[2] The Amalia Tuff is a weakly to densely welded ash-flow tuff erupted during the formation of the Questa caldera 25.39 ± 0.04 million years ago (Ma),[2] which erupted some 500–1,000 cubic kilometers (120–240 cu mi) of magma.

[4] The tuff is silicic and alkaline, though its silica content of 77-80% likely reflects some secondary silicification.

The tuff includes a lithic-rich lower facies that is nonwelded to partially welded and up to 30 meters (98 feet) thick.

[2] Isotope evidence suggests the original basaltic magma assimilated considerable crust material and underwent crystal fractionation at a deeper level of the crust than that of the magma chamber responsible for the Questa eruption.

[6] As originally defined, it included basalt flows and sedimentary beds dating from before the Questa eruption.