Where did the continents come from? Modern continental crust forms mainly above subduction zonesSubduction zone, where water-rich slabs of oceanic lithosphere drive melting in the mantle wedge and in the overlying crust. The resulting silica-rich magmas assemble into long-lived arcs; repeated addition, thickening, and partial melting of these arcs gradually builds continents. In this chapter we practice thinking in deep timeDeep time—thousands to billions of years—to follow that story from early oceans and stromatolitesStromatolites, through the Proterozoic “boring billion,” to the Cambrian diversification of life. We also show why the ocean floor is young though the ocean water is ancient, and how the Wilson CycleWilson Cycle explains the birth, growth, and demise of ocean basins. Along the way we examine why the fossilFossil record is patchy, how sea level varies on different timescales, and what plate motions look like when measured today.