The new crust tends to move horizontally from both sides of the divergent boundary, usually known as an oceanic ridge. A divergent boundary is thus a new crust zone. Most new crust zones are presently on the seafloor, producing sea-floor spreading. This may seem slow, but the process has been going on for millions of years and has caused a tiny inlet of water between the continents of Europe, Africa, and the Americas to grow into the vast Atlantic Ocean that exists today. The creation of new crust at a divergent boundary means that old crust must be destroyed somewhere else at the same rate, or else the earth would have a continuously expanding diameter.
Old crust is destroyed by returning to the asthenosphere at convergent boundaries.
The collision produces an elongated belt of down-bending called a subduction zone. The lithosphere of one plate, which contains the crust, is subducted beneath the second plate and partly melts, becoming part of the mantle. The more dense components of this subducted crust may become igneous materials that remain in the mantle. Some of it may eventually migrate to a spreading ridge to make new crust again. The less dense components may return to the surface as a silicon, potassium, and sodium-rich lava, forming volcanoes on the upper plate, or it may cool below the surface to form a body of granite.