Volcanoes. A perfect source of lava to cook your elves very well done from the beginning... and, well, that's it. To quote the wiki, "volcanoes do not explode, erupt, nor spew noxious fumes", and in general, the only volcano-y stuff they do is having lava. They are also tiny compared to the real deal! So without any further intro, I present you my ideas for revamped volcanoes.
Shape and Size
Volcanoes are BIG. They are mountains that can reach a height of several kilometers, and even moreso if you consider the underwater part if relevant. As a result, they should be a lot bigger than just one mere embark tile. Furthermore, while the "main attraction" is the central vent, DF volcanoes do not form any cone. I suggest adding two shapes:
-Stratovolcanoes form steep cones soaring up into the sky. They tower over the region's landscape, being around 30 to 50 z-levels tall, and occupy a circle of embark squares with a radius of around 5. They are made of various igneous extrusive layers (although generally weaker volcanoes are more mafic, while powerful ones are felsic), including two new ones, tuff and pumice. These stones are unremarkable economy-wise, but are very light due to being porous: pumice can even float! Any kind of volcano may be of this type.
-Shield Volcanoes are vast and very lightly sloped cones made of many layers of solidified lava. They are around 20 z-levels tall, and typically occupy a large chunk of the embark region. Running from the crater are large river-like formations, through which lava may flow. Only Strombolian volcanoes or weaker may be shields.
Both these types may have more than one funnel, resulting in a cluster of magma tubes at the summit rather than one. These may erupt separately, but always the same way.
Activity Types
Volcanoes are know in real life for being very fun. Let's go through a series of possible behaviors:
-Extinct: This kind isn't shown on the world map. Its core is just a large obsidian or basalt column, with no lava left. Sometimes, it doesn't even have a cone, if it was eroded away.
-Erebus Type/Lava Lake: These volcanos behave closely like the current ones: a constant lava lake sits in their craters, never going out or draining. They are also somewhat uncommon.
-Hawaii Type: These volcanos frequently (once a day to once a year) produce an excess of lava, resulting in a flow around the crater.
-Stromboli Type: These volcanos erupt frequently (up to many times a day in adventure mode, or down to once every few years). During that time (the duration of which depends on the frequency), the volcano may produce lava, as well as large blasts of magma mist above the crater. Various volcanic bombs are also ejected, making them an infinite source of tuff and pumice boulders. Lower frequency volcanos of this kind have their eruption announced a few days before by the message "[Volcano Name]'s activity seems to be increasing"
-Vulcano Type: These volcanos have a viscous lava, making them more violent. They erupt relatively rarely (every decade or so). Compared to the previous, they have a large lava dome blocking the conduit. Once the eruption starts, it is destroyed by a series of bursts sending a column of superheated ashes high into the sky, making any expedition outside dangerous due to the intense rain of rocks. After this, the eruption proceeds as before, but lasts longer. They are announced by the message "Tremors are coming from [Volcano Name]!", a few weeks before.
-Pelée Type: We're entering the fun zone! The lava here is very viscous, and can barely exit the volcano at all, resulting in the formation of a large lava dome. Every few decades, the pressure becomes so high it is disintegrated into a superheated pyroclastic flow. This extremely hot cloud of dust and lava behaves like an evil cloud but is affected by gravity, only stopping once the slopes becomes low for long enough. Anything unfortunate enough to be caught in is almost instantly cooked alive if it's not fully fireproof, and even then, it will get buried alive. The only solution if you're in the path is to bury yourself before, as it cannot go very far underground. The eruption may last for months, so you better prepare when the message "Powerful tremors are felt around [Volcano Name]!" is seen.
-Vesuvius/Plinian Type: If you're very (un)lucky, the lava of the volcano nearby may be so rich in gas and felsic rocks that, every century or so, that happens. A whole chunk of the volcano is blown to smithereens by the power of the blast, causing many very fun effects such as a huge quake that causes any cavity within the first 10 z-levels of the map to be immediately re-filled with rock. You want to live? You better build a bunker far below... Meanwhile on the surface, things are even funnier: a gigantic pyroclastic flows flattens and carbonizes everything in the whole embark region, and a massive pillar of ashes reaches kilometers into the sky. If you see the message "[Volcano Name]'s summit is changing in shape!", you better prepare for the worst... or flee. The massive amount of material in the atmosphere obscures the sky, causing any plant to grow slower, not just in the direct vincinity, but up to several regions away, as indicated by the message "[Volcano Name] is erupting, darkening the sky!". The volcanic winter may last up to a year.
-Tambora/Ultra-Plinian Type: When the message "[Volcano Name] is awakening! Flee while you can!" appears, I suggest you obey. In a year, a good chunk of the region is going to become history as a cataclysmic eruption erases it. Maybe if you shielded yourself in the deepest of hollows, you may live. If you're not in the blast zone, you're not safe either: a long volcanic winter ensnares the continent, dragging temperatures down and and causing pityful harvest. Ashes are everywhere, irritating lungs and poisoning livestock. May Armok be with you for the few coming years, as you will need him...
Embark Screen Announcements
The type of the volcano may be determined by a message while embarking. The message depends on the activity type:
-"This volcano has a permanent lava lake"
-"This volcano frequently vomits streams of lava"
-"This volcano often erupts lava and rocks"
-"This volcano has been seen ejecting large quantities of rocks and dust"
-"Many years ago, this mountain has been seen destroying entire cities"
-"It is said that, in long past ages, this mountain has detonated with an extreme force"
-"Legends say that once upon a time, this mountain was destroyed by a god"