As the summer wraps up, it is also time to wrap up my tour of the Cascade Range volcanoes as seen from space. The modern Cascades extend into Northern California and is the home of likely the largest volcano of the chain ... but the Earth's plates are plotting the end of the Cascades, starting from California.
The ocean plates that spawn the magma that forms the Cascade volcanoes are young and small. The Juan de Fuca plate is responsible for the volcanoes from Oregon to British Columbia (save for a sliver of the micro-micro Explorer Plate off Canada). However, once you cross into California, another micro-plate gets involved: the Gordo Plate. Sometimes lumped into the Juan de Fuca plate, the Gordo Plate is found off the very southernmost coast of Oregon and continues all the way down to area near Mendocino in northern California.
Triple the Fun
It is off the coast of Mendocino where something pretty unique exists: a triple junction. Here, three tectonic plates come into contact: the Gordo Plate, the Pacific Plate and the North American Plate. This triple junction means that south of Mendocino, subduction isn't happening anymore but instead the two plates (Pacific and North American) are sliding side by side - a motion called "strike-slip". This transform boundary is the reason the San Andreas fault system exists.
