It has a horseshoe-shaped caldera, which formed 30-40,000 years ago in a major landslide which covered an area of 500 km2 (193 sq mi) south of the volcano, underlying the city of Petropavlovsk-Kamchatsky.

[3][4][5] In his Journal of Captain Cook's Last Voyage, John Ledyard records the eruption of Avachinsky on 15 June 1779.

On the 15th it continued calm until noon when it clouded up and became very black and dark: the two mountains Peter and Paul were covered with the atmosphere near half way from their summits down, and at two o'clock we had again a small shock of an earthquake, and heard a hollow rumbling noise in the air, and the atmosphere continuing to condense, it became almost as dark as night, and the face of heaven looked very wild: we singled the stops of the sheet-anchor and eased the ship aloft at all the portentous appearances.

Between three and four the mountain Paul exploded with a tremendous shock that convulsed everything around us: The report that attended the explosion was very loud at first, but gradually decreased until it subsided to a sound like that of grumbling distant thunder.

Eruptions have generally been explosive, and pyroclastic flows and lahars have tended to be directed to the south west by the breached caldera.