Older volcanic activity occurred in the field during the Oligocene and Miocene, generating calderas (usually large craters formed by the explosion or collapse of a volcano) such as the Central Nevada Caldera Complex and the Lunar Lake caldera which underlies much of the northern Lunar Crater volcanic field. The volcanism produced ignimbrites, andesitic lavas and tuffs such as the 24 million years old Buckwheat Rim Tuff that Lunar Crater is embedded in; some of these volcanic rocks form structural blocks such as the Citadel Mountain block and the Pancake Range and others are correlated to ignimbrite sheets elsewhere in Nevada. An early caldera-forming ignimbritic phase was followed by an andesitic lavic phase, and in the last 11 million years basaltic eruptions have taken place in the Basin and Range.