Which event causes Yggdrasil to tremble?

Yggdrasil trembles during Ragnarök, the cataclysmic conflict at the end of the Norse mythic cycle.

In the Prose Edda’s account of Ragnarök, Odin seeks counsel from Mímir before the gods prepare for battle. Then Yggdrasil shakes, and fear spreads through heaven and earth. The trembling signals that the established cosmic order is breaking apart, not merely that one god has died or one enemy has attacked.

Ragnarök is a chain of disasters rather than a single battle. It includes the Fimbulwinter, the escape of monstrous beings, the deaths of major gods, and the final confrontation on the field of Vígríðr. Odin is killed by the wolf Fenrir, Thor defeats but is fatally poisoned by Jörmungandr, and Surtr’s fire devastates the world.

The tree’s trembling is therefore a symbolic and cosmic event. Yggdrasil is the central structure connecting the worlds, so its shaking conveys that the disturbance reaches every realm. After the destruction, the surviving gods and human pair Líf and Lífþrasir become part of the renewed world.