原文網址：www.sciencedaily.com/releases/2017/07/170727141758.htm
直達冰島熔岩起源的根部
這項發現結合前人研究提供的證據，顯示地底深處接近地球核幔邊界的熔融區域，可能形成了地表附近的玄武岩火山島鏈。在地球核幔邊界周圍有些稱作超低速帶(ULVZ)的區域，特點是其中含有液態岩石使得此區的地震波速比附近物質低了將近30%。
然而，由於超低速帶位在極深的地方，要確切描繪出它們的形狀相當困難。之前在玻里尼西亞的國家薩摩亞以及夏威夷群島下方有偵測到超低速帶，但是科學家卻無法成功清楚描繪出它們的形狀。由於這些超低速度很接近於火山島鏈的正下方，因此促成某些理論認為這些含有大量熔融岩石的區域提供地函柱原料，而在地表形成了火山島。
Kaiqing Yuan和Barbara Romanowicz在此利用震波層析成像法來探測冰島下方的超低速帶。震波層析成像法利用地震波來建構出地球內部類似X光的影像。
根據他們的結果，在核幔邊界附近有相當大一包圓形的部分熔融岩石，直徑大約有800公里，高度大概有15公里。它提供的地函柱直達這座玄武岩島嶼的下方。
作者強調這處超低速帶的地點、形狀，以及跟下部地函較淺處的地函柱寬度之間成正比的大半徑，都顯示該超低速帶和其上方湧升的地函柱有密切關係。
這些新數據加上之前已知位於薩摩亞和夏威夷下方的超低速帶，使得作者提出在提供活躍熱點材料的大型地函柱根部，會有一類特殊的大型超低速帶於此形成。
Getting to the root of Iceland's molten rock origins
New data reveal an unprecedented depiction of a region of
partially molten rock deep within the Earth, which appears to be feeding
material in the form of a plume to the surface, where Iceland is located.
The finding, in combination
with evidence from previous studies, suggests that these molten regions deep
below, near the core-mantle boundary of the Earth, may cause basaltic ocean island
chains to form along the surface. Around the Earth's core-mantle boundary are
regions called ultralow-velocity zones (ULVZs), which are characterized by
liquid rock with velocities up to 30% lower than surrounding material.
However, depicting ULVZs
has been particularly difficult given their extreme depths. ULVZs have been
detected below the Polynesian country of Samoa and the Hawaiian islands, yet a
clear depiction of their shape has eluded scientists. The proximity of these
ULVZs below volcanic island chains has prompted theories suggesting that the
giant reservoirs of molten rock feed the mantle plumes that create the islands
on Earth's surface.
Here, Kaiqing Yuan and
Barbara Romanowicz used seismic tomography, which constructs an x-ray-like
picture of the Earth's interior using seismic waves, to probe a ULVZ below
Iceland.
Based on their results,
there appears to be a massive circular blob of partially molten rock,
approximately 800 kilometers in diameter and 15 kilometers in height, along the
core-mantle boundary, feeding the plume directly below the basaltic island.
The authors note that this
ULVZ's location, shape and large diameter, which is proportionate with the
width of the plume higher up in the lower mantle, suggests a close link between
the ULVZ and the rising plume above it.
These new data, in
combination with the known presence of ULVZs below Samoa and Hawaii, led the
authors to propose that a specific class of large ULVZs form at the roots of
broad plumes that feed active hotspots.
原始論文：Kaiqing Yuan,
Barbara Romanowicz. Seismic evidence for partial melting at the root of
major hot spot plumes. Science, 2017; 357 (6349): 393
DOI: 10.1126/science.aan0760
引用自：American Association for the Advancement of
Science. "Getting to the root of Iceland's molten rock origins."
ScienceDaily. ScienceDaily, 27 July 2017.
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