The Volcano is one of new tiles that are introduced in Northgard: Ragnarok. It is a special tile that cannot be tamed or colonized.
Sometimes during the game, the Volcano will enter into eruption and throw out Molten Rocks all over the map. Some of them will fall into your territory, enable you to mine them. However, you must mine them completely before 4 months, as Rock Golems would spawn from them and will cause damage.
The number of Molten Rocks falling onto your territory will vary depending on the number of tile that you have colonized. If you didn’t manage to mine completely the rock before the deadline, the Rock Golem will spawn missing a few HP.