 The Incapillo ignimbrite is an <unk> ignimbrite that covers a surface area of 80 @.@ 47 square kilometres ( 31 @.@ 07 sq mi ) , extending to a distance of 15 kilometres ( 9 @.@ 3 mi ) from the caldera . The ignimbrite appears in an eastward @-@ heading ephemeral river valley and the southern Quebrada del Veladero , possibly also next to the Rio Salado headwaters . <unk> range from 250 to 10 metres ( 820 to 33 ft ) ; the ignimbrite is underlain by a lithic @-@ and @-@ ash rich surge deposit with a thickness of 5 centimetres ( 2 @.@ 0 in ) . The ignimbrite displays banding features away from the caldera and in Quebrada de Veladero football @-@ sized clasts are mixed within fine ash . Rocks from the ignimbrites farther away from their source indicate the ignimbrite probably formed from the mixing of less viscous dacitic magma with rhyolite . The total volume of the ignimbrite is about 20 @.@ 4 cubic kilometres ( 4 @.@ 9 cu mi ) . Ages have been found of 0 @.@ 52 ± 0 @.@ 03 and 0 @.@ 51 ± 0 @.@ 04 mya ago . It is a rhyodacitic to rhyolitic ignimbrite with a high crystal and pumice and low lithic content . The dense rock equivalent volume is about 14 cubic kilometres ( 3 @.@ 4 cu mi ) . The volume of the Incapillo ignimbrite is comparable to that of the Katmai ignimbrite . The ignimbrite was probably formed from a low @-@ height fountaining eruption without a high eruption column , forming a base surge first and pyroclastic flows later . The change from lava dome to ignimbrite @-@ forming eruptions may have been triggered by the injection of hotter magmas into the magma chamber or less likely by changes in the tectonic context . During the eruption , a piston @-@ like collapse formed the caldera . 
