano was erupted 0.4 million years ago, but younger phases, consisting of lava flows and subsidiary volcanoes with compositions of andesite and basaltic andesite are also present. Mount Silverthrone, an eroded lava dome on the northeast edge of Silverthrone Caldera, was episodically active during both Pemberton and Garibaldi stages of volcanism. The eroded Franklin Glacier Complex just to the southeast consists of dacite and andesite rocks that range in age from 3.9 to 2.2 million years old. Southeast of Franklin Glacier Complex, the Bridge River Cones comprise remnants of both andesitic and alkali basalt cones and lava flows. These range in age from about one million years old to 0.5 million years old and commonly display ice-contact features related to subglacial eruptions. The Mount Meager massif, the most persistent volcano in