Volcanoes are vents in the Earth's crust that allow liquid lava from inside the planet to travel to the surface. According to Reference.com, the word "volcano" applies to both the vent and the cylindrical cone the lava forms.
When lava is underground, it is magma, notes About.com. Magma is from the Earth's mantle and most magma that becomes lava comes from the first 100 kilometers of the mantle. Adding water or releasing pressure creates enough heat to melt rock.
Volcanoes form in subduction zones and continental rifts. Because magma is less dense than solid rock, it eventually works its way to the Earth's surface. While some volcanoes form by spewing ash and rocks from their central vents, others form in different ways. Smaller flows tend to form cinder cones, while larger ones build lava domes. Shield volcanoes form broad cones rather than tall ones. Shallow lava chambers form bowl-shaped calderas, which are the largest structures on earth. The word caldera comes from the Spanish word for cauldron, as these volcanoes form concave or dish-shaped bowls as the lava collapses.
Scientists measure volcanoes on the logarithmic Volcanic Explosivity Index (VEI). VEI 8 eruptions cause global changes in the climate and the population of humans.