Following the HDT eruption, the nature of magma generation and evolution beneath Toba changed significantly as large volumes of silicic magma began to accumulate and evolve in the shallow crust. Emplacement of this magma presumably was accompanied by regional inflation and development of a proto-Batak Tumor. While the duration of magma body assembly is unknown, it surely would have been associated with earthquakes and localized geothermal activity (hot springs and fumaroles) as the overlying rocks were uplifted and heated. At ~ 790,000 years ago, the roof of this shallow magma body foundered, collapsing into the giant magma body below, forcing silica-rich magma towards the surface along ring fractures, or linear vents along its axis. The ignimbrite generated by this eruption was the first to contain the mineral quartz, prompting George Walker to develop a nomenclature for the quartz-bearing Toba tuffs based upon their relative age. Accordingly, this ignimbrite was named the Oldest Toba Tuff (OTT), and the two subsequent quartz-bearing tuffs were named the Middle Toba Tuff (MTT) and the Youngest Toba Tuff (YTT).