Smoking Mountain is the term used by dinosaurs in The Land Before Time for volcanoes. It can specifically be used to refer to a group of volcanoes just outside the Great Valley as well as one on the edge of the Great Valley.
This volcano is first seen in The Land Before Time II: The Great Valley Adventure when Chomper wandered off towards it. Littlefoot lead the others to the volcano to find him. The group was ambushed by Ozzy and Strut while searching. The Egg Stealers tried to force the children into a sulphur pool. Cera tried to fool them by saying they are friends with a Sharptooth, which Ozzy doesn't believe. Strut reminds him of the Sharptooth shadow they had seen earlier, at which Cera spills the beans, revealing that the Sharptooth was a baby. As the Struthiomimus close in, the volcano starts erupting. The Gang, reunited with Chomper, escape the lava using a tree to cross a gorge, while the Egg Stealers jump down the gorge to get away.
In The Canyon of Shiny Stones, the Smoking Mountain reappears. Ducky says that this one is sometimes called the Booming Mountain. It is revealed to be close to the titular canyon. Oddly, Petrie says that he has never been close to the volcano before. He is also superstitious of it, claiming that it hates Flyers and that it gets angry if they fly over its crater. He tries to fly over all the same, but it does start shooting out lava bombs. Later, it erupts, filling the Canyon of Shiny Stones with lava. Yet again the gang escape the lava with a tree, this time pushed down by the adults for the children to use as a ramp. The children escape, but without the Shiny Stone she was planning on replacing the one from Tria's collection (which she'd lost) with.