找到大塔穆火山如何形成的新線索
By Jeannie Kever
位在日本東方約1600公里處的大塔穆火山(Tamu
Massif)在2013年受到了媒體的大量關注，原因為研究人員宣稱它是紀錄中地球最大的單一火山，面積約有新墨西哥州的大小(約30萬平方公里)。
但在本周(7/8當周)發表於《自然―地球科學》(Nature Geoscience)的新發現，結論指出大塔穆火山所屬的火山類型跟過往認為的不同，使得研究人員質疑宣稱其為地球最大單一火山的說法。
地殼中帶有磁性的岩石會干擾地磁而造成磁力異常。這篇研究的人員分析大塔穆火山的磁場數據之後，發現它的磁力異常模式類似於板塊交界的中洋脊會形成的模式。
休士頓大學的地球物理學家William
Sager是本研究的資深作者。他說研究人員從這項發現得到的結論指出，形成大塔穆火山的作用是地質學家所稱的中洋脊「擴張」，也就是屬於板塊邊界的中洋脊形成海洋地殼的過程，而不是之前認為的盾狀火山。盾狀火山是最常見的火山類型之一，它們是由大量液態熔岩流堆積而成。
由美國、中國、日本的研究人員組成的國際研究團隊，目標是找出位於三條擴張洋脊連接處的大塔穆火山如何形成。他們表示答案的關鍵即為磁力異常。
中洋脊是兩個海洋板塊彼此分離而形成的板塊邊界，本身就是一系列巨大的火山。熔岩流和岩漿會匯聚在中洋脊軸部而形成新的地殼，其中含有的磁性礦物可以記下地磁極性的倒轉，造成中洋脊的磁性異常呈現出平行洋脊的特殊線性分布模式。
對大塔穆火山的新理解
之前已經發現大塔穆火山的周遭地區具有由三條洋脊形成的線性磁性異常，但研究人員不確定此模式是否延續至火山內部。在2013年由Sager和其同事發表的一篇論文中，結論認為大塔穆火山是一座盾狀火山，由山頂噴發出來的廣大熔岩流所形成。
最近這篇研究的作者搭乘施密特海洋研究所的研究船Falkor，以現代GPS導航系統進行定位，做出新的磁力剖面網格。以此網格為基礎，他們接著運用了54年來長達72000公里的航線中，蒐集到的460萬筆地磁數據，彙整出大塔穆火山整體的磁性異常地圖。結果顯示出大塔穆火山周圍的線性磁性異常可以和火山本身的線性磁性異常融為一體，代表它的形成過程是一種特殊的、中洋脊形成地殼的作用。
Sager表示這項發現的重要之處，在於其傳達出大塔穆火山和其他海底高原的形成過程，可能跟以往認為的不同。目前廣為接受的模型認為，這些大型火山是由一大團稱為「地函柱」的岩漿從地函上升，到達地表時形成的。陸地上和這種模式類似的大型噴發產物稱為「陸上洪流玄武岩」，它們是由一層一層的熔岩流往上堆積而成。
洋脊擴張假說形成的火山隨著年代增長則是往側向擴張：新的物質總是從洋脊的中心添加進來，之前形成的物質則往兩側推送出去。這項假說意謂大塔穆火山的緩坡並非反映了熔岩流的形狀，而是洋脊火山活動從旺盛逐漸衰弱，使得產生的地殼厚度跟著變薄所造成。
陸上洪流玄武岩和海底高原的噴發活動常常互相作為類比。Sager表示這項新發現顯示兩者之間的形成機制其實不同，使得這種類比並不恰當。
毫無疑問是世上最大的火山之一
Sager表示這項發現使得大塔穆火山不再是世界上最大的盾狀火山。這項頭銜回到夏威夷群島中的茂納羅亞火山。
「世界上最大的火山其實是中洋脊系統，它們就跟棒球上的縫線一樣環繞了全世界，總長將近65000公里。」Sager表示，「中洋脊是相當大的火山系統，而非單一一座火山。」
他說研究人員現在認為大塔穆火山是中洋脊系統的其中一部份。「大塔穆火山絕對是世上最大的火山之一。」
Sager表示2013年的論文是研究人員根據當時他們擁有的知識寫出來的。「科學是個持續更新而進步的過程。當時我們做出來的解釋有些方面困擾著我，所以我提出要進行新的研究航次。回到大塔穆火山後我們取得新的地磁資料，而得出這項新成果。」
「從事科學研究時，我們必須一直質疑我們以為已經了解的事物，並且不停地反覆檢驗我們的推論。這樣我們才有辦法盡可能地接近真相――無論真相究竟是什麼。」
New clues to the origin of Tamu Massif
The discovery of Tamu Massif, a gigantic
volcano located about 1,000 miles east of Japan, made big news in 2013 when
researchers reported it was the largest single volcano documented on earth,
roughly the size of New Mexico.
New findings, reported this week in Nature Geoscience, conclude that it is a
different breed of volcanic mountain than earlier thought, throwing into doubt
the prior claim that it is the world’s largest single volcano.
The study analyzed magnetic field data over Tamu
Massif, finding that magnetic anomalies – perturbations to the field caused by
magnetic rocks in the Earth’s crust – resemble those formed at mid-ocean ridge
plate boundaries.
William Sager, a geophysicist at the University of
Houston and senior author for the paper, said the discovery led researchers to
conclude that Tamu Massif formed by mid-ocean ridge “spreading,” the
geologists’ term for creation of ocean crust at mid-ocean ridge plate
boundaries, rather than as a shield volcano, as previously thought. Shield
volcanos are formed primarily as stacks of fluid lava flows and are one of the
most common types of volcano.
An international group of researchers – from Texas,
China and Japan – sought to understand how the massive Tamu Massif volcano
formed near the nexus of three spreading ridges. The key, they report, is
magnetic anomalies.
Mid-ocean ridges – plate boundaries where oceanic
plates move apart – are themselves large volcanoes. These ridges record
distinctive linear magnetic anomalies, parallel to the ridge, as they form new
crust. This is a result of lava flows and magma being concentrated near the
ridge axis where the magnetic minerals in the new crust record reversals of the
magnetic field polarity.
A new
understanding of Tamu Massif
Linear magnetic anomalies formed by the three ridges
had previously been found around Tamu Massif, but it was unclear where they
stopped within the volcano. A paper published in 2013 by Sager and colleagues
concluded that Tamu Massif is an enormous shield volcano, formed by
far-reaching lava flows emanating from its summit.
The latest study compiled a magnetic anomaly map over
Tamu Massif, using 4.6 million magnetic field readings collected over 54 years
along 72,000 kilometers of ship tracks. The data set was anchored by a new grid
of magnetic profiles, positioned with modern GPS navigation, collected by the
study authors using the Schmidt Ocean Institute ship Falkor. The resulting map
shows that linear magnetic anomalies around Tamu Massif blend into linear
anomalies over the mountain itself – implying that the underwater volcano
formed by extraordinary mid-ocean ridge crustal formation.
Sager said the finding is important because it
demonstrates that Tamu Massif and other oceanic plateaus are formed by a
different process than previously thought. A widely-accepted model suggests a
large blob of magma, known as a “mantle plume,” rises through the mantle and
creates a massive volcano when it arrives at the surface. This eruption is
thought to be analogous to massive eruptions on land, called “continental flood
basalts” and it creates a vertical succession of lava flows.
The ocean-ridge-spreading hypothesis suggests the age
progression is instead lateral. New material is always added at the center of
the ridge as older material drifts laterally away. An implication is that the gradual
slopes of Tamu Massif are not caused by lava flow shape but instead by a
gradual inflation and then deflation of ridge volcanism as the crust became
thicker and then grew thinner.
The new finding also weakens the accepted analogy
between eruptions of continental flood basalts and oceanic plateaus because the
formation mechanisms are shown to be different, Sager said.
‘Certainly one of
the largest’
With the discovery, Sager said Tamu Massif can no
longer be considered the world’s largest shield volcano. That title reverts to
Mauna Loa, on the island of Hawaii.
“The largest volcano in the world is really the
mid-ocean ridge system, which stretches about 65,000 kilometers around the
world, like stitches on a baseball,” Sager said. “This is really a large
volcanic system, not a single volcano.”
Researchers now think Tamu Massif formed as part of
that mid-ocean ridge system, he said. “Tamu Massif is certainly one of the
largest volcanic mountains in the world.”
The 2013 paper was based on what researchers knew at
the time, Sager said. “Science is a process and is always changing. There were
aspects of that explanation that bugged me, so I proposed a new cruise and went
back to collect the new magnetic data set that led to this new result.
“In science, we always have to question what we think
we know and to check and double check our assumptions. In the end, it is about
getting as close to the truth as possible - no matter where that leads.”
原始論文：William W.
Sager, Yanming Huang, Masako Tominaga, John A. Greene, Masao Nakanishi,
Jinchang Zhang. Oceanic plateau formation by seafloor spreading implied
by Tamu Massif magnetic anomalies. Nature Geoscience, 2019;
DOI: 10.1038/s41561-019-0390-y
引用自：University of Houston. “Research Yields
New Clues to the Origin of Tamu Massif”
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