Delayed brittle-like fragmentation of vesicular magma analogue by decompression
Delayed brittle-like fragmentation of vesicular magma analogue by decompression
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DOI：
10.1016/j.jvolgeores.2013.04.008
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发表时间：
2013-05-15
影响因子：
2.9
通讯作者：
Shida, Tsukasa
中科院分区：
文献类型：
--
作者：
Kameda, Masaharu;Ichihara, Mie;Shida, Tsukasa
为了弄清火山喷发中的一个关键现象--泡状岩浆的破碎，进行了含气泡糖浆的快速减压实验。我们关注的是类似脆性的碎裂，它发生的时间比脆性碎裂发生的时间更长。在不同的初始压力、压力差和减压时间下，测试了气泡糖浆对不同粘度和孔隙率的减压的响应。用高速摄影观察到的响应用德博拉数(DT)进行分类，它定义为糖浆的粘弹性松弛时间与减压时间的比率，气泡表面的最大差应力Delta sigma(Max)，以及当表面的差应力达到临界断裂应力Delta sigma(C)时气泡表面的瞬时脆性度量。在许多实验运行中，De(Dt)和beta(C)都表明材料的延性响应，当Delta sigma(Max)大大超过Delta sigma(C)时，就会发生脆性碎裂。对于所有观察到脆性类破碎的运行，破碎的开始时间基本上比松弛时间延迟，而开始发生在气泡粘性膨胀的特征时间内。在许多脆性碎裂的情况下，碎裂在由单一减压事件触发的发病后继续发生。详细的分析表明，气泡糖浆中的气体通过表面裂纹突然释放，可能导致局部脆性增加，从而开始脆性破碎。岩浆破碎可以看作是连续的类似脆性的破碎。这一观点可以为观测到的在触发减压事件后爆发爆发的时间延迟提供解释。(C)2013爱思唯尔B.V.保留所有权利。
A rapid decompression experiment using syrup containing gas bubbles was conducted in order to clarify the fragmentation of vesicular magma, which is a key phenomenon in volcanic eruptions. We focus on brittle-like fragmentation, which occurs with a longer time scale than brittle fragmentation. The response of the bubbly syrup to the decompression for various viscosities and porosities was tested under various initial pressures, pressure differences, and decompression times. The response observed by high-speed photography was classified using the Deborah number De(DT), which is defined as the ratio of the viscoelastic relaxation time of the syrup to the decompression time, the maximum differential stress at the bubble surface Delta sigma(max), and a measure of instantaneous brittleness at the bubble surface beta(c) at the time when the differential stress at the surface reaches the critical fracture stress Delta sigma(c). In a number of the experimental runs in which both De(DT) and beta(c) indicated ductile response of the material, brittle-like fragmentation occurred when Delta sigma(max) substantially exceeded Delta sigma(c). For all of the runs in which brittle-like fragmentation was observed, the onset of fragmentation was delayed substantially from the relaxation time, whereas the onset occurred within the characteristic time for viscous expansion of the bubbles. In many cases of brittle-like fragmentation, fragmentation continued after the onset triggered by a single decompression event. Detailed analysis indicates that brittle-like fragmentation started with a sudden release of the gas in the bubbly syrup through a surface crack, which might lead to an increase in local brittleness. Magma fragmentation may be viewed as sequential brittle-like fragmentation. This view may provide an explanation for the observed time delay for the onset of an explosive eruption after a triggering decompression event. (C) 2013 Elsevier B.V. All rights reserved.