Violent and devastating, legendary in history and mythology, volcanoes in action are among the most powerful forces of nature. They have shaped the Earth for billions of years and continue to do so, sometimes with deadly consequences to life and property. Awesome to witness, erupting volcanoes are natural geologic structures providing a conduit to the Earth's surface for the extrusion of molten rock materials migrating upwards from the depths beneath the Earth's crust. Volcanoes can occur on land or on the seabed beneath the oceans. Molten rock, or magma, to use its proper geological name for a body of molten rock found at depth, exists under pressure deep within the Earth. When overlaying solid rocks provide a channel, perhaps because of the constantly occurring internal adjustments of the surrounding rocks, the magma may escape to move upward to regions of lower pressure until eventually emerging at the surface, acquiring the new name: lava. On the other hand, magma that cannot find a outlet will become trapped at some sub-surface location for ever. Magma and lava are similar but not the same. With the relief of pressure on the upward moving magma, much of the gaseous component is able to escape, thus changing its composition. This modified molten rock will emerge at the surface and then be called lava. In some cases the volcanic lava reaches the end of its upward journey with a volatile mixture of molten rock, gases, and water, expanding and exploding with such force that molten lava, rock fragments and ash are violently propelled into the air to great heights. Active volcanoes release carbon dioxide and other gasses into the atmosphere, including significant amounts of water vapor. Volcanoes are not rare, and with tens of thousands of volcanoes erupting over the hundreds of millions of years of the planet's early existence, the water vapor released by volcanoes provided the source of much of the Earth's original oceans and much of its atmosphere.
