Osrakum is a result of a major eruption contemporaneous with the uplift of the Guarded Land plateau.

The Osrakum caldera was formed by the collapse of an immense cinder cone. The extent of this cone is shown in the first diagram. This collapsed into an oval fault that produces a deep crater (green contour) which gradually filled with water.

Subsequent smaller eruptions occuring within the caldera produced the complex topography of contemporary Osrakum:

· first the Ydenrim which is almost submerged

· second the Pillar of Heaven which is a basaltic chimney that was the core of a cinder cone. This was wholly eroded away into the smaller lake contained by the Ydenrim and, where it rises above the water level, forms the Yden.

· thirdly the smaller, collapsed cone the interior of which is the Plain of Thrones.