世界各地有40到50座火山正在噴發或是蠢蠢欲動，這些火山可能會活動而造成災害，使得數億人正處於它們帶來的風險當中。然而，儘管火山噴發可以對人類的生命與財產造成重大災害，我們仍然無法做出精準且可信度高的預測。就算專家能給出精確的預報，還是有可能無法提供充裕的時間讓人民撤離或是做出緊急應對。
圖片來源：Daniel Rasmussen，史密森尼學會。照片拍攝經過阿拉斯加國家海洋野生動物避難所的許可，特別監測許可證編號為#74500-16-009。
做出精準且可信的火山噴發預報仍然是個難以達到的目標，很大一部份是因為火山學家還沒完全瞭解岩漿到達地表之前，它們在火山底下的動力學特性以及發生的作用。火山學家Dan
Rasmussen是史密森尼美國國家自然史博物館的研究員，最近由他主持的新研究結果或許能讓專家朝精準預測火山噴發的目標更近一步。
這篇研究3月10日發表在期刊《科學》(Science)，他們發現以世界上最常見的火山類型來說，水分含量高的岩漿傾向儲存於地殼較深的位置。這項結果證實了某些科學家猜測的，水分是控制岩漿儲存深度最重要的因素。
「這項研究的重要之處在於找出了水分和岩漿儲存深度有所關聯，因為火山爆發的原因跟威力大部份也取決於水，」Rasmussen表示。他解釋水分引發火山噴發的原理，就像是二氧化碳讓汽水搖晃之後噴出來一樣。
「當儲存在火山下方的岩漿有水溶於其中，如果壓力突然減低，便會形成氣泡，使得岩漿上升並從火山噴出，就像是把汽水搖晃之後突然打開會噴出來一樣，」Rasmussen表示。「岩漿的含水量越高意味著可以產生越多氣泡，噴發就有可能更加猛烈。」
「這些結果讓我們更加理解儲存在火山下方的岩漿所處的條件與物理特性。若要更加準確地預測火山噴發，就必須建立精細的物理模型，而這種理解便是基本原料，」Rasmussen表示。
研究人員完成這項研究除了透過新的野外工作及實驗室分析之外，還重新分析了現有的數據――來自史密森尼學會全球火山計畫追蹤過往火山爆發的成果。
Rasmussen的研究契機始於2015年，當時他從哥倫比亞大學的拉蒙特―多爾蒂地球觀測站完成博士學位。他的指導教授火山學家Terry
Plank建議他繼續探討一項還沒解開的謎團：為什麼一座火山和另一座火山的岩漿會儲存在不同的深度？又是受到什麼條件控制？
包括Rasmussen與卡內基科學研究所的地球物理學家Diana
Roman在內的團隊，前往阿拉斯加崎嶇且偏遠的阿留申群島進行野外調查。他們從當地的八座火山採集了火山物質。
研究人員在挑選火山來進行研究的時候，特別針對了某種地質背景：位在兩個板塊聚合邊界的火山弧。包括阿留申群島的在內，火山弧是地球上數量最多的火山類型；此外，環繞太平洋板塊、著名的「火環」也都屬之，因此研究人員要加強預測火山噴發的能力時，它們便是最顯眼的目標。
團隊運用船隻和直升機從這八座火山採取少量的火山灰，途中穿越了波濤洶湧的海洋，在烏尼馬克島還遭受了巨大的棕熊帶來的威脅。這趟遠征的主要目標是火山灰，因為它含有橄欖石這種綠色的晶體，每一顆橄欖石的直徑都只有一毫米左右，跟一張身分證差不多薄。
橄欖石晶體在地下形成的時候偶爾會捕捉微量的岩漿。火山噴發把這些特殊的橄欖石晶體送到地球表面之後，裡面的岩漿會冷卻而變成玻璃。將這些火山內部的微量岩漿冷卻所形成的玻璃進行化學分析之後，研究人員就能推算岩漿的含水量。
對於阿留申群島的八座火山，團隊推算了其中六座的樣品裡被捕獲的岩漿含水量，接著再結合科學文獻中世上另外56座火山岩漿含水量的估計值。最終對於岩漿含水量估計值的名單涵蓋了62座火山，3856個樣品。
為了探討這些岩漿庫的含水量估計值以及儲存深度的對應關係，研究人員翻遍科學文獻建立了另一份名單，列出112座火山的331個深度估計值。
Rasmussen表示史密森尼學會全球火山計畫建立的資料庫是「彙整出這些名單的關鍵。因為它是調查噴發歷史時相當好的來源，而且我們只想考慮最近噴發過的火山。」Rasmussen和研究人員著重在最近噴發的火山，是因為岩漿庫在剛噴發後應該不會有太大的移動，因此利用最近的噴發產物所做出的深度或含水量估計值，最有可能精確反映出火山岩漿庫目前的狀態。
經過數年的野外工作、地球化學分析與文獻回顧，團隊成功把世界各地28座火山的岩漿庫深度估計值和含水量估計值的對應關係作圖。結果顯而易見：岩漿庫的含水量和儲存深度有很強的相關性。換句話說，岩漿含有的水分越多，就越傾向儲存於地殼較深的地方。
研究也證實岩漿的含水量確實可以控制它的深度，不是只有相關性而已。團隊偵測到的化學示蹤劑顯示了含水的岩漿是在地函中形成，從而證明這種因果關係。
「如果是由儲存深度決定岩漿的含水量，那麼我們觀察到的深度和含水量之間的相關性依然成立，但是代表岩漿初始含水量的化學示蹤劑就不會形成而讓我們發現到，」Rasmussen表示。
至於含水量如何決定岩漿的儲存深度，Rasmussen與共同作者主張和脫氣作用有關。在此過程中水和岩漿混和之後會產生氣泡，接著岩漿從地殼往上升的時候會開始脫氣，並且變得更加黏稠。研究人員認為岩漿的上升速度因此減慢，最終停滯下來。
岩漿儲存深度絕大部分取決於水含量的證據推翻了目前此領域最廣為接受的解釋，該理論認為岩漿會從地殼中的裂縫上升是因為熔岩比周圍的地殼更容易上浮，當岩漿不比周遭環境更容易上浮時便會達到浮力平衡，因此停滯下來而儲存在地殼中。
Rasmussen表示這項研究的下個目標是探討他們的發現是否也能套用到其他地質背景的火山，比方說夏威夷群島等熱點火山，或是東非的裂谷火山。Rasmussen說除了將這項研究加以延伸，還有一個更大的謎題有待解決：「如果岩漿儲存深度是由含水量控制，那麼含水量又是由什麼來控制呢？」
研究經費來自史密森尼學會、美國國家科學基金會、華盛頓西南社區基金會以及美國地質調查局。
Study finds that water determines
magma depth, a key to accurate models of volcanic activity, eruption
Around the world, between 40 and 50
volcanoes are currently erupting or in states of unrest, and hundreds of
millions of people are at risk of hazards posed by these potentially active
volcanos. Yet, despite the profound hazards posed to human life and property by
volcanic eruptions, humanity still cannot reliably and accurately predict them,
and even when forecasts are accurately made by experts, they may not afford
ample time for people to evacuate and make emergency preparations.
Accurate and reliable predictions have remained an
elusive target largely because volcanologists do not fully understand the
natural dynamics and processes of the magma underneath a volcano before it
finds its way to the surface. Now, the results of a new study led by
volcanologist Dan Rasmussen, a Peter Buck Fellow at the Smithsonian’s National
Museum of Natural History, may bring experts one step closer to accurately
forecasting volcanic eruptions.
The study, published today, March 10, in the journal Science, finds that, for the world’s
most common type of volcano, magma with higher water content tends to be stored
deeper in the Earth’s crust. The finding identifies what some scientists expect
is the most important factor controlling the depth at which magma is stored.
“This study connects the depth at which magma is
stored to water, which is significant because water largely initiates and fuels
eruptions,” Rasmussen said. He explained that water drives eruptions
analogously to how carbon dioxide can make a shaken-up soda bottle explode.
“With water dissolved in magma that is stored beneath
a volcano, if there is a sudden decrease in pressure, like when a shaken soda
bottle cap is suddenly opened, gas bubbles form and those cause the magma to
rise and jet out the volcano, similar to when a soda shoots out of a bottle
top,” Rasmussen said. “More water content in magma means more gas bubbles and
potentially a more violent eruption.”
“These results move us closer to understanding the
physics and conditions of magma storage beneath volcanoes, and that is an
essential ingredient for the kinds of detailed physics-based models necessary
to more accurately forecast eruptions,” Rasmussen said.
The study was completed through new field work and
lab analyses in addition to reanalysis of existing data collected from past
volcanic eruptions tracked by the Smithsonian’s Global Volcanism Program.
Rasmussen began his research in 2015 while completing
his doctorate at Columbia University’s Lamont-Doherty Earth Observatory with
his advisor, volcanologist Terry Plank, who suggested he pursue the still-open
question of why magma storage depth varies from one volcano to the next and
what controls that depth.
Along with a team that included geophysicist Diana
Roman of the Carnegie Institution for Science, Rasmussen went into the field to
collect volcanic material from eight volcanoes located in the rugged and remote
Aleutian Islands of Alaska.
The researchers focused on a particular geological
setting when selecting volcanoes for this study: so-called arc volcanoes that
occur at the intersection of two converging tectonic plates. Arc volcanoes,
like those found in the Aleutians, are the most numerous type of volcano on
Earth and comprise the entirety of the infamous “Ring of Fire” encircling the
Pacific Plate, making them the most obvious target for improving predictive
capacities.
Using ships and helicopters, the team collected bits
of volcanic ash from these eight volcanoes amid rough seas and, on the island
of Unimak, the threat of giant brown bears. Volcanic ash was the primary target
of the expedition because it can contain green crystals made of olivine—each
one with a diameter of about 1 millimeter, about the thickness of a plastic ID
card.
Underground, these olivine crystals sometimes trap
tiny bits of magma when they form. After an eruption sends these special
olivine crystals to Earth’s surface, the magma inside them cools and becomes
glass. By analyzing the chemical composition of these miniscule pieces of
cooled magma from the inside of a volcano, the researchers were able to
estimate the magma’s water content.
After estimating the water content from the entrapped
pieces of magma collected from six of the eight Aleutian volcanoes, the team
then combined those data with other estimates of magmatic water content taken
from the scientific literature for an additional 56 volcanoes from around the
world. The final list of estimated magmatic water content spanned 3,856
individual samples from 62 volcanoes.
To examine the relationship between the estimated
water content of these magma reservoirs and their respective storage depths,
the researchers scoured the scientific literature and created an accompanying
list of 331 depth estimates for 112 volcanoes.
Rasmussen said the Smithsonian’s Global Volcanism
Program’s database “was key in compiling these lists because it’s a really good
resource for eruption history, and we only wanted to consider volcanoes that
had recently erupted.” Rasmussen and the research team focused on recent
eruptions because magma reservoirs do not appear to move a lot following an
eruption, and so any estimates of depth or water content that were made using
recently erupted material have the highest likelihood of accurately reflecting
the current state of the volcano’s magma reservoir.
After years of field work, geochemical analysis and
literature review, the team was able to plot the estimated magma storage depths
for 28 volcanoes from around the world against their respective estimated
magmatic water contents. The results were strikingly clear: a magma reservoir’s
water content strongly correlated with its storage depth. In other words,
magmas that contained more water tended to be stored deeper in the Earth’s
crust.
The study also shows that a magma’s water content is
responsible for controlling its depth, rather than merely correlating to it.
The team showed this causal relationship by detecting the presence of chemical
tracers associated with the formation of water-containing magmas in Earth’s
mantle.
“If storage depth determined water content in magma,
it could still create the correlation between water content and depth that we
observed, but it wouldn’t produce the chemical tracers of the magma’s initial
water content that we found,” Rasmussen said.
As for how water content might determine magma
storage depth, Rasmussen and his co-authors argue that it has to do with a
process known as degassing in which the water mixed in with the magma forms
bubbles of gas. When magma rising through the Earth’s crust begins to degas, it
becomes more viscous, which the researchers suggest causes the magma’s ascent
to slow and stall.
The evidence that water content largely controls
magma storage depth overturns the most widely accepted explanation in the field
today, which contends that magma rises through cracks in Earth’s crust because
the molten rock is more buoyant than the surrounding crust, settling at its
storage depth because it reaches neutral buoyancy where magma is no more
buoyant than its surroundings.
Rasmussen said the next step for this research is to
see if these findings hold for volcanoes in other geologic settings such as
hot-spot volcanoes like the Hawaiian Islands or rift volcanoes like those in
East Africa. Beyond this extension of the research, Rasmussen said an even
larger question looms: “If magma water content controls magma storage depth,
what controls magma water content?”
Funding and support for this research were provided
by the Smithsonian, the National Science Foundation, the Community Foundation
for Southwest Washington and the U.S. Geological Survey.
原始論文：Daniel J.
Rasmussen, Terry A. Plank, Diana C. Roman, Mindy M. Zimmer. Magmatic
water content controls the pre-eruptive depth of arc magmas. Science,
2022; 375 (6585): 1169 DOI: 10.1126/science.abm5174
引用自：Smithsonian. "Water determines magma
depth, a key to accurate models of volcanic activity, eruption."
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