Volcanoes

Forces of both creation and destruction, volcanoes have a carved themselves a significant past in the lore of Britannia, and many other worlds.

History of Volcanoes in Britannia
The first known volcanic disturbances came to light in the last third of the Age of Darkness, when the daemon-human hybrid, Exodus, first appeared. The Isle of Fire rose from the sea in a hail of lava and steam, curtaining a castle within. Not until Exodus' arrival did scattered pools of lava appear throughout the lands of Sosaria, and even after his defeat, they remain.

In Ultima IV, a set of three volcanoes marked the spot where the extremely powerful skull of Mondain the Wizard could be recovered from the ocean depths. (The Avatar was later able to dispose of the Skull by casting it into the fiery depths of the Abyss.) By the time of Ultima V, a full-fledged island had emerged at the site of the volcanoes, and was where the tyrant Blackthorn chose to build his castle. After Blackthorn's banishment, the island was given to the gargoyles following the destruction of their old home. There they remained until the Great Cataclysm struck, forcing the gargoyles to abandon it and thus leaving it open to claim by the Guardian, where he built his own fortress.

A colony was started in the Abyss by Sir Cabirus, who had intended it to be a testiment to virtue. Without his leadership, the colony fell into disarray and ruin. In the final events of Ultima Underworld, the colony was destroyed by an immense volcanic eruption from which the Avatar narrowly escaped.

Prior to Ultima IX, the town of Jhelom was destroyed by a volcanic eruption, and was subsequently rebuilt around the volcano's summit, being rechristened Valoria. 

Volcanoes in Other Lands
The hidden valley of Eodon hosted at least one active volcano, as evidenced by the intro sequence to Savage Empire. The Avatar also heard the pleas of Jumu, chief of the Jukari Tribe who had lost his father and village shaman to a recently-activated volcano. The Jukari had become separated from their sacred hide by rivers of flowing lava, and tasked the hero with recovering it.

Furnace, an ancient Ophidian city on the Serpent Isle, was so superheated by volcanic activity that the Chill spell, was required to prevent any visitors from bursting into flames upon arrival.

The island of Morgaelin in Pagan is home to the Elemental Titans, and is really just the extended cone of the still-active volcano (also named Morgaelin). The volcano is an ever-present threat to the people of that land. Daemon's Crag, the sorcerer's enclave in Pagan, surrounded by an active lava river, is accessible only through the Mouth of the Daemon. 