The 16 large plates that make up the Earth's hard upper crust, or lithosphere, constantly grind against each other or subduct, pushing one plate beneath the other. Beneath the lithosphere lies the athenosphere, a layer of hot, partially liquid rock called magma. When magma rises, it breaks through to the Earth's surface, most often along plate boundaries. Pressure from rising magma can also create fissures in areas distant from plate edges.
Melting rock in the athenosphere drives a volcanic eruption. When rock liquefies under high temperatures to become magma, gases are produced. These gases, bubbling in the magma, drive it upward, creating pressure that is released in an explosion. Magma can be very thick (high-viscosity) , or thinner and runny (low-viscosity). Low-viscosity magma with low levels of gases produces a nonexplosive eruption in which magma simply flows out of a volcanic cone or fissure; high viscosity magma propelled by large quantities of gases explodes to the surface, sometimes with extreme violence.
Many mountains throughout the world began life as active volcanoes. Magma rising to the surface pushes rock upward, creating a cone. Pressure building within the volcano causes an explosion that breaks through rock or old lava, allowing hot magma to flow out as lava, along with gases, ash and particles. Volcanoes grow from repeated episodes of eruption and lava flow, which form and re-form the shape of the cone. Some volcanic eruptions occurring along fissures in the earth's surface create lava flows over a large area without forming a typical volcanic cone.
Volcanologists have identified several types of eruptions, which aids in classifying volcanoes around the world. These include the weakest, the Hawaiian, in which low-level eruptions are nearly continuous, and mid-level events such as the Strombolian and Vulcanian types which occur over months or years. At the other extreme are the extremely violent Plinian and Ultraplinian eruptions which occur on a time scale of centuries or even thousands of years. Volcanoes may exhibit characteristics of more than one type during an eruption event, which can continue for a long period of time.