Although typical Sunda Arc volcanism had been underway in the Toba area for quite some time (as recorded by the basaltic andesites in the northern caldera walls), significant explosive eruptions did not begin until about 1.2 million years ago when a dacitic composition ignimbrite was deposited across north-central Toba proper. Because the HDT outcrops occur in the northeast and northwest caldera walls, its source vents are presumed to have been within the northern portion of the current caldera and were later obliterated by the YTT eruption. Whether these vents were associated with a pre-existing stratovolcano or defined a new locus of volcanic activity has yet to be established. However, rock fragments (lithics) torn from the vent walls during eruption, and now found in the tuffs, suggest that the HDT may have erupted from an area underlain mostly by older metamorphic and sedimentary basement rocks. Compared to the eruptions that would follow, the HDT was relatively small, perhaps on the order of 35 km3, approaching the size of some larger stratovolcano eruptions like Crater Lake in the USA or Tambora.