Volcanoes can and do form underwater, and in fact most volcanoes are on the ocean floors, not on land. A chain of volcanoes below the sea circles the earth for more than 30,000 miles (48,300 km), standing, on average, about 18,000 feet (5,486 m) above the sea floor. This chain is called the Mid-Ocean Ridge. Here, tectonic plates are spreading apart and volcanoes are forming new crust. Mid-ocean ridges are geologically active, with new magma constantly emerging onto the ocean floor. The crystallized magma forms new crust of basalt and gabbro. It is estimated that about 20 volcanic eruptions occur each year along earth’s mid-ocean ridges, and that every year 2.5 square kilometers of new sea floor is formed by this process.