Nanoscale liquid immiscibility is observed in the 2018–2021 Fani Maoré submarine lavas (Comoros archipelago). Heat transfer calculations, Raman spectroscopy, scanning and transmission electron microscopy reveal that in contrast to thin (500 µm) outer rims of homogeneous glassy lava (rapidly quenched upon eruption, >1000 °C s−1), widespread liquid immiscibility is observed in thick (1 cm) inner lava rims (moderately quenched, 1–1000 °C s−1), which exhibit a nanoscale coexistence of Si- and Al-rich vs. Ca-, Fe-, and Ti-rich melt phases. In this zone, rapid nanolite crystallization contrasts with the classical crystallization process inferred for the slower cooled ( < 1 °C s−1) lava interiors. The occurrence of such metastable liquid immiscibility at eruptive conditions controls physicochemical characteristics of nanolites and residual melt compositions. This mechanism represents a common yet frequently unobserved feature in volcanic products, with the potential for major impacts on syn-eruptive magma degassing and rheology, and thus on eruptive dynamics.