Identifying the processes that initiate and control large explosive eruptions on time scales of human interest is challenging. We present evidence for syneruptive, subsurface interactions between rhyolite magmas derived from distinctive sources that reflect tectonic processes starting, then temporarily stopping, the 27 ka Oruanui supereruption (530 km3 magma; Taupo, New Zealand). Subordinate (3%–16%) biotite-bearing pumice lapilli occur in fall deposits from phases 1 and 2 (of 10) among the biotite-free “normal” pumices in the Oruanui deposits. The biotite-bearing magma is sourced to an independent magmatic system 10–15 km northeast of Taupo. Lateral migration of the biotite-bearing magma was initiated during a rifting episode that also triggered the Oruanui eruption. Limited physical mingling indicates that the biotite-bearing and Oruanui biotite-free magmas interacted in the conduit simultaneously with eruption. After phase 1, a time break of months elapsed before phase 2 explosive activity resumed from the same vent, again with intermixed biotite-bearing magma. A rift-related tectonic event, with its accompanying stress change, is inferred to have initiated diking from northeast to southwest, transporting biotite-bearing magma and triggering the Oruanui eruption. Stress relaxation following the rifting event terminated phase 1 before a tectonic event caused renewed diking and activity in phase 2. Tectonics may trigger large silicic eruptions through rupturing of shallow crustal magma chambers and lateral magma transport, especially in extensional regimes, without necessarily leaving any discernible geological imprint. Only the presence of a “foreign” magma in the Oruanui deposits allows the tectonic control to be identified.