為什麼地底下的岩漿庫會特別偏好出現在某個深度？

原文網址：https://www.brown.edu/news/2019-08-19/magmachamber

為什麼地底下的岩漿庫會特別偏好出現在某個深度？

某些反覆噴發的火山經常發生劇烈活動。供應這類火山的岩漿庫位在地殼的深度，大部分都集中在一個相當特定的範圍。發表於期刊《自然―地球科學》的新研究解釋了為何會有此現象。這項發現可以讓科學家更加了解世界各地的火山作用。

岩漿庫是地殼內部的岩石處於部分熔融狀態的地方，通常位在地下6到10公里處。研究人員運用的電腦模型，可以呈現出岩漿庫的物理性質隨著時間經過如何演變。結果顯示限制岩漿庫所在深度的兩項關鍵因子，分別是水氣是否會從岩漿裡冒出，以及地殼能否擴張來容納持續成長的岩漿庫。

「我們從觀察結果得知，似乎有個深度特別適合岩漿庫一再噴發。」研究主要作者，美國布朗大學的地質學家Christian Huber表示。「長久以來都沒有人能夠解答為何會有這種最適深度。而這篇研究首度解釋了決定此現象的作用是什麼。」

在地下6到10公里的地方，從淺至深的壓力一般是對應到1.5千巴至2.5千巴。模型顯示壓力小於1.5千巴的時候，岩漿含有的水很容易就會形成氣泡，造成十分劇烈的火山爆發，使得岩漿庫噴出的岩漿比補充進來的還多。這類岩漿庫很快就會停止活動。而壓力大於2.5千巴的時候，地球深處的高溫使得岩漿庫周圍的岩石變軟而容易變形，使得岩漿庫不需要噴出岩漿到地表，就可以毫不費力變得更大。這類系統會漸漸冷卻並凝固，過程中甚至完全不會噴發。

「1.5公里至2.5公里深是火山系統感到最快活的地方。」Huber表示。「它們可以噴出岩漿，接著補充新的，如此反覆進行。」

Huber表示這項模型的重要之處，在於可以同時呈現岩漿庫所處的地殼以及內部岩漿的動力學特性。之前科學家已經相當了解深部的岩漿庫可以不噴發就繼續成長，卻沒有理解出水氣對淺層岩漿庫造成的限制。

「最適區域的界限為何是1.5千巴？一直以來都沒有好的解釋。」Huber表示。「我們證實氣體的行為事實上相當重要，雖然它不過是造成噴出的岩漿比補充進來的還多。」

Huber說這項發現有助於科學家瞭解全球岩漿的存量有多少。

「岩漿待在地殼和噴發到地表的比例為何，是個相當重大的問題。」Huber表示。「岩漿可以把二氧化碳和其他氣體帶到大氣當中，進而影響氣候。因此瞭解哪些岩漿會待在地殼當中，哪些會噴發出來便相當重要。」

論文共同作者包括Meredith Townsend、WimDegruyter和Olivier Bachmann。研究經費來自美國國家科學基金會(NSF-EAR 1760004)，以及瑞士國家科學基金會(200021_178928)。

Why there’s a ‘sweet spot’ depth for underground magma chambers

A new study reveals why the magma chambers that feed recurrent and often explosive volcanic eruptions tend to reside in a very narrow depth range within the Earth’s crust. The findings, published in Nature Geoscience, could help scientists to better understand volcanic processes the world over.

The research makes use of computer models that capture the physics of how magma chambers, reservoirs in the crust that contain partially molten rock, evolve over time. The models showed that two factors — the ability of water vapor to bubble out of the magma, and the ability of the crust to expand to accommodate chamber growth — are the key factors constraining the depth of magma chambers, which are generally found between six and 10 kilometers deep.

“We know from observations that there seems to be a sweet spot in terms of depth for magma chambers that erupt repeatedly,” said Christian Huber, a geologist at Brown University and the study’s lead author. “Why that sweet spot exists has been an open question for a long time, and this is the first study that explains the processes that control it.”

Depths of six to 10 kilometers generally correspond to pressures of about 1.5 kilobars on the shallow side and 2.5 kilobars on deep side. The models showed that at pressures less than 1.5 kilobars, water trapped within the magma forms bubbles readily, leading to violent volcanic explosions that blast more magma out of a chamber than can be replaced. These chambers quickly cease to exist. At pressures more than 2.5 kilobars, warm temperatures deep inside the Earth make the rocks surrounding the magma chamber soft and pliable, which enables the chamber to grow comfortably without erupting to the surface. These systems cool and solidify over time without ever erupting.

“Between 1.5 and 2.5, the systems are happy,” Huber said. “They can erupt, recharge and keep going.”

The key to the models, Huber said, is that they capture the dynamics of both the host crust and of the magma in the chamber itself. The ability of deep magma chamber to grow without erupting was fairly well understood, but the limit that water vapor exerts on shallow magma chambers hadn’t been appreciated.

“There hadn’t been a good explanation for why this habitable zone should end at 1.5 kilobars,” Huber said. “We show that the behavior of the gas is really important. It simply causes more mass to erupt out than can be recharged.”

Huber says the findings will be helpful in understanding the global magma budget.

“The ratio of magma that stays in the crust versus how much is erupted to the surface is a huge question,” Huber said. “Magma supplies CO₂ and other gases to the atmosphere, which influences the climate. So having a guide to understand what comes out and what stays in is important.”

Coauthors on the paper Meredith Townsend, WimDegruyter and Olivier Bachmann. The work was supported by the National Science Foundation (NSF-EAR 1760004) and the Swiss National Fund (200021_178928).

原始論文：Christian Huber, Meredith Townsend, Wim Degruyter, Olivier Bachmann. Optimal depth of subvolcanic magma chamber growth controlled by volatiles and crust rheology. Nature Geoscience, 2019; DOI: 10.1038/s41561-019-0415-6

引用自：Brown University. "Why there's a 'sweet spot' depth for underground magma chambers."