The Smartville Volcanics are much older than most of the Franciscan rocks of the Coast Ranges. Whereas the Franciscan was subducted about 100 Ma, the Smartville rocks date from the Jurassic, 200 to 170 Ma, and were subducted 175165 Ma. They represent an oceanic island arc, like those in today's western Pacific, that was carried to the edge of the North American plate and plastered onto it. Subduction zones capture more than just deep-sea chertthey also include island arcs, oceanic plateaus, seamounts, chunks of oceanic crust and whatever else may exist on the deep seafloor. The Smartville block forms much of the western part of the northern Sierra.