Two stages of volcanic activity constructed the Itcha Range. The first stage, referred to as the felsic shield-building stage, occurred between 3.8 and 3.0 million years ago. Three phases comprise this stage; a pre-explosive phase, an explosive phase and a post-explosive phase. Analysis of the first trachyte magmas to erupt during the pre-explosive phase suggest they were 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.