岩漿化學中的臨界點決定了火山是否會猛烈爆發
在活躍行星內部進行的作用於行星表面最壯觀的表現方式便是火山爆發。
在溢流式噴發中，熔岩流會緩緩地持續流出至地表；反之，爆裂式噴發則是一種劇烈的自然現象，可以將熾熱的物質噴發至大氣上空數公里處。
這兩種噴發類型的轉變也代表了最危險的自然災害之一。
過去數十年來，為了瞭解這樣的轉變背後機制究竟為何，而啟發了無數地球科學研究。
在一篇新研究中，由布里斯托大學地球科學院的Danilo
Di Genova博士領導的國際科學家團隊，首度提出證據顯示岩漿化學中極度微小的差異，就能將世界各地的寧靜式噴發和爆裂式噴發明確區分開來。
此外，他們也指出岩漿在奈米尺度上的變化也能劇烈增加火山發生爆裂式噴發的可能性。
Di Genova博士表示：「我們在研究中呈現了新的實驗數據、熱力學模擬，並分析全球火山紀錄中有關成分方面的資料，這些證據結合起來顯示流紋岩質岩漿的流動特性中出現的急遽轉變，決定了一座火山的噴發方式是溢流式或爆裂式。」
「我們發現在流紋岩質岩漿中，些微的成分變化就能讓流動性質產生重大差異。成分變化的原因可能是結晶作用、同化作用、岩漿補充或者混合作用。」
「由成分引起的流動特性變化也可能是源自於岩漿本身性質的改變，像是溫度、壓力或者氧逸度。」
這可能會讓先前已經「關閉」的岩漿庫因為化學作用造成的液化而重新活動；或者降低脫氣作用的進行效率，使得岩漿因為化學作用而「固化」，造成它發生爆裂式噴發的可能性增加。
此外，研究也顯示最近在火山岩中發現含鐵奈米晶體迅速沉澱的現象，會使得熔岩結構中的鐵變少，並提供引起爆裂式噴發的氣泡形成時所需的成核中心，而增加岩漿發生爆裂式噴發的機率。
Chemical tipping point of magma determines explosive potential of
volcanoes
Volcanic eruptions are the
most spectacular expression of the processes acting in the interior of any
active planet.
Effusive eruptions consist of a gentle and steady flow of lava
on the surface, while explosive eruptions are violent phenomena that can eject
hot materials up to several kilometres into the atmosphere.
The transition between these eruptions represents one of the
most dangerous natural hazards.
Understanding the mechanisms governing such transition has
inspired countless studies in Earth Sciences over the last decades.
In a new study led by Dr Danilo Di Genova, from the
University of Bristol’s School of Earth Sciences, an international team of
scientists provide evidence, for the first time, that a subtle tipping point of
the chemistry of magmas clearly separates effusive from explosive eruptions
worldwide.
Moreover, they demonstrate that variabilities at the nanoscale
of magmas can dramatically increase the explosive potential of volcanoes.
Dr Di Genova said: “The new experimental data,
thermodynamic modelling and analysis of compositional data from the global
volcanic record we presented in our study provide combined evidence for a
sudden discontinuity in the flow behaviour of rhyolitic magmas that guides
whether a volcano erupts effusively or explosively.
“The identified flow-discontinuity can be crossed by small
compositional changes in rhyolitic magmas and can be induced by
crystallisation, assimilation, magma replenishment or mixing.
“Composition-induced flow behaviour variations may also
originate from changes in magmas intrinsic parameters such as temperature,
pressure or oxygen fugacity.”
These can result in revitalization of a previously “locked”
magma chamber via chemical fluidification or may hinder efficient degassing and
lead to increased explosive potential via chemical “stiffening” of a magma.
Furthermore, the study showed how the sudden precipitation of
iron-bearing nanocrystals, which have been recently found in volcanic rocks,
can increase the explosive potential of a magma via both depletion of iron in
the melt structure and providing nucleation points for gas bubbles which drive
explosive eruption.
原始論文：D. Di Genova,
S. Kolzenburg, S. Wiesmaier, E. Dallanave, D. R. Neuville, K. U. Hess, D. B.
Dingwell. A compositional tipping point governing the mobilization and
eruption style of rhyolitic magma. Nature, 2017; 552 (7684):
235 DOI: 10.1038/nature24488
引用自；University of Bristol. "Chemical tipping
point of magma determines explosive potential of volcanoes."
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