In the northern walls of the current caldera between Haranggaol and Silalahi, another ignimbrite, known as the Middle Toba Tuff, is exposed. This densely welded rhyolite tuff erupted ~ 500,000 years ago and overlies older tuffs, lavas, and pre-caldera metamorphic/sedimentary basement rocks. The MTT is at least 180 m thick, but its limited exposure leaves much to speculation, including its estimated volume of ~ 60 km3. As with the HDT, any caldera that may have formed during the MTT eruption has been engulfed by the YTT caldera. At Haranggaol, the MTT contains abundant fragments of older volcanic rocks, consistent with eruption from a stratovolcano. However, rock fragments found in the MTT in the northwest caldera walls suggest eruption through mostly non-volcanic basement rocks. Such circumstantial evidence indicates that the MTT vents were not confined to a pre-existing stratovolcano. Assuming vent locations in northern Toba, the MTT represents extension of Toba silicic magmatism northward. Distal MTT ash has tentatively been identified in the Indian Ocean deep-sea cores, but based on the limited exposure of proximal tuff, the MTT eruption is thought to be more like a Tambora size eruption than the earlier OTT super-eruption.