Petrologically the volcanic Kaiserstuhl is an alkali-carbonate rock formation. The volcanic rocks making up a large part of the central and western Kaiserstuhl were the result of numerous volcanic eruptions during the Miocene, about 19 to 16 million years ago. They cover parts of the sedimentary base of the eastern Kaiserstuhl, which is why in some places changes in the base's mineralogical composition occurred. The driving force behind this process of contact metamorphism was an increase in temperature. Due to the alternate eruption of tephra and lava flows from several vents a complex stratovolcano came into existence. Some of the rising magma solidified as volcanic intrusions below the surface - and today forms the central part of the Kaiserstuhl. Laterally rising phonolite magma also intruded into the sedimentary base of the Eastern Kaiserstuhl. Several hundred metres of the original volcano have been eroded.