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Abstract Investigating the conditions behind the formation of pyroclast textures and lava flow morphologies is important to understand the dynamics of submarine volcanic eruptions, which are hard to observe. The development of clast textures and lava morphologies depends on the competing effects of their eruption rates and the rates of solidification. While eruption rates are governed by subsurface magmatic processes, the solidification timescales depend on the rate of heat loss from lava to the external water. However, the effect of the speed of lava flow or clast on their solidification timescales under two‐phase (liquid water and vapor bubbles) water boiling conditions is poorly constrained. Using laboratory experiments with remelted igneous rocks, we investigate the effect of the relative motion between lava and external water on its cooling timescale. We use a range of water speed (0–12.5 cm s⁻¹) in our experiments while keeping our sample stationary to simulate a range of relative speed between lava and ambient water. Using transient heat transfer modeling, we find that heat flux from the surface of the sample to the external water overall increases with increasing water speed. We find heat transfer coefficients of up to ∼1.72 × 10³ W m⁻² K⁻¹. The implications of high heat flux on the formation of solid lava crust under submarine conditions are discussed. 

Pre-mixing of magma and external water plays a key role in driving explosive phreatomagmatic and submarine volcanic eruptions. A thin film of water vapor forms at the magma–water interface as soon as hot magma comes in direct contact with the cold water (Leidenfrost effect). The presence of a stable vapor film drives efficient mixing and mingling between magma and water, as well as magma and wet and water-saturated sediments. Such mixing occurs before explosive molten fuel–coolant type interactions. Using high-temperature laboratory experiments, we investigate the effect of magma and water temperatures on the stability of vapor film, which has not been performed systematically for a magmatic heat source. The experiments were performed with re-melted volcanic rock material, from which spherically-shaped rock samples were produced. These samples were heated to 1,110°C and then submerged in a water pool with a constant temperature (3–93°C). The experiments were recorded on video, and, synchronously, sample and water temperatures were measured using thermocouples. The time-dependent thickness of the vapor film was measured from the video material. The vapor film tends to oscillate with time on the order of 10 2  Hz. We find that the vertical collapse rates of vapor films along the sample–water interfaces are 13.7 mm s −1 and 4.2 mm s −1 for water temperatures of 3.0°C and 65°C, respectively. For a given initial sample temperature, the thickness and stability time scales decrease with decreasing water temperature, which has implications for the efficiency of pre-mixing required for explosive eruptions. Using thermodynamics and previously measured material parameters, it is shown that a sudden collapse of the vapor film can start brittle fragmentation of the melt and thus serves as the starting point of thermohydraulic explosions.