超級火山爆發後的餘波顯示出多巴火山的岩漿系統有多麼龐大

原文網址：www.sciencedaily.com/releases/2017/05/170516143417.htm

超級火山爆發後的餘波顯示出多巴火山的岩漿系統有多麼龐大

超級火山久久一次但十分宏偉的噴發，會造成巨大的破壞並在全球尺度上影響氣候模式長達數十年。新研究顯示這些地點在事件之後的數萬年間，儘管規模小上許多，但火山噴發仍會持續發生。

事實上，美國奧勒岡州立大學的研究人員可以將最近發生於蘇門答臘北邊的錫納朋火山噴發，跟距其25英里處，多巴(Toba)火山臼發生於74000年前的最近一次超級火山噴發畫上關聯。

這項研究本周發表在期刊《自然通訊》(Nature Communications)。

此研究的主要作者，奧勒岡州立大學的博士候選人Adonara Mucek表示：「超級火山噴發之後，火山和岩漿系統會試著重回平衡狀態――就像是一池水被丟入一顆石頭而受到擾動之後，會再次回到平靜一樣――過程是很漫長的。」

「多巴火山至少超級噴發的15000年至20000年過後，似乎都還有噴發活動持續進行，而構造適應(structural adjustment)則持續到至少數百年之前――有可能今日仍在發生。這等同於發生在岩漿系統中，一場地震過後的餘震一般。」

這是科學家首度可以精確指出在超級火山噴後之後發生了什麼事情。若要被定義為是座超級火山，則噴發規模必須達到火山爆發指數(Volcano Explosivity Index)的規模8以上，意謂噴發出來的物質體積經測量後要超過1000立方公里，也就是240立方英里。

多巴火山噴發時釋出的岩漿體積，是美國1980年代的聖海倫火山噴發的28000倍以上。由於其規模實在過於巨大，科學家認為它使地球形成了長達數年的火山冬天，而可能促成了人類演化史上的瓶頸效應(bottleneck)。

其他著名的超級火山所在位置包括了美國的黃石公園、紐西蘭的陶波火山臼以及義大利的坎皮佛萊格瑞。

此篇研究的共同作者，奧勒岡州立大學的火山學家Shanaka "Shan" de Silva表示：「超級火山的生命週期長達數百萬年，在其一生中可能會發生數次超級噴發事件。但在這些事件之間，火山並沒有死亡。科學家長久以來便猜測在最初的噴發過後，還是會有噴發活動持續發生，但此研究是我們首度能精確定出這些噴發事件的年代。」

研究人員表示雖然過往的氬定年法研究給出了多巴火山噴發的粗略年代，但這些定年結果的誤差範圍實在太大。在他們的研究中，奧勒岡大學的研究人員和澳洲、德國、美國與印尼的合作學者，量測了噴發出來的浮石與熔岩中，鋯石晶體含有的氦有多少，而可以解讀出多巴火山最近這段時期的活動歷史。

礦物晶體中的氦是鈾衰變過程中留下的產物，科學家對於鈾的放射性衰變途徑以及半衰期已有相當了解。

Mucek表示：「多巴火山的年紀至少有130萬年，而它的超級噴發事件發生在74000年前，在此之後，至少可以確定有六起噴發事件――實際上可能還有數起。我們找到的最近一次噴發事件大約發生於56000年前，但還有其他起噴發事件等著我們去研究。」

研究人員也利用了自形成之後，已經抬升600公尺的湖泊中的沉積物，對其進行碳14定年，而成功估算出多巴火山構造適應的歷史。數據顯示構造適應至少從3萬年前開始，進行到至少2000年前――或許今日仍持續發生。

錫納朋火山是座目前仍在噴發的火山，且跟其他蘇門答臘的火山相比顯得相當特殊。研究同時發現錫納朋火山和多巴火山系統的化學指紋和鋯石結晶歷程一模一樣。de Silva強調，這代表多巴火山系統或許比之前認為的還要大，且分布範圍更廣。

他說：「我們的數據指出錫納朋火山近期且仍在持續的噴發，是多巴火山系統從超級噴發事件回復時的部分過程。」

研究人員強調發現兩者之間有關聯，並非代表多巴火山臼正處於短期之內隨時都要發生毀滅性噴發的危急狀態。de Silva表示：「對於正處於復原過程中的超級火山來說，這可能是相當『稀鬆平常的』。」但研究人員指出，這也強調出對該處的地面抬升現象，以及岩漿系統的影像進行更頻繁且更精密的監測是很重要的。

de Silva表示：「超級火山造成的危害不會在最初噴發後就停止下來。它在接下來的數萬年間，可能會轉而繼續對附近地區規律地造成區域性災害，像是火山噴發、地震、山崩和海嘯。」

「多巴火山至今仍活著且相當活躍。」

研究人員表示雖然多巴火山噴發的規模已經相當龐大，但火山臼之下的岩漿庫還要大上非常、非常多。從地球上其他火山臼，像是黃石公園的研究中，科學家估計超級火山噴發時，火山臼的體積是噴發出來岩漿量的10到50倍之間。

Aftermath of supereruption shows Toba magma system's great size

The rare but spectacular eruptions of supervolcanoes can cause massive destruction and affect climate patterns on a global scale for decades -- and a new study has found that these sites also may experience ongoing, albeit smaller eruptions for tens of thousands of years after.

In fact, Oregon State University researchers were able to link recent eruptions at Mt. Sinabung in northern Sumatra to the last eruption on Earth of a supervolcano 74,000 years ago at the Toba Caldera some 25 miles away.

The findings are being reported this week in the journal Nature Communications.

"The recovery from a supervolcanic eruption is a long process, as the volcano and the magmatic system try to re-establish equilibrium -- like a body of water that has been disrupted by a rock being dropped into it," said Adonara Mucek, an Oregon State doctoral candidate and lead author on the study.

"At Toba, it appears that the eruptions continued for at least 15,000 to 20,000 years after the supereruption and the structural adjustment continued at least until a few centuries ago -- and probably is continuing today. It is the magmatic equivalent to aftershocks following an earthquake."

This is the first time that scientists have been able to pinpoint what happens following the eruption of a supervolcano. To qualify as a supervolcano, the eruption must reach at least magnitude 8 on the Volcano Explosivity Index, which means the measured deposits for that eruption are greater than 1,000 cubic kilometers, or 240 cubic miles.

When Toba erupted, it emitted a volume of magma 28,000 times greater than that of the 1980 eruption of Mount St. Helens in Washington state. It was so massive, it is thought to have created a volcanic winter on Earth lasting years, and possibly triggering a bottleneck in human evolution.

Other well-known supervolcano sites include Yellowstone Park in the United States, Taupo Caldera in New Zealand, and Campi Flegrei in Italy.

"Supervolcanoes have lifetimes of millions of years during which there can be several supereruptions," said Shanaka "Shan" de Silva, an Oregon State University volcanologist and co-author on the study. "Between those eruptions, they don't die. Scientists have long suspected that eruptions continue after the initial eruption, but this is the first time we've been able to put accurate ages with those eruptions."

Previous argon dating studies had provided rough ages of eruptions at Toba, but those eruption dates had too much range of error, the researchers say. In their study, the OSU researchers and their colleagues from Australia, Germany, the United States and Indonesia were able to decipher the most recent volcanic history of Toba by measuring the amount of helium remaining in zircon crystals in erupted pumice and lava.

The helium remaining in the crystals is a remnant of the decaying process of uranium, which has a well-understood radioactive decay path and half-life.

"Toba is at least 1.3 million years old, its supereruption took place about 74,000 years ago, and it had at least six definitive eruptions after that -- and probably several more," Mucek said. "The last eruption we have detected occurred about 56,000 years ago, but there are other eruptions that remain to be studied."

The researchers also managed to estimate the history of structural adjustment at Toba using carbon-14 dating of lake sediment that has been uplifted up to 600 meters above the lake in which they formed. These data show that structural adjustment continued from at least 30,000 years ago until 2,000 years ago -- and may be continuing today.

The study also found that the magma in Toba's system has an identical chemical fingerprint and zircon crystallization history to Mt. Sinabung, which is currently erupting and is distinct from other volcanoes in Sumatra. This suggests that the Toba system may be larger and more widespread than previously thought, de Silva noted.

"Our data suggest that the recent and ongoing eruptions of Mt. Sinabung are part of the Toba system's recovery process from the supereruption," he said.

The discovery of the connection does not suggest that the Toba Caldera is in danger of erupting on a catastrophic scale any time soon, the researchers emphasized. "This is probably 'business as usual' for a recovering supervolcano," de Silva said. It does emphasize the importance of having more sophisticated and frequent monitoring of the site to measure the uplift of the ground and image the magma system, the researchers note.

"The hazards from a supervolcano don't stop after the initial eruption," de Silva said. "They change to more local and regional hazards from eruptions, earthquakes, landslides and tsunamis that may continue regularly for several tens of thousands of years.

"Toba remains alive and active today."

As large as the Toba eruption was, the reservoir of magma below the caldera is much, much greater, the researchers say. Studies at other calderas around Earth, such as Yellowstone, have estimated that there is between 10 and 50 times as much magma than is erupted during a supereruption.

原始論文：Adonara E. Mucek, Martin Danišík, Shanaka L. de Silva, Axel K. Schmitt, Indyo Pratomo, Matthew A. Coble. Post-supereruption recovery at Toba Caldera. Nature Communications, 2017; 8: 15248 DOI: 10.1038/ncomms15248

引用自：Oregon State University. "Aftermath of supereruption shows Toba magma system's great size." ScienceDaily. ScienceDaily, 16 May 2017.