原文網址：https://www.insidescience.org/news/scientists-find-surprisingly-cool-hotspots-under-earths-crust
這項發現顯示某些火山如何形成的現行理論可能太過簡化
Charles Q. Choi
夏威夷、冰島、加拉巴哥群島等火山島都是由「熱點」創造出來，新研究發現經常有證據顯示它們的溫度可能低得驚人。
冰島的法格拉達爾火山。圖片來源：ImageBank4u via Shutterstock
科學家強調這些發現顯示熱點的來源或許和之前認為的不同，並非都是來自巨大的地函柱，也就是從地核附近湧上來的灼熱岩石。
火山一般發現在板塊邊界的附近。板塊是漂浮在地函(地殼與地核之間的區域)頂部的巨大岩石，當它們互相碰撞時便會產生火山。環太平洋地區組成「火環」的火山便是典型的範例。
然而，火山有時候也會從板塊中央噴發出來。這些熱點的來源可能是地函柱――也就是狀似蘑菇的高溫柱狀岩石，它們會從地函深處上湧，並且像噴燈一樣灼燒蓋在上方的物質。地質學家認為當板塊在地函柱的上方游移，就會出現鍊狀的火山島。
先前的研究提出火山熱點的溫度大概比中洋脊高出100到300℃，後者是板塊在海底張裂、岩漿上湧的地方。這支持了以下觀點：熱點的熱源是地核附近的高溫物質，而中洋脊的熱源則是較為低溫的地函岩石。
最近科學家發現許多熱點的溫度比之前認為的低了許多，使得它們的起源成為一道問題。「有很大一部份的熱點並不符合傳統的地函柱模型，」馬里蘭大學帕克分校的地震學家Vedran
Lekic表示。他並未參與此研究。
在這項新研究，研究人員分析了海洋熱點和洋脊下方的地函當中，地震波傳遞的速度來估計這兩個地方的溫度。(地震波在低溫的岩石裡傳播得比較快)
大約45%的熱點比中洋脊高出155℃以上。不過，40%左右的溫度卻沒有特別高，只比中洋脊高出50到136℃，因此它們的浮力不足以支持深部地函的岩石持續上湧。不只如此，還有大約15%的熱點溫度特別低，只比中洋脊高出36℃甚至不到。
為了闡明這些不同類型熱點的起源，科學家也檢視了岩石裡較為稀少的氦-3與較為常見的氦-4之間的比例。(氦-3的原子核只有一個中子，而氦-4的則有兩個中子)
在地殼找到的氦大多為氦-4，它們來自鈾以及其他放射性同位素隨時間衰變的過程；而地球深處的氦則富含氦-3，可能是因為此處還有非常古老的物質，保存了地球剛誕生的時候這兩種同位素之間最初的比例。研究人員發現高溫熱點氦-3對氦-4的比例比低溫熱點高出許多。
傳統模型認為熱點起源於從地函深部上湧的地函柱，雖然它也許可以解釋高溫熱點――最有名的熱點大多屬於此類，像是夏威夷、冰島、加拉巴哥群島、薩摩亞和復活節島――不過「或許實際上只有一些熱點的行為真正符合地函柱與熱點的傳統模型，」研究共同作者，加州大學洛杉磯分校的地球動力學家Carolina
Lithgow-Bertelloni表示。
「這鞏固了之前某些研究人員所主張的，他們認為『熱點』這個詞會讓人誤解，那些不符合標準板塊構造學說的火山，應該改稱為『異常熔融區』(melting anomalies)，」新墨西哥大學的地震學家Ross
Maguire表示。他並未參與此研究。
低溫熱點的起源可能是上部地函、移動緩慢而有更多時間冷卻的深部地函柱、或者是和翻攪的地函岩石互相作用而冷卻的深部地函柱。「如果結果無誤，那麼解釋這類發現便是地球動力學家的挑戰，」德國地球科學研究中心的地球動力學家Bernhard
Steinberger表示。他並未參與此研究。「這些結果毫無疑問會激發出新的研究。」
從各方面來說，「傳統的地函柱觀點與它在30到50年前提出時相比並沒有出現更多缺陷，而是變得更加複雜，」Lithgow-Bertelloni表示。
反之，這項成果「指出地函柱之間的差異其實高出許多，」Steinberger表示。「就像每次你對一顆行星或衛星拍下距離更近的影像，雖然裡面會有些完全意想不到的景物，但它是圓的這點卻不會改變。」
Lithgow-Bertelloni表示他們未來希望可以更加詳細地分析每個熱點，使他們對於熱點的溫度有更加全面的認知。她也提出未來的目標之一是進行更多電腦模擬，來驗證產生低溫熱點的各種情境。
詳述他們發現的研究論文發表在1月7號這期的《科學》(Science)期刊。
Scientists find surprisingly cool
'hotspots' under Earth's crust
The findings suggest current theories
of how some volcanoes form may be too simple
The hotspots that created volcanic
islands such as those of Hawaii, Iceland and the Galapagos Islands may often
prove surprisingly cool, a new study finds.
These findings suggest that such hotspots may not
always originate from giant plumes of scorching hot rock welling up from near
Earth's core as previously thought, scientists noted.
Volcanoes are typically found near the borders of
tectonic plates, born from clashes between those giant slabs of rock as they
drift on top of the mantle layer between Earth's core and crust. Classic
examples of such volcanoes are those that make up the so-called Ring of Fire on
the Pacific Rim.
However, volcanoes sometimes erupt in the middle of
tectonic plates. The sources of these hotspots might be mantle plumes,
mushroom-shaped pillars of hot rock ascending from the deep mantle to sear
overlying material like a blowtorch. As tectonic plates wander over such
plumes, geologists think chains of volcanic isles can emerge.
Previous research suggested volcanic hotspots are
roughly 100 to 300 degrees Celsius (180-540 F) hotter than mid-ocean ridges,
where magma rises as tectonic plates spread apart underwater. This supported
the view that hotspots were heated by matter from near Earth's hot core and
mid-ocean ridges by cooler mantle rock.
Now scientists find that many hotspots are
dramatically cooler than previously thought, raising questions about their
origins. "A substantial fraction of hotspots do not fit the classical
plume model," said Vedran Lekic, a seismologist at the University of
Maryland, College Park, who did not participate in this study.
In the new study, researchers analyzed the velocity
of seismic waves rippling through the mantle underneath oceanic hotspots and
ridges to estimate temperatures at those sites. (Seismic waves travel faster
through cold rock.)
Roughly 45% of hotspots are more than 155 C (279 F)
hotter than mid-ocean ridges. However, about 40% are only 50 to 136 C (90-245
F) hotter than mid-ocean ridges, not particularly hot and therefore not buoyant
enough to support the active upwelling of rock from the deep mantle. What's
more, roughly 15% of hotspots are especially cold, only 36 C hotter or less
than mid-ocean ridges.
To shed light on the origins of these different
varieties of hotspots, the scientists also examined the ratio of rarer helium-3
to more common helium-4 in their rock. (The atomic cores of helium-3 each
possess just one neutron, whereas helium-4 nuclei each have two.)
Helium found in Earth's crust is mostly helium-4
arising from the breakdown of uranium and other radioactive isotopes over time,
whereas helium from deep within Earth is richer in helium-3, likely from
reservoirs of ancient material preserving the original ratio found between
these isotopes during Earth's first days. The researchers discovered hot
hotspots possessed a much higher ratio of helium-3 to helium-4 than cold
hotspots did.
Although the classic model of hotspots originating
from plumes welling up from the deep mantle may explain hot hotspots, including
most of the famous ones, such as those underlying Hawaii, Iceland, the
Galapagos, Samoa and Easter Island, "perhaps the truth is that only a few
hotspots truly behave like our classical model of mantle plumes and
hotspots," said study co-author Carolina Lithgow-Bertelloni, a
geodynamicist at the University of California, Los Angeles.
"This reinforces what some researchers have
argued previously, which is that the term 'hotspot' is misleading and that
volcanoes that don't fit the plate tectonic paradigm should rather be referred
to as 'melting anomalies,'" said seismologist Ross Maguire from the
University of New Mexico, who did not take part in this research.
Cooler hotspots may instead originate in the upper
mantle, or from slow-moving deep plumes that have more time to cool, or from
deep plumes that interact with and get cooled by swirling mantle rock. "If
this is real, it will be a challenge for geodynamicists to explain such a
finding," said Bernhard Steinberger, a geodynamicist at the German
Research Center for Geosciences in Potsdam, who was not a part of this work.
"These results will doubtlessly trigger new research."
All in all, "the classical view of plumes is not
so much flawed than more complex than presented 30 to 50 years ago,"
Lithgow-Bertelloni said.
Instead, this work "points to a much greater
variety among plumes," Steinberger said. "It is kind of like whenever
you get a new close-up view of a planet or moon. It has some totally unexpected
features. But it is still round."
In the future, the scientists would like to analyze
every hotspot in more detail to get an even better sense of their temperatures,
Lithgow-Bertelloni said. They also aim to conduct more computer simulations
testing various cool hotspot scenarios, she added.
The researchers detailed their findings in the Jan. 7
issue of the journal Science.
原始論文：Xiyuan
Bao, Carolina R. Lithgow-Bertelloni, Matthew G. Jacksonand, Barbara Romanowicz.
On the relative temperatures of Earth’s
volcanic hotspots and mid-ocean ridges. Science,
2022; 375 (6576) DOI:10.1126/science.abj8944
引用自：Inside
Science. “Scientists find surprisingly cool 'hotspots' under Earth's crust.”
沒有留言:
張貼留言