|This article is about an older version of DF.|
During world generation, areas of the map have a "volcanism" rating, (see advanced world generation for more information) much like they have a rainfall or drainage rating. In areas with high volcanism, volcanoes and magma have a higher chance of rising to the surface. A tile with 100 volcanism may become a volcano.
To start on a map that includes a volcano, you will have to search for one on the fortress location selection screen. Most of the time they are not visible on the world map. Scroll around the world looking for a red ^ in the regional map. Select that space, and in the local map, move your starting area to include the square with a dark red ≈, which is the caldera. A volcano is not a bright red ≈ - those tiles indicate red sand. There may also be additional local magma pipes or magma pools in nearby tiles around the volcano. Using the site finder assists in finding volcanoes; see its page for more information.
Sometimes a magma pipe will exist in an area with no volcanoes nearby.
Living on a volcano
Volcanoes provide ready access to obsidian, which a craftsdwarf can form into a rock short sword; these are as strong as steel shortswords. It can also be used to make more valuable rock crafts and furniture, as obsidian has a value of 3, compared to 2 for flux stones and 1 for other ordinary stones. It's possible to divert water into magma to form your own obsidian, although obsidian is often already present in great quantities.
The volcanic activity also leads to an abundance of heavy extrusive igneous rock (such as basalt, felsite and andesite) under a layer of farmable materials. The nature of these rock layers also provides for many useful ores such as iron (through hematite or other means), gold, galena, and other non-precious metals. The rock also provides useful gems such as turquoise and zircons, and occasionally diamonds. Genuine volcanoes sometimes have other interesting features, similar to named mountains: they are frequently sites for caves, often have a wider than usual variety of stone and ore, and may include unusual features such as cave rivers, chasms or bottomless pits.
Because of the natural tendency for Magma to turn into obsidian when coming in contact with water, digging alongside the magma chamber may provide an easy route underneath an aquifer and into the perfectly dry stone and ore deep below.
Due to the random nature of map generation, occasionally your group may end up embarking directly over lava, and it will shortly be time to select a new fort location. Other times a message about the cavern collapsing may occur the instant you embark - this is because the volcano is inserted into the already existing landscape, and it can cut through a supporting wall. You can disable the pause and centre aspects of the announcement related to cavern collapses to speed up the start of the game.
Creatures like fire imps can and maybe will climb out of volcanoes, where they'll attack everything they see, which may leads to a deadly and nearly unstoppable forestfire, so it's possibly a good idea to dig a channel or erect a wall around open volcanoes in fortress mode as soon as possible.
In the real world
A volcano, in the real world, is a vent in the planet's crust through which lava and pyroclastic materials are expelled. They're usually created through plate tectonics in subduction zones, but they can form anywhere magma is capable of breaking through to the surface.