Asuncion is located in the far northern regions of the Mariana arc, situated 101 km northwest of Agrihan and 37 km southeast of the Maug Islands; the island of Pagan lies 175 km to the southwest.
The island takes the form a large, 857 m tall, emergent stratovolcano measuring 3.4 km in length and up to 2.6 km in width, with an area of 7.4 km². The volcano is assymmetrical in outlook, with the northeast slopes being the steepest and terminating in high cliffs. The southwestern slopes, by contrast, are shallower and meet the sea in low cliffs of a few metres in height. Coastlines, where at sea level, are generally rocky.
Asuncion is thickly vegetated with Coconut palm plantations on the lower slopes, mixed-broadleaf forest (dominated by Terminalia spp.) found also on lower slopes as well as at elevations above the Coconut groves. Upper slopes are colonised by grasses and ferns. The deep ravines that descend radially from the summit are thickly vegetated with a mixture of woody species that include Pandanus, Psionia and Hibiscus.