The Chirpoy islands are the remains of a partially submerged volcanic caldera which measures 8–9 km wide.

[1] Chirpoy, the northernmost of the two islands, has an area of approximately 21 km2, and consists of three overlapping stratovolcanoes named Chyorny and Snow.

Chyorny (Japanese: 大崩山, romanized: Okuzureyama), with a height of 691 metres (2,267 ft) is the tallest point on the island.

Brat Chirpoy is the site of the southernmost of five major Steller sea lion rookeries on the Kuril Islands.

In 1801, the Japanese government officially claimed control of the islands, incorporating them into Ezo Province (now Hokkaidō), but sovereignty passed to the Empire of Russia under the terms of the Treaty of Shimoda in 1855.