Molten lava hardened into pumice and cinder, and scientists declared that a modern-day miracle had occurred. In 1963, the island of Surtsey emerged near Iceland from a spot 425 feet below the ocean surface. Millions of years ago, this miracle was common place as hundreds of volcanic islands, such as the Hawaiian Island chain, were born in oceans throughout the world. Undersea volcanoes are still found in all the oceans with approximately 10,000 of them on the floor of the Pacific Ocean. Like their counterparts on land, underwater volcanoes are usually cone-shaped.
Volcanoes alternate active periods which can last for millions of years with long dormant phases.
During active periods, the lava cone builds up and eventually explodes onto the ocean floor as molten rock. The lava solidifies and turns into volcanic rock. These volcanoes are enormous. Hawaii, the largest mountain on earth, has a relief of 32,024 feet above its base - over 3,000 feet higher than Mt. Everest. Cone erosion at sea level is the cause of the flat-topped undersea volcanoes known as guyots.