Both mid-oceanic ridges and subduction zones are caused by plate tectonics. The Earth's crust (lithosphere) is composed of various plates that float on the surface of the asthenosphere and move very slowly over time. Mid-oceanic ridges occur at divergent plate boundaries, or where two plate edges under the ocean move away from one another. Magma rises up at the boundary to form an underwater mountain chain as it cools to form new crust. Subduction zones occur at convergent plate boundaries where two plates (usually an ocean plate and a land plate) are colliding. One plate is pushed underneath (subducted) the other plate. This causes mountain chains to form by thrusting pieces of the crust upward.