Approximately 80 percent of all volcanic activity on Earth occurs underwater. Mid-ocean ridges, the longest mountain chains on the planet, are sites of continuous volcanic activity where tectonic plates diverge. Submarine eruptions create new ocean floor at a rate of about 3.4 square kilometers per year. Black smokers — towering hydrothermal chimneys reaching heights of 60 meters — form when superheated mineral-rich water meets cold seawater, precipitating dissolved metals into dramatic structures.