[4] It sits atop a quartzite and sandstone basement that was lifted from the seaground during the Acadian orogeny and is of Ordovician age.

The international road through Sico Pass[10] connecting Salta in Argentina with Calama in Chile runs close to El Laco.

A number of tourist sites are found in the Atacama Desert adjacent to El Laco, and the dry climate also makes the area suitable for astronomy facilities.

Laquito and Cristales Grandes, two abyssal iron magma structures, date back to that era.

[15] Pico Laco is an andesitic lava dome with a height of 400 metres (1,300 ft) above the surrounding terrain.

An age of 5.3±1.9 mya on lavas in the northern part of the volcanic complex is the oldest obtained date.

[9] Another date from Pico Laco is 2.0±0.3 mya;[16] one study suggested four separate episodes of volcano development.

[17][3]: 684  Hydrothermal alteration has also been described for lower portions of the volcanic pile and probably occurred because of gases escaping from intruded magma.

[3]: 684  Reports exist of continuing fumarolic activity and hot springs with the deposition of clay and other minerals.

[12] On the flank of the volcano, apatite, hematite, and magnetite deposits are found[18] at altitudes of 4,600–5,200 metres (15,100–17,100 ft).

[21] The deposits lie on top of flat lava flows of andesitic composition,[22] concentrically around Pico Laco.

[3]: 684  The iron-containing rocks include lava flows, ash, and lapilli,[24] as well as ore breccias formed presumably when the volcano collapsed.

The El Laco magnetite lava flows have drawn scientific attention for decades[25][26] and are unique in the world;[11] they were deemed a IUGS Geological Heritage Site in 2024.

It was erupted from feeder dykes on its southern and eastern end and forms a table-shaped body on a spur, in the shape of a mesa.

[26][19] A magnetic layer of rock spreads north from the volcano,[28] and a large magnetite body has been modelled beneath Pasos Blancos.

[30] Centimeter-sized vesicles to meter-sized tubes coated on the inside by euhedral magnetite were formed in the lava by escaping volcanic gas.

[39][40] A third hypothesis envisages the crystallization of magnetite within a silicic magma and its subsequent extraction through flotation attached to volatile bubbles.

[8][5]: 525  The role of a post-magmatic fluid phase, which was inferred from inclusions in crystals,[40] or of a combination of magmatic and hydrothermal mechanisms has also been suggested.

A favourable tectonic context associated with the compression of the magma chamber and the presence of faults helped with the eruption of the magnetite.

[44][45] Mining in Laco Sur removed about two million tons of magnetite between the 1970s and 1990s, leaving an open pit exposing 30 metres (98 ft) of rock.

[1] In 2009, these mineral reserves were mined by Cia Minera del Pacifico S.A.[46] It is estimated that the deposit contains 733.9 million tons of ore,[7] consisting of 50% iron.

[50] The main rocks of the volcano are andesite and dacite, which contain biotite and pyroxene as well as blebs containing iron oxide.

[3]: 681–682  Magnetite, and in lesser measure hematite, are the most abundant iron minerals;[3]: 685  anhydrite,[51] diopside,[52] goethite, limonite, maghemite, pyrite,[3]: 685  scapolite[8] and diadochite are also found.

[24] High oxygen-18 amounts in the Laco magmas indicate either crustal contamination or isotopic effects during fractional crystallization.

[40] Hydrothermal alteration of the central lava dome and iron-bearing deposits has generated alunite, anatase, bassanite, chlorite, copper veinlets, gypsum, illite, jarosite, kaolinite, labradorite, quartz, rutile, sanidine, smectite, sulfur and tridymite.

[11] A nearby weather station (23°45′S 67°20′W / 23.750°S 67.333°W / -23.750; -67.333) at 4,500 metres (14,800 ft) altitude showed an average temperature of 2.3 °C (36.1 °F) in 1991, with strong short-term variability.