東加火山爆發

 原文網址：https://www.news.ucsb.edu/2022/020589/tonga-eruption

加州大學聖塔芭芭拉分校的地質學家主持的研究成果，描述了東加火山劇烈的噴發過程

By Harrison Tasoff

1月15日，洪加湯君—洪加哈派(Hunga Tonga-Hunga Ha’apai)火山對東加造成了重大災情。噴發造成的海嘯遠到加勒比海都可以感受到，產生的大氣波則繞行了地球好幾圈。同時，往上竄升的火山煙流則讓氣體和火山灰一路穿過平流層，直達中氣層底部。

火山閃電劃過了洪加湯君—洪加哈派的上空，這座火山在2022年1月15日發生了一場特別猛烈的噴發。圖片來源：東加地質局

噴發結束不到二個月之後，地質學家便對這次事件是如何引發整理出一套初步的解釋。加州大學聖塔芭芭拉分校的Melissa Scruggs與名譽教授Frank Spera是這組國際研究團隊的成員，他們發表在期刊《地震研究進展》(Earthquake Research Advances)的論文對此事件首次做出了全面的論述。作者認為當天噴發如此劇烈的緣由為前一天發生的噴發，其造成主要的火山口沉到海面之下，熔岩因此可以蒸發大量的海水，使得隔天的火山爆發變得更加劇烈。

「無庸置疑地，這肯定是1991年皮納圖博火山之後最大的火山爆發，」通訊作者Scruggs表示。她最近剛從加州大學聖塔芭芭拉分校完成博士學位，研究領域是岩漿混和作用和引發火山噴發的機制。Scruggs將一月的這場事件和位在3000英里之外，1883年爆發的喀拉喀托火山比較。

洪加湯君—洪加哈派(HTHH)是座層狀火山，這種大型的錐狀火山具有週期性劇烈噴發的傾向，但通常只有溫和的活動。HTHH是湯加火山島弧的一部份，這條火山鍊的岩漿來源為隱沒至印澳板塊下方的太平洋板塊。當板塊下沉，岩石會受到熱和壓力的作用而被逼出水分和其他揮發性物質。這些水分會讓上方的岩石熔點下降，使得距離板塊邊界約100公里處出現排成鍊狀的火山。

水下的威脅

洪加湯君島與洪加哈派島(兩座火山的名稱來源)是這個火山臼邊緣僅有的兩個最高點――曾經如此，直到火山爆發將兩座島的大部分地方炸到天空之前。

Scruggs首次得知東加發生火山爆發是在睡前滑推特的動態消息。「我看到一張火山爆發衛星影像的GIF，讓我的心跳幾乎停止，」她停頓了一下，思考該怎麼說。她立刻明白這次事件勢必會造成重大災害，「最可怕的部分是整個東加都失去聯繫了，我們完全不知道發生了什麼事情。」

事件還沒落幕的時候她便通知了其他火山學家，試著去理解衛星清楚捕捉到的影像。「我們立刻展開行動來試著瞭解發生了什麼事，」Scruggs表示，「我們蒐集了我們能取得的所有資訊，在最初這幾週可以拿到的都不放過。」作者依據他們能找到的各類資源，包括了公開取用的數據、影片甚至是推特，很快地對這場爆發做出了敘述。

團隊透過許多資料庫計算出1月15日的事件發生在當地時間下午5:02 (0402 ±1 UTC)。美國地質調查局大約13分鐘後記錄到來自火山口所在地點的震波事件。噴發的前兩個小時特別劇烈，並在約12個小時之後逐漸平息下來。

但實際上，噴發活動的起源可以一路追溯至2021年12月20日。而在更早之前，火山在2009年以及2014、2015年便有噴發過的紀錄。Scruggs相信那些較早的活動是瞭解HTHH最近這次噴發為什麼如此劇烈的關鍵，可能和地下深處的熔岩通道系統改變，或是熔岩的化學性質隨時間變化有關。

洪加湯君和洪加哈派過往是兩座分開的島嶼，之後主火山口發生噴發，創造出陸橋才連結在一起。「這座島嶼2015年才誕生，」Scruggs說，「但現在已經消失了。若非現在是衛星時代，我們可能永遠不會知道它存在過。」

2022年1月14日，主火山口噴發徹底摧毀了陸橋，造成火山口沉到海面下方。「如果這座陸橋還在，1月15日的噴發情形或許就跟前一天的差不多，因為不會有大量的海水參與其中，」Scruggs強調。

驚天動地的爆發

同樣的火山，一天的差別：星期五還露出水面的火山口，到了星期六卻沉在水裡。「這讓事情整個不一樣了，」Scruggs表示。

上方：洪加湯君和洪加哈派過往是分開的島嶼，在七年之間逐漸變大然後連在一起。下方：2022年1月14日的噴發使得主要火山口沉到海中，造成隔天的噴發讓整個島嶼幾乎消失。日期：2021.11.16、2022.01.07、2022.01.15、2022.01.18。圖片來源：PLANET LABS PBC

團隊相信1月15日的噴發力道如此猛烈有很大一部份是因為海水。就跟玩具水火箭一樣，此等規模的噴發需要比例正確的水和氣體，才能提供衝向天空的力量。

而且它確實就像火箭一樣。「噴發一路衝到前往太空的半途，」Scruggs的語氣充滿驚訝。火山灰柱衝到大氣58公里高的地方，貫穿平流層而到達中氣層底部，比1980年聖海倫火山的火山灰柱到達的高度還多出一倍。這次事件也因此成為紀錄中最高的火山煙流。

噴發也伴隨著數目極為驚人的閃電。作者猜測蒸發的海水造成岩漿碎裂成微小的火山灰微粒，加上蒸氣到達高層大氣之後會凝結成極小的冰晶。這些顆粒的運動、溫度變化與大小產生了極為大量的靜電荷分離區域，使得噴發地點上空出現閃電。在噴發的最初兩個小時，地球上的雷擊大約有80%劃過了洪加湯君—洪加哈派的天空。

作者預估1月15號HTHH噴發出的物質體積大約為1.9立方公里，重量為2900兆克。「噴發物質的體積其實並不重要，」論文共同作者Spera表示，他也是Scruggs的指導教授。「這場噴發的能量與海洋和大氣的連動才是特別之處：以全球尺度來看有許多能量傳輸到大氣與海水當中。」

行經海水的衝擊波引發的海嘯傳遍了整個太平洋，甚至到達更遠的地方。不只這樣，波浪抵達的時間比海嘯預警模型預測的還要更快，因為這些模型並不是用來測定火山爆發――它們根據的方程式是用來描述地震產生的海嘯。

第二波海嘯則隨著大氣壓力波發生，甚至在跟南太平洋沒有直接連結的加勒比海，也出現了衝擊波引發的氣象海嘯。Scruggs表示這是前所未見的，「基本上整個海洋噴發五天之後都處在餘波盪漾的狀態。」

滿滿的後續工作

科學家仍在拼湊火山當時發生了什麼事，因此他們還無法完全理解海嘯波。然而，更新海嘯傳播的預警系統仍是一項非常重要的任務，如此才能將這種機制囊括進去，不然下次發生類似HTHH的火山噴發時，預警系統可能會出現錯誤，使得許多人喪生。

這場事件確實顯現了未受監視的海底火山造成的危害。雖然1月15日的噴發造成了重創，東加人民的應對還算不錯。政府根據火山前一天的活動預先發布了警報，而且該國對於火山噴發與海嘯早就有所計畫。

HTHH在過去也經歷過類似的劇烈噴發。紐西蘭奧塔哥大學的研究人員最近發表的論文發現大約1000年前，一場大型噴發摧毀了這座海底火山頂部的火山臼。而相似的火山可能會以類似的方式噴發。比方說另一座位在格瑞納達島北方僅8公里，主噴發口距離水面只有150公尺的海底火山基克姆詹尼(Kick ‘em Jenny)。「想像類似東加火山爆發的事件發生在加勒比海會怎麼樣，」Scruggs表示。

研究人員僅用目前公開的資料來快速完成這項研究。他們計畫之後有更多資訊及樣本可供使用，而且其他研究人員對這場驚天動地的事件發表更多結果之後，再回頭審視目前得出的所有發現。他們的主要目標是讓未來有關這項主題的研究有個出發點。

Scruggs特別想要探討採自這次噴發的火山灰。對於訓練有素的地質學家來說，火山岩可以提供相當豐富的資訊。檢視這些物質可以闡明噴發的岩漿種類、有多少岩漿，甚至可以知道噴發過程中有多少海水參與在內。

「有這麼多問題被提了出來，」Scruggs表示，「或許連我們認為不可能保存的事物都已經有所記錄。」

在10月丹佛舉辦的美國地質學會2022年度大會中，加州大學聖塔芭芭拉分校將主持洪加湯君—洪加哈派噴發的特別受邀議程。「可以看到其他地球科學家對於這座特別的火山有何發現一定很令人興奮，」Spera表示，「我們的研究不過是開端而已。」

 

Tonga Eruption

UC Santa Barbara geologists lead the effort to describe the devastating eruption in Tonga

On January 15, the volcano Hunga Tonga-Hunga Ha’apai devastated the nation of Tonga. The eruption triggered tsunamis as far afield as the Caribbean and generated atmospheric waves that travelled around the globe several times. Meanwhile, the volcano’s plume shot gas and ash through the stratosphere into the lower mesosphere.

Just two months after the eruption, geologists have put together a preliminary account of how it unfolded. UC Santa Barbara’s Melissa Scruggs and emeritus Professor Frank Spera were part of an international team of researchers that published the first holistic account of the event in the journal Earthquake Research Advances. The authors think that an eruption the day before may have primed the volcano for the violent explosion by sinking its main vent below the ocean’s surface. This enabled molten rock to vaporize a large volume of seawater, intensifying the volcanic eruption the very next day.

“This is definitely, without a doubt, the largest eruption since Mt. Pinatubo in 1991,” said corresponding author Scruggs, who studies magma mixing and eruption triggering mechanisms, and recently completed her doctorate at UC Santa Barbara. She compared January’s event to the 1883 eruption of Krakatoa, which was heard 3,000 miles away.

Hunga Tonga-Hunga Ha’apai (HTHH) is a stratovolcano: a large, cone-shaped mountain that is prone to periodic violent eruptions, but which usually experiences milder activity. It’s one of many along the Tofua Volcanic Arc, a line of volcanoes fed by magma from the Pacific Plate diving beneath the Indo-Australian Plate. Heat and pressure cook the rocks of the descending plate, driving out water and other volatiles. That same water decreases the melting temperature of the rock above, leading to a chain of volcanoes about 100 kilometers from the plate boundary.

A submerged danger

The islands of Hunga Tonga and Hunga Ha’apai — after which the volcano is named — are merely the two highest points along the rim of the caldera, or central crater. Or they were, until the eruption blew most of the islands sky high.

Scruggs first heard about the eruption as she scrolled through her Twitter feed while getting ready for bed. “I saw a GIF of the satellite eruption, and my heart just stopped,” she said, pausing to find her words. She immediately knew that the event would cause massive devastation. “The scariest part was that the entire country was cut off, and we didn’t know what had happened.”

She was already messaging other volcanologists as the events unfolded, trying to understand the images that satellites had so clearly captured. “We really just set out to try to understand what happened,” Scruggs said. “So, we gathered all the information that we could, anything that was available within the first few weeks.” The authors drew on whatever resources they could find to quickly characterize this eruption, including publicly available data, videos and even tweets.

Using a variety of data sets, the team calculated that the January 15 event began at 5:02 p.m. local time (0402 ±1 UTC). The U.S. Geological Survey recorded a seismic event around 13 minutes later at the vent location. The first two hours of the eruption were particularly violent, with activity fading after about 12 hours.

But eruption activity had actually started all the way back on December 20, 2021. And before that, the volcano had erupted in 2009 and again in 2014 and 2015. Scruggs believes these earlier episodes are key to understanding the violence behind HTHH’s recent eruption, perhaps related to changes in the magma plumbing system at depth or the chemistry of the magma over time.

Hunga Tonga and Hunga Ha’apai used to be separate islands until they were united by eruptions from the volcano’s main vent, which created a land bridge. “This island was just born in 2015,” said Scruggs. “And now it’s gone. Were it not for the satellite era, we would not have even known it ever existed.”

On January 14, 2022 an explosion from the main vent razed this connection, sinking the vent beneath the ocean’s surface. “Had that land bridge not been taken out, the January 15 eruption might have behaved just like the day before because it would not have had that excess seawater,” Scruggs remarked.

A staggering explosion

Same volcano, one day’s difference: On Friday the vent was above the water, and by Saturday it was below. “That made all of the difference in the world,” Scruggs said.

The team believes that the seawater played a large part in the violence and force behind the Jan. 15 eruption. Much like a bottle rocket, an eruption of this scale takes the right ratio of water and gas to provide the force to send it skyward.

And it took off like a rocket, too. “It went halfway to space,” Scruggs exclaimed. The ash plume shot 58 kilometers into the atmosphere, past the stratosphere and into the lower mesosphere. This is more than twice the height reached by the plume from Mt. Saint Helens in 1980. It was the tallest volcanic plume ever recorded.

A truly staggering amount of lightning also accompanied the eruption. The authors suspect that vaporizing seawater caused the lava to fragment into microscopic ash particles, which were joined by tiny ice crystals once the steam froze in the upper atmosphere. The motion, temperature change and size of the particles generated incredible amounts of static charge separation that flashed above the eruption. For the first two hours of the eruption, about 80% of all lightning strikes on Earth split the sky above Hunga Tonga-Hunga Ha’apai.

The authors estimate around 1.9 km³ of material, weighing 2,900 teragrams, erupted from HTHH on Jan. 15. “But the volume of the eruption was not the big deal,” said Spera, a coauthor on the paper and Scruggs’ doctoral advisor. “What was special is how the energy of the eruption coupled to the atmosphere and oceans: A lot of the energy went into moving air and water on a global scale.”

The shockwave traveling through the ocean triggered tsunamis throughout the Pacific, and beyond. What’s more, the wave arrived faster than tsunami warning models predicted because the models aren’t calibrated for volcanic eruptions — they're based on equations that describe tsunamis generated by earthquakes.

A second tsunami followed the atmospheric pressure wave. This shockwave even triggered a meteo-tsunami in the Caribbean, which has no direct connection to the South Pacific. Scruggs called it unprecedented: “Basically the whole ocean just kind of sloshed around for five days after the eruption,” she added.

Plenty of work to do

Scientists are still piecing together what happened at the volcano, so they have yet to develop a complete understanding of the tsunami wave. However, it’s an important task needed to update tsunami travel forecast systems so they account for this type of mechanism. Otherwise, warnings could be incorrect the next time a volcano like HTHH erupts, potentially costing more lives.

Indeed, the event highlights the danger posed by unmonitored submarine volcanoes. Despite the devastation, the people of Tonga were relatively well prepared for the Jan. 15 eruption. The government had issued warnings based on the previous day’s activity, and the nation had plans in place for eruptions and tsunamis.

HTHH has experienced similarly violent eruptions in the past. A recent paper by researchers at the University of Otago, New Zealand revealed that a large eruption destroyed the caldera at the summit of the undersea volcano about 1,000 years ago. And similar volcanoes could well erupt in the same manner. Consider Kick ‘em Jenny, another submarine volcano whose main vent is a mere 150 meters underwater. It’s located just 8 km north of the island of Grenada. “Imagine if something like the Tonga eruption happened in the Caribbean,” Scruggs said.

The researchers worked quickly with only publicly available data. They plan to revisit all their findings as more information and samples become available and as more researchers publish their own findings on this groundbreaking eruption. Their primary goal was to provide a point of departure for future work on the topic.

Scruggs is particularly keen on learning about the ash collected from this eruption. Volcanic rock provides a wealth of information to a trained geologist. Examining the material could shed light on the type of magma that erupted, how much of it there was and perhaps even how much seawater was involved in the eruption.

“There’s so many questions that have been raised,” said Scruggs. “Things we didn’t even think were possible have now been recorded.”

The UC Santa Barbara researchers will lead a special invited session on the Hunga Tonga-Hunga Ha’apai eruption at the Geological Society of America’s 2022 annual meeting in Denver this October. “It will be exciting to see what scores of other earth scientists can discover about this unique volcano,” Spera said. “We are just at the beginning.”

原始論文：David A. Yuen, Melissa A. Scruggs, Frank J. Spera, Yingcai Zheng, Hao Hu, Stephen R. McNutt, Glenn Thompson, Kyle Mandli, Barry R. Keller, Songqiao Shawn Wei, Zhigang Peng, Zili Zhou, Francesco Mulargia, Yuichiro Tanioka. Under the Surface: Pressure-Induced Planetary-Scale Waves, Volcanic Lightning, and Gaseous Clouds Caused by the Submarine Eruption of Hunga Tonga-Hunga Ha’apai Volcano Provide an Excellent Research Opportunity. Earthquake Research Advances, 2022; 100134 DOI: 10.1016/j.eqrea.2022.100134

引用自：University of California - Santa Barbara. “Tonga Eruption.”