The Karkas Mountains were formed mainly during the Eocene volcanism. Located in the Sahand-Bazman volcanic and plutonic belt, this mountain range is composed predominantly of the Eocene volcanic rocks of the Tertiary volcanism. Aside from the Tertiary igneous (plutonic and volcanic) rocks, there are also pyroclastic and sedimentary rocks. There is a Precambrian-Paleozoic basement in the central part of this mountain chain.