供應火山的不是融化的岩漿庫，而是軟糊糊的岩石
by Hayley Dunning
一項新的研究顛覆了關於火山爆發的傳統想法，他們發現火山的供應來源不是在巨大的岩漿庫中形成的熔岩。
反之，這項研究提出火山的供應來源是所謂的「軟岩庫」(mush reservoirs)――此處大部分都還是固態的礦物晶體，但晶體之間的微小空間有岩漿存在。
包括規模最大的火山噴發在內，我們對岩漿作用的認知都建立於岩漿是儲存在充滿液體的「岩漿庫」(magma chamber)中，一座巨大、裝滿液態岩漿的地下洞窟。然而，從來沒有人真的觀察到岩漿庫。
英國倫敦帝國學院和布里斯托大學的研究人員今日發表在《自然》(Nature)的研究提出，我們需要重新思考關於岩漿庫的基本假設。
主要作者，倫敦帝國學院地球科學與工程學系的Matthew
Jackson教授表示：「現在我們需要重新檢視軟岩庫導致火山噴發的原因與方式。運用這些發現我們可以對火山爆發有更多理解，從而增進民眾的安全，同時也能讓我們瞭解跟火山系統有關的金屬礦藏如何產生。」
軟岩庫產生的噴發
火山要噴發就需要岩漿供應，這些融化的液體岩石所含的固體礦物結晶相對來說較少。傳統上，科學家認為形成岩漿並儲存的地點是在地底的巨大岩漿庫中。
近期對於岩漿化學的研究挑戰了此觀點，使得軟岩庫模型隨之誕生，他們認為岩漿存在於礦物晶體之間的微小空隙，形成體積較小的岩漿池。然而，軟岩庫模型無法解釋如果要讓這些所含晶體較少的岩漿噴發到地表，它們要如何上升並進入到火山當中。
研究團隊利用對軟岩庫的最新模擬結果而得到了解答。在軟岩庫的環境下，岩漿的密度較周圍的晶體小，使得它們可以透過晶體之間的縫隙而往淺處移動。
當岩漿上升會跟晶體反應使其融化，造成局部地區含有晶體相對而言較少的岩漿。這些短暫出現含有岩漿較多的區域造成了火山噴發。
解開重大謎團
共同作者，布里斯托大學地球科學院的教授Stephen Sparks表示：「關於火山有個重大的謎題是人們認為火山底下有個充滿熔岩的巨大空間；然而，這種所謂的岩漿庫卻相當難以發現。」
「由帝國學院和布里斯托大學的地質學家發展出來的新理論，認為熔岩是在大都仍為晶體的高熱岩石中形成，它們大部分的時間都存在於微小的孔隙當中而非巨大的岩漿庫。不過，這些熔岩會緩緩地從岩石中擠出而形成熔岩池，接著噴發出來或是形成暫時性的岩漿庫。」
新的軟岩庫模型不但能解釋噴發如何開始，也能解釋其他跟火山系統有關的現象，像是岩漿化學成分的演變過程，以及先形成的晶體有多少可以跟後形成的岩漿一起噴發出來。
Volcanoes fed by ‘mush’ reservoirs
rather than molten magma chambers
Volcanoes are not
fed by molten magma formed in large chambers finds a new study, overturning
classic ideas about volcanic eruptions.
Instead,
the study suggests that volcanoes are fed by so-called ‘mush reservoirs’ –
areas of mostly solid crystals with magma in the small spaces between the
crystals.
Our understanding of volcanic processes, including those leading to the
largest eruptions, has been based on magma being stored in liquid-filled
‘magma’ chambers – large, underground caves full of liquid magma. However,
these have never been observed.
The new study, by researchers at Imperial College London and the
University of Bristol and published today in Nature,
suggests the fundamental assumption of a magma chamber needs a re-think.
Lead
author Professor Matthew Jackson, from the Department of Earth Sciences
and Engineering at Imperial, said: “We now need to look again at how and why
eruptions occur from mush reservoirs. We can apply our findings to
understanding volcanic eruptions with implications for public safety and also
to understand the formation of metal ore deposits associated with volcanic
systems.”
Erupting from mush reservoirs
In
order to erupt, volcanoes need a source of magma – melted, liquid rock –
containing relatively few solid crystals. Traditionally, this magma was thought
to be formed and stored in large underground magma chambers.
Recent
studies of magma chemistry have challenged this view, leading to the suggestion
of the mush reservoir model, where smaller pools of magma sit in the small gaps
between solid crystals. However, the mush reservoir model could not explain how
magmas containing relatively few crystals arise and are delivered to volcanoes
in order for them to erupt at the surface.
Now, with
sophisticated modelling of mush reservoirs, the research team has come up with
a solution. Within the mush reservoir scenario, the magma is less dense than
the crystals, causing it to rise up through the spaces between them.
As it rises, the
magma reacts with the crystals, melting them and leading to local areas
containing magma with relatively few crystals. It is these short-lived areas of
increased magma that can lead to eruptions.
A
major mystery solved
Co-author Professor
Stephen Sparks, from the University of Bristol’s School of Earth Sciences,
said: “A major mystery about volcanoes is that they were thought to be
underlain by large chambers of molten rock. Such magma chambers, however, were
very difficult to find.
“The new
idea developed by geologists at Imperial and Bristol is that molten rock forms
within largely crystalline hot rocks, spending most of its time in little pores
within the rock rather than in large magma chambers. However, the rock melt is
slowly squeezed out to form pools of melt, which can then erupt or form
ephemeral magma chambers.”
As well
as the initiation of eruptions, the new mush reservoir model can help explain
other phenomena in volcanic systems, such as how the magma chemical composition
evolves and how much older crystals can be erupted within younger magmas.
原始論文：M. D. Jackson,
J. Blundy, R. S. J. Sparks. Chemical differentiation, cold storage and
remobilization of magma in the Earth’s crust. Nature, 2018;
DOI: 10.1038/s41586-018-0746-2
引用自：Imperial College London. "Volcanoes fed by
'mush' reservoirs rather than molten magma chambers."
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