High-SiO₂ (75-77 wt% SiO₂) rhyolite is the most differentiated magma type on Earth and constitutes some of the largest explosive eruptions (100-1000’s km³) over the last 1 Myr, including those from Yellowstone, WY and Long Valley, CA. What is perhaps most surprising about high-SiO₂ rhyolites is that they are relatively scarce at subduction zones, both in volcanic arcs and granitoid batholiths. Their scarcity is reflected in the dacitic composition of the upper 1/3 of continental crust. Instead, high-SiO₂ rhyolites are more commonly found in regions of continental extension, often in association with basalt (i.e., bimodal volcanism). An outstanding question is why high-SiO₂ rhyolites (eutectic melts) fail to segregate and erupt in significant quantities at subduction zones, but efficiently do so, often in surprisingly large volumes, in regions of continental extension. The answer to this and related questions is addressed by examining the kinetics of phenocryst nucleation and growth in the Long Valley high-SiO₂ rhyolites, including the ≥600 km³ Bishop Tuff and the preceding ≥ 100 km³ Glass Mountain complex, all of which are relatively cold (≥ 700°C) and hydrous (≤7 wt% H₂O). It is shown, through the results of thermometry, hygrometry, and both phase-equilibrium and decompression experiments, as well as available geochemistry, that their origin must be driven by at least two stages of partial melting of a mixed basalt:granitoid lithology in the upper crust. Moreover, their segregation and ascent must be initially fluid-undersaturated, such that when fluid-saturation inevitably occurs during ascent, the melts cross their liquidus without pre-existing nuclei, enabling large undercoolings and the rapid growth of large sparse phenocrysts. Thus phenocryst growth occurs during transit to the surface and not in a long-lived magma chamber, constraining the timescale between melt segregation and eruption to be extremely short, on the order of several weeks or less. In contrast, the reason why aplite dikes in granitoids fail to erupt is because they segregate under H₂O fluid-saturated conditions from crystallizing magma bodies and thus contain numerous nuclei. During fluid-saturated ascent, the loss of only a small amount of dissolved water from these eutectic melts causes them to immediately crystallize.