Brittle fragmentation by rapid gas separation in a Hawaiian fountain
Brittle fragmentation by rapid gas separation in a Hawaiian fountain
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DOI：
10.1038/s41561-021-00709-0
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发表时间：
2021-03-29
影响因子：
18.3
通讯作者：
Houghton, Bruce F.
中科院分区：
文献类型：
--
作者：
Namiki, Atsuko;Patrick, Matthew R.;Houghton, Bruce F.
脆性碎裂作用是决定火山喷发方式的关键过程，它是岩浆生成小火山碎屑的过程。然而，低粘度岩浆如何在上升的喷泉中以脆性的方式破碎，目前还不清楚。在这里，我们根据2018年夏威夷基拉韦厄火山下东裂谷带喷发的观测结果描述了夏威夷喷泉的破碎过程。主要的破碎机制是惯性驱动，并产生一个人口的大型流体火山碎屑。然而，当喷泉中释放出足够的火山气体时，就会产生一个更小、更呈泡状的火成碎屑亚群，并被夹带到以气体为主的对流羽流中。这些火山碎屑的粒度分布与脆性破碎固体物质的粒度分布相似。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，后期快速膨胀降低气体温度热释光和冷却的玻璃化转变温度以下的液体火山碎屑的外表面。当热的内部由于从熔体到气泡的进一步挥发性扩散而试图膨胀时，刚性外壳破碎。火山气体的绝热膨胀发生在所有的喷发中。快速绝热冷却引起的脆性碎裂可能是一个普遍的过程，虽然不同的重要性，在爆炸爆发。
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 Kilauea 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.