Volcanoes form when molten rock, created inside the crust or upper mantle of the Earth, moves upward and erupts on the Earth's surface. Molten rock is less dense than the surrounding rock, so it is buoyant and rises, just like hot air. Each eruption can produce layers of lava that will later become volcanic rock. These layers build the volcano.
Volcanoes have several shapes, which are controlled by the composition of the magma and the nature of its eruption. If a volcano produces very fluid lava (low in the compound SiO2, or silica, which is what glass and quartz and most of sand is made of), the magma flows a long distance before it cools, making a flat, shield-shaped volcano. If the volcano produces very sticky magma (high in silica) it tends to have an explosive eruptive style that includes lava, pyroclastic flows, and ash. This material piles up right around the volcano, forming a steep cone, a classic volcano shape.
Depending on the pressure that forces the magma up, and depending on the viscosity (how liquid the magma is) and amount of gas in the magna, eruptions vary in size, explosiveness and danger.
Caldera, plinian, and vulcanian eruptions are the most dangerous and explosive. Caldera-forming eruptions are the largest of the violent and explosive volcanoes. As a result of releasing high volumes of magma, these volcanoes will actually collapse back into the earth, into the void created by the emptied magma chamber. The plinian eruption carries magma high in viscosity and gas content to a lava plume up to 30 miles high in the air. Vulcanian eruptions carry material over a wide area and are commonly considered the early phase in the eruption of a volcano, sometimes referred to as "clearing their throats" before a more violent eruption.(Abbott, 2004)
Non-explosive type eruptions are divided into Strombolian, Hawaiian, and Icelandic. Strombolian eruptions are characterized by the intermittent explosion of lava from a single vent or crater. Each episode is caused by the release of volcanic gases, and they typically occur every few minutes or so, sometimes rhythmically and sometimes irregularly. Each eruption tosses magma and volcanic rock tens to hundreds of meter in the air. Hawaiian-type eruptions, after an initial venting of gas, have lava outpouring onto the ground. These eruptions can last for days or up to a year or more and are less dangerous. The magma released in these eruptions has lower viscosity and levels of gas. Icelandic-type or fissure eruptions are considered the most peaceful as they occur when magma flows up through cracks in the ground and leaks out onto the surface. These often occur where plate movement has caused large fractures in the earth's crust.