火山礦物晶體給出了關於岩漿的新觀點

原文網址：www.sciencedaily.com/releases/2017/06/170615142757.htm

火山礦物晶體給出了關於岩漿的新觀點

根據6月16日發表於期刊《科學》(Science)的新研究，火山學家對活火山之下的岩漿庫中正在進行的活動有了新認識，他們發現此處比過往認為得還要冷，也更為凝滯。這是對於火山如何運作的新見解，或許之後可以幫助火山學家更加清楚得知，火山會於什麼時候造成重大危害。

本篇文章的通訊作者Kari Cooper表示：「我們對於岩漿庫樣貌的觀念必須要做出改變。」他是加州大學戴維斯分校的地球科學和物理學教授。

直接研究岩漿是很困難的。即使在有火山活動的地方，岩漿也是位於地表以下數公里處。雖然地質學家有時候能在偶然或是刻意為之的情況下鑽探到岩漿所在的地方，但此處的高溫高壓可以摧毀科學家試著想放進去的任何儀器。

Cooper和她的同僚轉而去蒐集紐西蘭塔拉威拉山周圍，700年前一場火山噴發所堆積的碎屑中的鋯石晶體。這場噴發的規模大約是1980年聖海倫火山的5倍，它帶到地表的岩漿曾經待在岩漿庫裡，而記錄了其內的溫度與化學性質。一旦岩漿噴發到地表，這些過去的紀錄就會在原地保存下來。

Cooper表示其中的晶體就像飛航記錄器，也就是「黑盒子」一樣，可以用來研究火山爆發。「我們不用試著去將殘骸拼湊起來，這些晶體就可以告訴我們它們還在地底時，包括噴發上升到地表的過程中經歷了什麼事件。」

經由研究七顆鋯石晶體成分中的稀有元素，他們可以得出晶體最初形成的年代，以及晶體待在岩漿庫的歲月中，有多久處在高溫(超過700℃)環境下。科學家從晶體提供的資訊可以瞭解岩漿庫中晶體所處那一部份的狀態。

研究人員發現除了一顆之外，其他六顆晶體的年代至少都有數萬年之久，但它們的一生中僅有少部分時間(大約少於百分之四)處在融化的岩漿裡面。

比較像是雪堆而非熔岩湖

Cooper表示研究得出的整體圖像顯示岩漿庫不像是一團滾沸的熔岩，比較像是一座雪堆：大多是固體或晶體，並有少量的液體在內部流動。

要讓噴發形成，需要有一定量呈現固體或結晶態的岩漿融化並流動，可以藉由跟岩漿庫中儲存在它處的高溫液體作用而達成。噴發前的岩漿可能會到處吸收岩漿庫中不同部分的物質，這會在相當短的地質時間――數十至數百年內發生。意味著我們或許可以找出岩漿最容易流動的地點，來判斷最有可能發生火山爆發的高風險區。

研究中用到的所有晶體是在700年前被較小型的噴發炸到地表，有趣的是，在前一次發生於25000年前的巨型噴發中，它們仍然是以未熔融的狀態待在塔拉威拉山的岩漿庫裡。顯示岩漿的流動過程必定牽涉到相當複雜的作用。

Jennifer Wade表示：「要瞭解火山爆發，我們必須要先解開噴發之前火山傳達給我們的訊息。」他是贊助此研究，國家科學基金會地球科學部門的計畫主任。「這項研究將時光回溯到噴發之前，利用晶體中保留的訊息來瞭解岩漿如何從安置在岩漿庫的狀態，轉變成開始流動而預備噴發。」

Volcanic crystals give a new view of magma

Volcanologists are gaining a new understanding of what's going on inside the magma reservoir that lies below an active volcano and they're finding a colder, more solid place than previously thought, according to new research published June 16 in the journal Science. It's a new view of how volcanoes work, and could eventually help volcanologists get a better idea of when a volcano poses the most risk.

"Our concept of what a magma reservoir looks like has to change," said Kari Cooper, professor of earth and physical sciences at the University of California, Davis and corresponding author on the paper.

It's hard to study magma directly. Even at volcanic sites, it lies miles beneath the Earth's surface and while geologists have occasionally drilled into magma by accident or design, heat and pressure destroy any instrument you could try to put into it.

Instead, Cooper and her colleagues collected zircon crystals from debris deposited around Mount Tarawera in New Zealand by an eruption about 700 years ago. That eruption, roughly five times the size of Mount St. Helens in 1980, brought lava to the surface that had resided in the reservoir, exposed to its temperature and chemistry. Once on the surface, that record of the past was frozen in place.

The crystals are like a "black box" flight recorder for studying volcanic eruptions, Cooper said. "Instead of trying to piece together the wreckage, the crystals can tell us what was going on while they were below the surface, including the run up to an eruption."

By studying trace components elements within seven zircon crystals, they could determine when the crystals first formed and how long during their life within the magma reservoir they were exposed to high heat (over 700 degrees Celsius). The crystals give information about the state of the part of the magma reservoir in which they resided.

The researchers found that all but one of the seven crystals were at least tens of thousands of years old, but had spent only a small percentage (less than about four percent) exposed to molten magma.

A Snow Cone Not a Molten Lake

The picture that emerges, Cooper said, is less a seething mass of molten rock than something like a snow cone: mostly solid and crystalline, with a little liquid seeping through it.

To create an eruption, a certain amount of that solid, crystalline magma has to melt and mobilize, possibly by interacting with hotter liquid stored elsewhere in the reservoir. The pre-eruption magma likely draws material from different parts of the reservoir, and it happens very quickly in geological time -- over decades to centuries. That implies that it may be possible to identify volcanoes at highest risk of eruption by looking for those where the magma is most mobile.

Interestingly, all the crystals studied had remained unmelted in Mount Tarawera's magma reservoir through a gigantic eruption that occurred about 25,000 years ago, before being blown out in the smaller eruption 700 years ago. That shows that magma mobilization must be a complex process.

"To understand volcanic eruptions, we need to be able to decipher signals the volcano gives us before it erupts," says Jennifer Wade, a program director in the National Science Foundation's Division of Earth Sciences, which funded the research. "This study backs up the clock to the time before an eruption, and uses signals in crystals to understand when magma goes from being stored to being mobilized for an eruption."

原始論文；Allison E. Rubin, Kari M. Cooper, Christy B. Till, Adam J. R. Kent, Fidel Costa, Maitrayee Bose, Darren Gravley, Chad Deering, Jim Cole. Report Rapid cooling and cold storage in a silicic magma reservoir recorded in individual crystals. Science, 16 Jun 2017 DOI: 10.1126/science.aam8720

引用自：University of California - Davis. "Volcanic crystals give a new view of magma." ScienceDaily. ScienceDaily, 15 June 2017.