Volcanoes are openings in the Earth's crust through which magma, ash and volcanic gases erupt. The term can also be used to refer to the landform formed by an eruption. The term can also be used for small conical formations of mud (mud volcanoes), which form in volcanic areas or places that are under tectonic stress. Volcanoes occur along the Earth's plate boundaries in subduction and rift zones and in the middle of plates over hotspots. The following resources provide information about volcanoes throughout the world.