原文網址：https://www.uu.nl/en/news/subterranean-islands-strongholds-in-a-potentially-less-turbulent-world
在地函深處藏有兩座跟陸塊一樣龐大的巨大「島嶼」。由烏特勒支大學發表在《自然》(Nature)的新研究顯示，相較於周圍溫度較低的隱沒板塊墳場，這兩個區域不只較為高溫，而且年代勢必也相當古老——至少有五千萬年甚至更久。這些觀察結果牴觸了一項越來越多人質疑的理論：地函是均勻混和且快速流動的說法。「地函並不如一般想法中的容易流動。」
此圖表示了板塊的隱沒過程(藍色)以及從LLSVP升起的地函柱(紅色)。組成後者的礦物顆粒比隱沒板塊大非常多。圖片來源：Utrecht University
大地震會讓整個地球像座大鐘般地低鳴，如同樂器一樣發出許多不同性質的音調。這些由整個地球發出的振盪在遭遇地球內部的異常帶時聽起來會「變調」或降低音量，因此地震學家探討有多少音調變調就可以研究地球深處的樣貌。這種地震學家用來得到地球內部影像的方法，就像醫生運用X光來透視我們的身體一樣。在上個世紀末，分析這些振盪的結果顯示地表之下有兩座「超大陸」：一座位在非洲下方，另一座則在太平洋下方，兩者都藏在距離地表兩千多公里深的地方。Arwen
Deuss表示：「沒有人知道它們究竟是什麼，也不知道它們是否只是暫時存在的現象，或者已經待在那裡數百萬年，甚至數十億年，」Deuss是烏特勒支大學的地震學家與教授，研究專長為地球深處的構造與成分。「圍繞這兩座大島的是由隱沒作用運送至此的板塊所形成的墳場。隱沒作用造成一個板塊下潛到另一個板塊的下方，並且從地表一路沉沒到地下將近三千多公里深的地方。」
震波減速
「許多年前我們就已經知道這兩座島嶼位在地核與地函的邊界，並且看到地震波經過它們時會減速。」因為如此，地震學家稱呼這些區域為「大型低震波速帶」(Large
Low Seismic Velocity Provinces，LLSVPs)。「地震波在LLSVP會變慢是因為此處的溫度較高，就像在大熱天底下我們很難跑得跟天氣涼爽時一樣快。」Deuss和她的同僚Sujania
Talavera-Soza很想知道他們是否還能對這些區域有更多發現。「我們加入的新資訊稱為地震波的『阻尼』(damping)，其為地震波在地球內部傳播時損失的能量大小。為了得到這項新資訊我們不僅研究了有多少音調變調，還有它們的音量變化。」Talavera-Soza接著表示：「跟我們預期的不同，我們發現LLSVP的阻尼並不高，使得震波在該處聽起來很大聲；但我們發現低溫的板塊墳場阻尼相當高，使同樣的音調在這裡變得十分輕柔。上部地函就不一樣了，我們在此的發現跟預期相同：此處的溫度很高，震波的阻尼也很大。就像你在天氣很熱時出門跑步一樣，你不只跑得比天氣涼爽時還慢，而且也更容易氣喘吁吁。」
顆粒大小
他們的同事Laura
Cobden專長為研究地球深處的礦物，她建議可以研究LLSVP的礦物顆粒大小。而美國的同僚Ulrich
Faul也指出單憑溫度並無法解釋LLSVP的阻尼為什麼不高。Deuss說：「礦物顆粒大小極為重要。由於隱沒板塊前往地球深處的旅途中會再結晶，因此最終到達板塊墳場的時候礦物顆粒變得比較小。顆粒尺寸變小意味著顆粒數量變多，彼此之間的交界也變得更加密集。由於地震波每通過一次礦物顆粒的交界就會損失一點能量，因此我們看到板塊墳場的阻尼較高，便是來自於此處顆粒之間的交界更為密集。而LLSVP的阻尼相當低，也意味著組成它們的礦物顆粒勢必大上很多。」
年代久遠
LLSVP的礦物顆粒不可能是一夕之間長成的，因此只能代表一件事：它們的年紀比周圍的板塊墳場老非常非常多。更重要的是，由於組成LLSVP的材料相當大塊，因此它們也相當堅固。從此可以得出LLSVP並不會參與地函對流——所以跟地科課本教我們的相反，地函實際上並不是均勻混和的。Talavera-Soza解釋：「無論如何，LLSVP勢必要透過某種方式成功在地函對流中存活下來。」
動力來源
關於地函的知識對於了解地球的演化過程來說是不可或缺的。「對於了解其他發生在地表的現象來說也是如此，比方說火山活動與造山運動，」Deuss補充。「這些現象的動力來源都是地函。舉個例子吧：地函柱是從地球深處往上升的高熱物質，就像是熔岩燈裡面往上浮的巨大泡泡，」當它們觸及地表便會引發火山活動，夏威夷就是一個範例。「而我們認為這些地函柱的起源，便是LLSVP的外緣。」
大型地震
在這類研究當中，地震學家充分利用了超級大地震所造成的振盪。他們尤其喜歡發生深度相當深的地震，比方說1994年的波利維亞大地震。「這場地震完全沒有登上新聞版面，因為它發生在地下650公里深的地方，而且很幸運地沒有對地表人類的生命財產造成損害，」Deuss解釋。整個地球因此而發出的振盪，亦即音調，可以用數學的方法來描述，藉此我們能輕易「看出」因為特殊構造而造成的阻尼(也就是振盪的音量變化)，並且運用波速(也就是走調的程度)來把該構造分離出來。「這項成果令人印象深刻，因為從這些振盪的訊號我們所能揭露出來的訊息當中，阻尼只不過是全部的十分之一而已。」對於這類研究來說，並不需要等到下一次地震發生，過去的地震所提供的數據就很有用了。「我們能用的數據可以追溯到1975年，因為從當年開始，地震儀就足以提供高品質的數據來讓我們進行研究。」
Subterranean
‘islands’: strongholds in a potentially less turbulent world
Deeply hidden in Earth’s mantle there are
two huge ‘islands’ with the size of a continent. New research from Utrecht
University, published in Nature,
shows that these regions are not only hotter than the surrounding graveyard of
cold sunken tectonic plates, but also that they must be ancient: at least half
a billion years old, perhaps even older. These observations contradict the idea
of a well-mixed and fast flowing Earth’s mantle, a theory that is becoming more
and more questioned. “There is less flow in Earth’s mantle than is commonly
thought.”
Large earthquakes make the whole Earth ring like a
bell with different tones, just like a musical instrument. Seismologists study
Earth’s deep interior by investigating how much these tones are ‘out of tune’,
because whole Earth oscillations will sound out of tune or less loud when they
encounter anomalies. This way seismologists will be able to make images of the
interior of our planet, just like a hospital doctor can ‘see’ through your body
with X-rays. At the end of the last century, an analysis of these oscillations
showed the existence of two subsurface ‘super-continents’: one under Africa and
the other one under the Pacific Ocean, both hidden more than two thousand
kilometres below the Earth’s surface. “Nobody knew what they are, and whether
they are only a temporary phenomenon, or if they have been sitting there for
millions or perhaps even billions of years,” says Arwen Deuss, seismologist and
professor of Structure and composition of Earth’s deep interior. “These two
large islands are surrounded by a graveyard of tectonic plates which have been
transported there by a process called ‘subduction’, where one tectonic plate
dives below another plate and sinks all the way from the Earth’s surface down
to a depth of almost three thousand kilometres.”
Slow waves
“We have known for years that these islands are
located at the boundary between the Earth’s core and mantle. And we see that
seismic waves slow down there.” Earth scientists therefore call these regions
‘Large Low Seismic Velocity Provinces’ or LLSVPs. “The waves slow down because
the LLSVPs are hot, just like you can’t run as fast in hot weather as you can
when it’s colder.” Deuss and her colleague Sujania Talavera-Soza were keen to
find out if they could discover more about these regions. “We added new information,
the so-called ‘damping’ of seismic waves, which is the amount of energy that
waves lose when they travel through the Earth. In order to do so, we did not
only investigate how much the tones where out of tune, we also studied their
sound volume.” Talavera-Soza adds: “Against our expectations, we found little
damping in the LLSVP, which made the tones sound very loud there. But we did
find a lot of damping in the cold slab graveyard, where the tones sounded very
soft. Unlike the upper mantle, where we found exactly what we expected: it is
hot, and the waves are damped. Just like when the weather is hot outside and
you go for a run, you don’t only slow down but you also get more tired than
when it is cold outside.”
Grain size
Their colleague Laura Cobden, who specializes in the
minerals that we find deep in the Earth, suggested to study the grain size of
the LLSVPs. According to their American colleague Ulrich Faul, temperature
alone cannot explain the absence of high damping in the LLSVPs. Deuss: ”Grain
size is much more important. Subducting tectonic plates that end up in the slab
graveyard consist of small grains because they recrystallize on their journey
deep into the Earth. A small grain size means a larger number of grains and
therefore also a larger number of boundaries between the grains. Due to the
large number of grain boundaries between the grains in the slab graveyard, we
find more damping, because waves loose energy at each boundary they cross. The
fact that the LLSVPs show very little damping, means that they must consist of
much larger grains.”
Ancient
Those mineral grains do not grow overnight, which can
only mean one thing: LLSVPs are lots and lots older than the surrounding slab
graveyards. Even more so: the LLSVPs, with their much larger building blocks,
are very rigid. Therefore, they do not take part in mantle convection (the flow
in the Earth’s mantle). Thus, contrary to what the geography books teach us, the
mantle cannot be well-mixed either. Talavera-Soza clarifies: “After all, the
LLSVPs must be able to survive mantle convection one way or another.”
Engine
Knowledge of the Earth’s mantle is essential to
understand the evolution of our planet. “And also to understand other phenomena
at the Earth’s surface, such as vulcanism and mountain building,” Deuss adds.
“The Earth’s mantle is the engine that drives all these phenomena. Take, for
example, mantle plumes, which are large bubbles of hot material that rise from
the Earth’s deep interior as in a lava lamp.” Once they finally reach the
surface, they cause vulcanism, like under Hawaii. “And we think that those
mantle plumes originate at the edges of the LLSVPs.”
Large earthquakes
In this type of research, seismologists make good use
of oscillations caused by really large earthquakes, preferably quakes that take
place at great depths, such as the great Bolivia earthquake of 1994. “It never
made it into the newspapers, because it took place at a large depth of 650 km
and luckily did not result in any damage or casualties at the Earth’s surface,”
Deuss explains. The whole Earth oscillations, or tones, are mathematically
described in such a way that we can easily ‘read’ the damping (i.e. how loud
the oscillation is) due to a specific structure and separate it from the wave
speed (i.e. how much out of tune it is). “Which is impressive, because the
damping of the signal is only one-tenth of the total amount of information that
we can unravel from these oscillations.” For this type of research, it is not
necessary to wait until another earthquake occurs. The data from previous
earthquakes is just as useful. “We can go back to 1975, because from that year
onwards, seismometers became good enough to give us data of such high quality
that they are useful for our research.”
原始論文：Sujania
Talavera-Soza, Laura Cobden, Ulrich H. Faul, Arwen Deuss. Global 3D model of
mantle attenuation using seismic normal modes. Nature, 2025; DOI:
10.1038/s41586-024-08322-y
引用自：Utrecht
University. “Subterranean ‘islands’: strongholds in a potentially less
turbulent world”
沒有留言:
張貼留言