relatively fluid as shown by their areal extent. Eruptions became more viscous during the explosive phase, followed by more viscosity during the post-explosive phase. As a result, the volume of erupted material became smaller over time. The increasing viscousness of felsic lava during the shield-building stage suggests a maturing plumbing system underneath the Itcha Range, which may have consisted of multiple, isolated, cupola-type magma chambers. A 900,000‑year‑long period of quiescence followed after the felsic shield-building stage, during which time erosion ate away at the shield's gentle slopes. This quiescence was followed by a mafic capping stage between 2.2 and 0.8 million years ago, but renewed activity might have occurred in the last 340,000 years. Alkali olivine basalts of the mafic capping stage were derived from the fractionation of a clinopyroxene, olivine and oxide assemblage. However, the associated hawaiite lavas may have derived from an alkali olivine basalt parent by the fractionation of a clinopyroxene-dominated assemblage at higher pressures. As volcanic activity waned during the mafic capping stage, lava flows became more viscous and decreased in volume. This suggests that parental magmas may have been derived by increasingly smaller degrees of partial melting with time. If volcanic activity of the Itcha Range is related to a hotspot, this temporal and spatial evolution would suggest a waning heat source. The main body of the shield erupted over an area of about . Volcanic activity associated with the Itcha Range extended south to the Satah Mountain area where lavas erupted along a north-northwest trending fault system and covered an additional area of . Although the Satah Mountain volcanic field is not part of the Itcha Range, it is linked to the range by a volcanic ridge. Felsic shield-building stage The felsic shield-building stage began with the eruption of phonolite, trachyte, phonolitic trachyte, quartz-trachyte and rhyolite lava. Subsequent volcanism of the pre-explosive phase produced a basal sequence of aphyric trachyte lava flows and domes with minor altered flow-banded rhyolite, sulfide-bearing rhyolitic tuffs and a few thin hawaiite lava flows. This activity was concentrated at the summit of the volcano as shown by the increasing thickness of volcanic material towards the summit. A hydrothermally altered aphyric trachyte dike, which forms a narrow ridge linking Mount Downton and Itcha Mountain, might have been the source for these eruptions. After the basal trachytes were erupted, the explosive phase produced alkali-feldspar porphyritic trachytes, which erupted as pyroclastic deposits