The overwhelming part of the Zillertal Alps consists of the crystalline primary rocks, plutons and metamorphic rock, that do not support the formation of caves. The only exceptions are pockets of rock capable of karstification such as limestone, dolomite, gypsum and other carbonates. These pockets are generally of quite low thickness, and only allow small caves to be formed. The creation of caves also requires water solubility. The Spannagel Cave has arisen in a large vein of Jurassic calcite marble of the Hochstegen Formation, a marble with over 90% total carbonate content, that runs through the slate mantle of the Western Tauern window. The formation of this extensive cave as a result of geological corrosion (and mixing corrosion) is explained by the tectonics of the region.