A volcano is a fissure in the Earth's crust, above which a Cone of volcanic material has accumulated. At the top of the cone is a bowl-shaped vent called a crater.
During eruptions, solid ash falls around the vent on the slopes of the cone, and lava oozes from the vent as well as from cracks on the side of the cone. Thus, the mountain is built up from layers of fragmental materials and the flow of lava.
Until recently, Volcanoes were thought to be the homes of vengeful gods who vented their anger on the people below. It is now understood that volcanoes are a visible sign of the Earth's tectonic forces. Most people associate volcanoes with the destruction they cause, but they've been very helpful in teaching us about the Earth's interior.
The Earth's crust is composed of about 20 Tectonic Plates, which move from a few millimeters to several centimeters per year. As plates move away from each other, Molten Rock emerges from the Earth's super-hot interior. Most volcanoes are found where two tectonic plates meet, such as the "Ring of Fire" around the Pacific Ocean. A few, such as the volcanoes that formed the Hawaiian Islands, are over hot spot's inside the Earth, where magma flows upward with enough force to burn through the crust.
Magma is molten rock below the surface of the Earth. When it erupts onto the surface it is called lava. We tend to think of magma as a liquid, but some components can separate from the liquid to form a gas. Although it is usually present in relatively small quantities, magmatic gas is extremely important because its presence will cause a volcanic eruption. During an eruption, 10 million to 1,000 million tons of volcanic gas are discharged into the atmosphere over a period lasting from a few hours to several days.
Vesuvius the only volcano on the European mainland, erupted on August 24, 79. The top of the mountain was blown off by an explosion, and the cities of Herculaneum, Pompeii, and Stabiae were destroyed. For more than 1,500 years these cities lay undisturbed, until 1798 when excavations began. The showers of wet ash and cinders formed a seal around the towns, preserving many public buildings and private dwellings. In addition, remnants of more than 2,000 victims were found, including several gladiators who had been chained to prevent them from escaping or committing suicide.