Mount Garibaldi began erupting about 250,000 years ago and has grown steadily since then. Like all of the Cascade volcanoes, Mount Garibaldi has its origins in the Cascadia subduction zone—a long convergent plate boundary that stretches from mid-Vancouver Island to Northern California. The subduction zone separates the Juan de Fuca, Explorer, Gorda and North American Plates. Here, the oceanic crust of the Pacific Ocean sinks beneath North America at a rate of per year. Hot magma upwelling above the descending oceanic plate creates volcanoes, and each individual volcano erupts for a few million years.