Mid-ocean ridge

Mid-ocean ridge is an underwater mountain range formed by plate tectonics. At mid-ocean ridge, molten material rises from the mantle and erupts. The molten material then spreads out, pushing older rock to both sides of the ridge. As it cools, it forms a strip of solid rock in the center of the ridge. Then more molten material splits apart the strip of solid rock that formed before, pushing it aside. The process, called sea-floor spreading, continually adds new material to the ocean floor. The ocean floor does not just keep spreading. Instead, it sinks beneath deep underwater canyons called deep-ocean trenches. Then subduction takes place. Subduction is the process when the ocean floor sinks beneath a deep-ocean trench and back into the mantle. At deep-ocean trenches, subduction allows part of the ocean floor to sink back into the mantle, over tens of millions of years. This process can change the size and shape of the oceans, and the ocean floor is renewed about every 200 million years.
(the following picture shows the mid-ocean ridge in red, to give you the idea of how big it is. Click on it to make it bigger.)