The most common place where a composite volcano forms is near areas with high tectonic plate activity. Tectonic plates either converge, move towards each other, or diverge, move apart. Both of these actions cause a weakening of the Earth's crust. This allows the immense pressure placed on the mantle by the weight of the Earth's crust to push magma through the weakened area. When the magma reaches the surface, it cools and forms the steep, conical shape of a composite volcano.
A composite volcano is not a solid piece of rock. It is a stratified mound formed mainly of cooled lava and volcanic ash. When a volcanic eruption occurs, lava and ash spew into the air. Because lava is a heavier material, it falls to the ground first and hardens into the first layer of material created during an eruption cycle. The ash settles at a later point and forms the second layer. As this process repeats over time, the layers eventually build the volcano formation.
When a composite volcano erupts, it emits a variety of dangerous materials into the atmosphere: lava, ash, cinder, sulfur dioxide and carbon dioxide. The most dangerous effect of an eruption is a phenomenon called a pyroclastic flow. This flow, which is a combination of rock and hot gases, can reach temperatures nearing 2,000 degrees Fahrenheit. Gravity and the force of the eruption cause a pyroclastic flow to travel down the side of the volcano at over 400 miles per hour, destroying everything in its path.
Mount Vesuvius, located in Naples, Italy, is one of the most famous composite volcanoes in history. In 79 AD, it erupted and sent a pyroclastic flow racing toward the nearby cities of Herculaneum and Pompeii. Both cities were completely destroyed and buried in ash. The heat from the flow was so intense that it instantly carbonized the cities' residents and turned them into living statues, revealing the horrific moment immediately before death.