A volcanic eruption is caused by magma (molten rock) moving from deep in the earth into a magma chamber within the volcano. To move upward, the magma must push aside rock, creating small earthquakes. Also, as the magma moves upward from deep within the earth, the pressure on it lessens so that gases release, just as removing the cap from a soda bottle releases the gases dissolved within. Once the magma is in the volcanic chamber, the additional volume of material may cause the volcano's slopes to swell.
Volcanic eruptions are powerful events, and they are capable of injecting hydrogen chloride (HCl) high into the atmosphere. Similarly, oceans produce large volumes of sea salt, which contains chlorine, on a daily basis. If these compounds accumulated in large quantities in the stratosphere, they might produce ozone depletion.
When magma is runny, eruptions are "effusive", which means they ooze lava gently all the time.
When magma is sticky, eruptions are explosive. The magma clogs the volcano's vent until so much pressure builds up that the magma burst out, like a popping champagne cork. The explosion shatters the plug of hard magma that blocks the volcano's vent, reducing it to ash and cinder.
Explosive eruptions are driven by expanding bubbles of carbon dioxide gas and steam inside the magma. An explosive eruption blasts globs of hot magma, ash, cinder, gas and steam high up into the air.
Volcanoes usually erupt again and again. The interval between eruptions, called the repose time, varies from a few minutes to thousands of years.
Have Eruptions Been Predicted?
Yes! On December 18, 2000 volcanologists predicted an eruption of Popocatepetl, near Mexico City. Some 40,000 people were evacuated from the surrounding region and just hours later, the volcano exploded more violently than it had in centuries.