Avachinsky, one of Kamchatka's most active volcanoes, rises above Petropavlovsk, Kamchatka's largest city. The shoulder at the right is the rim of a large horseshoe-shaped caldera, breached to the SW, that was formed about 30,000-40,000 years ago when a major debris avalanche buried an area of about 500 sq km to the south underlying the …