After eruption of the YTT and Samosir lava domes, some crystal-rich rhyolitic magma mush likely remained beneath the collapsed structure. The presence of such magma below silicic calderas is often cited as a barrier that prevents less evolved magmas from erupting inside the caldera ring fracture. Indeed at Toba, no post-YTT lavas (besides the Samosir lava domes) are known to have erupted in the interior of the caldera. However, the ring fractures of many calderas are commonly the sites of less evolved post-caldera lava extrusions. Along Toba’s southwestern ring fracture, dacite lavas erupted to form the Pardepur lava domes, while both andesite and dacite lava flows built Pusikbukit volcano. To the north, just beyond the topographic rim of the caldera, Tandukbenua volcano consists of even less evolved basaltic andesite and andesite lavas.