Brittle fragmentation, generating small pyroclasts from magma, is a key process determining eruptive style. How low-viscosity magma fragments within a rising fountain in a brittle manner, however, is not well understood. Here we describe a fragmentation process in Hawaiian fountains on the basis of observations from the 2018 lower East Rift Zone eruption of Kīlauea Volcano, Hawai’i. The dominant fragmentation mechanism is inertia driven and produces a population of large fluidal pyroclasts. However, when sufficient volcanic gas is released in the fountain, a subpopulation of smaller and more vesicular pyroclasts is generated and entrained into the gas-dominant convective plume. The size distribution of these pyroclasts is similar to that of brittlely fragmented solid materials. The erupted high-vesicularity pyroclasts sometimes preserve a deformed shape. These observations suggest that late-stage rapid expansion lowers the gas temperature adiabatically and cools the outer surface of liquid pyroclasts below the glass transition temperature. The rigid crust fragments as the hot interior attempts to expand due to further volatile diffusion from the melt into bubbles. Adiabatic expansion of volcanic gas occurs in all eruptions. Brittle fragmentation induced by rapid adiabatic cooling may be a widespread process, although of varying importance, in explosive eruptions.

脆性破碎，从岩浆中产生小的火山碎屑，是决定喷发方式的关键过程。然而，低粘度岩浆如何在上升的喷泉中以脆性方式碎裂，尚不清楚。在这里，我们根据夏威夷基拉韦厄火山 2018 年下东裂谷区喷发的观察描述了夏威夷喷泉的破碎过程。主要的碎裂机制是惯性驱动的，并产生大量流体火山碎屑。然而，当喷泉中释放出足够的火山气体时，会产生一个更小、更多泡状的火山碎屑亚群，并被夹带到以气体为主的对流羽流中。这些火山碎屑的尺寸分布与易碎的固体材料的尺寸分布相似。喷发的高泡火山碎屑有时会保持变形的形状。这些观察结果表明，后期快速膨胀会绝热地降低气体温度，并将液体火山碎屑的外表面冷却到玻璃化转变温度以下。由于从熔体进一步挥发扩散到气泡中，当热的内部试图膨胀时，坚硬的地壳碎裂。火山气体的绝热膨胀发生在所有的喷发中。由快速绝热冷却引起的脆性碎裂在爆炸性喷发中可能是一个普遍的过程，尽管重要性各不相同。由于从熔体进一步挥发扩散到气泡中，当热的内部试图膨胀时，坚硬的地壳碎裂。火山气体的绝热膨胀发生在所有的喷发中。由快速绝热冷却引起的脆性碎裂在爆炸性喷发中可能是一个普遍的过程，尽管重要性各不相同。由于从熔体进一步挥发扩散到气泡中，当热的内部试图膨胀时，坚硬的地壳碎裂。火山气体的绝热膨胀发生在所有的喷发中。由快速绝热冷却引起的脆性碎裂在爆炸性喷发中可能是一个普遍的过程，尽管重要性各不相同。