On the conditions of sheet injections and eruptions in stratovolcanoes
On the conditions of sheet injections and eruptions in stratovolcanoes
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成层火山片状喷射和喷发的条件
DOI：
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发表时间：
2005
期刊：
影响因子：
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通讯作者：
S. Brenner
中科院分区：
文献类型：
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作者：
Agust Gudmundsson;S. Brenner
几乎所有成层火山（复合火山、中央火山）的喷发都是通过裂缝提供岩浆的。因此，成层火山喷发的主要物理条件是岩浆驱动的裂缝能够传播到地表。充满岩浆的裂缝，无论是冻结的还是流体的，都被称为片状侵入体。更具体地说，当它们接近垂直时，它们被称为堤坝；当它们倾斜时，它们被称为倾斜（或圆锥）板。现场观察表明，大多数板块侵入不会到达地表以促进喷发，而是在不同的地壳深度处被阻止。因此，火山喷发期间的火山动荡比火山喷发更为常见。片状侵入是否被阻止或传播到地表主要取决于成层火山中的应力场。成层火山通常由具有对比机械特性的层组成，例如软（低杨氏模量）火山碎屑单元和硬（高杨氏模量）熔岩流。我们提出的数值模型表明，由此类层组成的火山通常会产生应力场，从而促进片状和岩脉的滞留。模型表明，板块侵入到达地表并促进火山喷发的必要条件是沿板块路径的应力场变得均匀。我们认为，火山周期期间成层火山的大部分活动都会促进应力场均质化。实地研究表明，个别成层火山的片状侵入体具有不同的倾角：有些是垂直的岩脉，有些是倾斜的片状，还有一些是水平的岩坎。分析模型表明，到达地表的岩片倾角会对喷发过程中的岩浆输送产生巨大影响。对于平坦的火山（例如塌陷火山口），这种影响通常比形成地形高点的成层火山更大。我们得出的结论是，片状侵入体的倾角越浅，其体积岩浆输送到成层火山表面的量就越少。
Nearly all eruptions in stratovolcanoes (composite volcanoes, central volcanoes) are supplied with magma through fractures. Consequently, a primary physical condition for an eruption to occur in a stratovolcano is that a magma-driven fracture is able to propagate to the surface. Magma-filled fractures, frozen or fluid, are referred to as sheet intrusions. More specifically, they are named dykes when subvertical, and inclined (or cone) sheets when inclined. Field observations indicate that most sheet intrusions do not reach the surface to feed eruptions but rather become arrested at various crustal depths. For this reason periods of volcanic unrest with sheet injections are much more common than volcanic eruptions. Whether a sheet intrusion becomes arrested or, alternatively, propagates to the surface depends primarily on the stress field in the stratovolcano. A stratovolcano normally consists of layers of contrasting mechanical properties, such as soft (low Young’s modulus) pyroclastic units and stiff (high Young’s modulus) lava flows. We present numerical models indicating that volcanoes composed of such layers commonly develop stress fields encouraging sheet and dyke arrest. The models indicate that a necessary condition for a sheet intrusion to reach the surface and feed a volcanic eruption is that the stress field along the sheet pathway becomes homogenised. We propose that much of the activity in a stratovolcano during a volcanic cycle encourages stress-field homogenisation. Field studies show that the sheet intrusions in individual stratovolcanoes have various dips: some are vertical dykes, others inclined sheets, and still others horizontal sills. Analytical models indicate that the dip of a sheet reaching the surface can have great effects on the magma transport during an eruption. This effect is normally greater for a flat volcano such as a collapse caldera than for a stratovolcano that forms a topographic high. We conclude that the shallower the dip of a sheet intrusion, the less will be its volumetric magma transport to the surface of a stratovolcano.