火山是地表下在岩浆库中的高温岩浆及其有关的气体、碎屑从行星的地壳中喷出而形成的，具有特殊形态的地质结构。

地球上的火山发生是因为地壳被分裂成17个主要的和刚性的地壳板块，它们漂浮在地幔的一个更热和更软的层。火山可以分为死火山和活火山。在一段时间内，没有出现喷发事件的活火山叫做睡火山（休眠火山）。另外还有一种泥火山，它在科学上严格来说不属于火山，但是许多社会大众也把它看作是火山的一种类型。火山爆发是一种很严重的自然灾害，它常常伴有地震。因此火山喷发会对人类造成危害，但同时它也带来一些好处。例如：可以促进宝石的形成；扩大陆地的面积（夏威夷群岛就是由火山喷发而形成的）；作为观光旅游考察景点，推动旅游业，如日本的富士山。

A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.

Earth's volcanoes occur because its crust is broken into 17 major, rigid tectonic plates that float on a hotter, softer layer in its mantle. Therefore, on Earth, volcanoes are generally found where tectonic plates are diverging or converging. For example, a mid-oceanic ridge, such as the Mid-Atlantic Ridge, has volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has volcanoes caused by convergent tectonic plates coming together. Volcanoes can also form where there is stretching and thinning of the crust's interior plates, e.g., in the East African Rift and the Wells Gray-Clearwater volcanic field and Rio Grande Rift in North America. This type of volcanism falls under the umbrella of "plate hypothesis" volcanism. Volcanism away from plate boundaries has also been explained as mantle plumes. These so-called "hotspots", for example Hawaii, are postulated to arise from upwelling diapirs with magma from the core–mantle boundary, 3,000 km deep in the Earth. Volcanoes are usually not created where two tectonic plates slide past one another.