Volcanoes are mountains, but they are very different from the towering peaks that we typically think of when we hear the word “mountain.” Volcanoes are formed when hot molten rock (magma) and ash escape from an opening in the Earth’s surface. The molten rock and ash become solid as they cool, and over time, this material builds up around the opening to form a volcano.
There are three main types of volcanoes: cinder cones, shield volcanoes, and composite volcanoes. Cinder cones are the simplest type of volcano. They are small, steep-sided hills composed of fragments of lava (cinders) that have been ejected from a single central crater. Shield volcanoes are large domes with gentle slopes produced by lava flows that originate from numerous craters or fissures on their flanks. Composite volcanoes, also called stratovolcanoes, have steeper sides than shield volcanoes and consist of alternating layers of lava flows and pyroclastic debris (fragments of rock blasted into the air by explosive eruptions). The most famous composite volcano is Mount Vesuvius in Italy, which erupted in 79 AD and buried the city of Pompeii.
Volcanic eruptions occur when magma rises to the surface and is ejected through an opening in the Earth’s crust. Magma can rise to the surface through cracks (fissures) or through vents that connect to chambers below ground (called magma chambers). When magma enters a fissure, it can cause an explosive eruption if the gas bubbles trapped inside the magma expand too rapidly. If magma rises slowly through a vent, it can produce a non-explosive eruption characterized by lava flows. The type of eruption depends on several factors including: 1) The amount of gas bubbles present in the magma; 2) The viscosity (thickness) of the magma; 3) The rate at which magma rises to the surface; 4) The size and shape ofthe volcanic vent; 5)The interaction between groundwaterand rising magma; 6 )The presence or absenceof an obstruction(such as a plugof cooled lava) blockingthe flowof magmafromthe vent; 7 )The angle at whichmagmareaches themouthofthe vent . All these factors must be considered when predicting volcanic activity