Think of Vinicunca – Rainbow Mountain – as the product of a sort of vast open-air cement mixer. Tectonic plates beneath the Andes region of Peru heave up and down. Wind and rain oxidise exposed minerals. Goethite and limonite turn sandstone brown, iron sulphite adds a yellow hue, chlorite lends a shade of green. Hence Rainbow Mountain’s attraction to geologists; Peruvians, who worship the 5,200-metre edifice as the deity of Cusco, the nearest city; and an increasing number of trekkers who either make a day trip to fill their Instagram feeds, or include it as part of the 70-kilometre Ausangate circuit, rated as one of South America’s most challenging hikes.