礦物晶體幫助火山對付壓力

原文網址：https://news.uaf.edu/crystals-help-volcanoes-cope-with-pressure/

礦物晶體幫助火山對付壓力

Meghan Murphy

阿拉斯加大學費爾班克斯分校的研究人員發現火山透過一種獨門方法來處理壓力――礦物晶體。

根據發表在《地質學期刊》(Journal of Geology)的一篇新研究，由微小晶體組成的結構可以減少岩漿上升時累積的內部壓力，而降低火山噴發的猛烈程度。

礦物晶體可以在短短18分鐘之內形成於上升的熔岩當中。如果岩漿中有超過百分之二十是晶體，它們就能像護欄一般地將氣體疏導至火山內部的潛在裂隙，或者是通往地表的通道。

主要作者，阿拉斯加大學費爾班克斯分校的地質科學博士生Amanda Lindoo表示：「當氣體無法排出會造成壓力持續累積而造成麻煩，結果可能是發生相當猛烈的火山噴發並產生火山灰柱。不過礦物晶體可以減緩這種情形。」

共同作者，阿拉斯加大學費爾班克斯分校地球物理所的火山學家Jessica Larsen表示這項發現挑戰了普遍接受的預測中，認為岩漿的矽含量是影響氣體逸散的主要因素。

她說一項常見的經驗法則是矽含量相當高的岩漿流動速度比較慢，造成內部的氣體較難逸散。雖然科學家知道這種岩漿也傾向生成較少的礦物晶體，但她表示並沒有太多的研究將主題放在晶體在火山噴發中扮演的腳色。

阿留申群島、喀斯開山脈和中美洲的火山激起了Larsen的好奇心。這些地區的火山有些跟理論一致地具有高含量的矽，但有些火山的岩漿矽含量卻不高。

她說：「若依照經驗法則，岩漿矽含量不高的火山應該不會發生災難性的猛烈噴發。但它們的確發生了。我們想要知道是什麼使這項法則的預測有所偏離，因為瞭解火山爆發造成的災害是很重要的。」

為了研究礦物晶體，Lindoo和Larsen一同在地球物理所的實驗岩石學實驗室中，以高溫爐將火山岩加熱至1300℃來將它們重新融化成液態熔岩。另外實驗室中還有加壓幫浦、壓力管線和閥門。

Lindoo利用從阿留申群島噴出的火山產物來製造岩漿。她對岩漿施予極大的壓力來模擬地球內部的壓力，接著將壓力減少以模仿矽含量低的岩漿往上升的過程。

隨著岩漿上升，溶於其中的水會形成氣泡――就像打開加壓過的汽水瓶時會冒出氣泡一樣。於此同時，熔岩內部也會生成礦物晶體。Lindoo接著比較實驗室樣品跟火山噴發的產物，她發現在生成許多結晶的樣品中，晶體的連結模式可以將氣體疏導出去。

Larsen表示溫度、岩漿的水含量以及岩漿上升的速度都會影響礦物結晶的形成。

Larsen說：「近來對於礦物晶體的形成方式我們已經有所瞭解，但是晶體對氣體逸散有多麼深遠的影響我們卻一無所知。」

Larsen表示她會繼續進行研究，但下一階段是要探討不同形狀和大小的晶體對氣體的逸散方式會有什麼樣的影響。

共同研究人員為英國布里斯托大學的科學家。美國國家科學基金會資助了本項研究。

Crystals help volcanoes cope with pressure

University of Alaska Fairbanks researchers have discovered that volcanoes have a unique way of dealing with pressure — through crystals.

According to a new study published in the Journal of Geology, a network of microscopic crystals can lessen the internal pressure of rising magma and reduce the explosiveness of eruptions.

Crystals can form in the rising molten rock in as little as 18 minutes. If the magma becomes more than 20 percent crystals, they can act like guard rails that funnel gas to possible cracks within the volcano or to the opening at the Earth’s surface.

“The problem is when the gas can’t get out,” said Amanda Lindoo, lead author and UAF geosciences doctoral student. “That causes a buildup in pressure that can lead to the very explosive eruptions that shoot ash plumes. The crystals can alleviate that.”

Co-author Jessica Larsen, a volcanologist with the UAF Geophysical Institute, said the findings challenge the prevailing assumption that the amount of silica in magma is the major driver in gas escape.

The usual rule of thumb, she said, is that magmas with lots of silica are slow-moving, which can make it hard for gas to escape. While scientists know that these magmas tend to form fewer crystals, she said not much research has focused on the crystal’s role in eruptions.

Volcanoes in the Aleutian Islands, the Cascade Range and Central America aroused Larsen’s curiosity. Some volcanoes in those regions have magma consistently high in silica, while others have low-silica magma.

“If you follow the rule of thumb, then the volcanoes with low-silica magma shouldn’t produce hazardous, explosive eruptions,” she said. “And yet they do. We wanted to know what was swinging the pendulum, because it’s important to understanding the hazards of eruptions.”

To study the crystals, Lindoo worked with Larsen in the Geophysical Institute’s Experimental Petrology Lab, which has a furnace that can superheat volcanic rocks up to 2,400 F and melt them back into molten lava. It also has pressurizing pumps, pressure lines and valves.

Lindoo created magma from eruptive materials from the Aleutian Islands. She applied extreme pressure to the magma to simulate pressures in the Earth, but then reduced pressure to mimic the way low-silica magma rises.

As the magma “rose,” dissolved water formed into gas bubbles — much as bubbles form when opening a bottle of pressurized soda. Crystals also grew in the molten part. Lindoo then compared lab samples to those taken from volcanic explosions and found patterns of crystal networks channeling gas where crystal formation was high.

Larsen said temperature, the amount of water in the magma and the speed of the magma’s rise all play a role in crystal formation.

“For awhile we’ve understood how crystals form,” said Larsen. “But we didn’t know how profoundly the crystals influenced gas escape.”

Larsen said she will continue the research, but the next phase will look at how the different sizes and shapes of crystals influence gas escape.

The researchers collaborated with scientists at the University of Bristol in the United Kingdom. The National Science Foundation funded the study.

原始論文：A. Lindoo J.F. Larsen K.V. Cashman J. Oppenheimer. Crystal controls on permeability development and degassing in basaltic andesite magma. Journal of Geology, 2017 DOI: 10.1130/G39157.1

引用自：University of Alaska Fairbanks. "Crystals help volcanoes cope with pressure: Crystal networks in magma can act like guard rails to funnel gas out."