Andesites from the Lauca volcano, active in the Late Miocene as of 10.5 million years ago,[3] exist in two groups,
plagioclase rich ones which underwent alteration and fine grained silicic ones that are fresher.[1] The territory is underpinned with a crust formed from rocks of
Precambrian to
Cretaceous age, with thicknesses up to 70 kilometres (43 mi).[2]Amphibole rich lava flows formed a uniform
volcanic cone. A later major
ignimbrite eruption caused the collapse of the centre and formed the Lauca ignimbrite.[1] A
caldera is present, with a
lava dome at the northeastern rim.[1]
The Lauca ignimbrite has a thickness up to 150 metres (490 ft), covering the
El Diablo Formation in parts and filling the
Lluta Canyon.
Ar-Ar ages for this
rhyolitic ignimbrite have been determined at 2.73 ± 0.02 Ma based on
sanidine analysis and 2.87 ± 0.05 Ma from
biotite.[4] The Lauca ignimbrite has been correlated to the Pérez ignimbrite in Bolivia and the Pachía ignimbrite in southern Peru.[5] Along the Copaquilla-Tignámar Fault, uplifting the eastern side over the western side,[6] the Lauca ignimbrite has been displaced over 100 to 150 metres (330 to 490 ft), indicative of tectonic activity after deposition in the Late Pliocene.[7] In parts, the Lauca ignimbrite is covered by
conglomeratic terrace deposits.[8]