地球古代磁場的樣貌為何？

原始網址：www.sciencedaily.com/releases/2016/06/160624135852.htm

What did Earth's ancient magnetic field look like?

地球古代磁場的樣貌為何？

New work from Carnegie's Peter Driscoll suggests Earth's ancient magnetic field was significantly different than the present day field, originating from several poles rather than the familiar two. It is published in Geophysical Research Letters.

卡內基科學研究院的 Peter Driscoll進行的新研究提出地球古代磁場跟現今磁場之間有相當大的差異。他認為過去的磁場是從數個磁極散發出來而非我們熟知的兩個。這項研究刊登於期刊《地球物理通訊》(Geophysical Research Letters)之上。

Earth generates a strong magnetic field extending from the core out into space that shields the atmosphere and deflects harmful high-energy particles from the Sun and the cosmos. Without it, our planet would be bombarded by cosmic radiation, and life on Earth's surface might not exist. The motion of liquid iron in Earth's outer core drives a phenomenon called the geodynamo, which creates Earth's magnetic field. This motion is driven by the loss of heat from the core and the solidification of the inner core.

地球產生的強烈磁場會從地核延伸至太空，這道磁場不只能保護大氣層，還能偏折從宇宙和太陽散發出的有害高能粒子。沒有這道磁場，我們的星球會被宇宙輻射轟炸，地球上的生物也可能不復存在。地球外核液態鐵的流動會驅動所謂的「地球發電機」(geodynamo)現象而產生磁場，這種運動的能量來源為地核本身以及內核凝固時產生的熱。

But the planet's inner core was not always solid. What effect did the initial solidification of the inner core have on the magnetic field? Figuring out when it happened and how the field responded has created a particularly vexing and elusive problem for those trying to understand our planet's geologic evolution, a problem that Driscoll set out to resolve.

但是地球內核並非一直以來都是固態的。內核開始凝固時會對地磁產生什麼樣的效應？解開此現象何時發生以及對磁場的影響，對於想瞭解地球的地質活動是如何演變的科學家來說是道相當令人困惑且費解的難題，而Driscoll決心要解決這道謎題。

Here's the issue: Scientists are able to reconstruct the planet's magnetic record through analysis of ancient rocks that still bear a signature of the magnetic polarity of the era in which they were formed. This record suggests that the field has been active and dipolar--having two poles--through much of our planet's history. The geological record also doesn't show much evidence for major changes in the intensity of the ancient magnetic field over the past 4 billion years. A critical exception is in the Neoproterozoic Era, 0.5 to 1 billion years ago, where gaps in the intensity record and anomalous directions exist. Could this exception be explained by a major event like the solidification of the planet's inner core?

問題在此：有些古代岩石能保留它們形成年代時的磁極資訊，科學家經由分析它們可以重建地球磁場的相關紀錄。這些紀錄呈現出在地球歷史上大多時間地球磁場都相當活躍且具備雙極性(dipolar)，意味著其具備兩個磁極。40億年來地質紀錄中也並未有太多證據指出過往地球磁場強度有相當大的變化。有一個重大例外出現在5億年至10億年前的新元古代(Neoproterozoic Era)，磁場強度的紀錄在此出現一段缺失，且磁場方向也變得相當異常。像是地球內核開始凝固這類的重大事件可以解釋這件例外嗎？

In order to address this question, Driscoll modeled the planet's thermal history going back 4.5 billion years. His models indicate that the inner core should have begun to solidify around 650 million years ago. Using further 3-D dynamo simulations, which model the generation of magnetic field by turbulent fluid motions, Driscoll looked more carefully at the expected changes in the magnetic field over this period.

為了處理這個問題，Driscoll模擬45億年來地球熱能的歷史變化。他的模型指出內核應該是在大約6億5000萬年前開始凝固。Driscoll進一步運用3-D發電機模型來模擬由紊流液體運動產生的磁場，如此他可以更仔細地觀察磁場在這段期間伴隨產生的變化。

"What I found was a surprising amount of variability," Driscoll said. "These new models do not support the assumption of a stable dipole field at all times, contrary to what we'd previously believed."

「我發現這段期間的變化十分驚人。」Driscoll說。「跟我們原先認為的相反，新的模擬結果並不支持地磁總是雙極性的假說。」

His results showed that around 1 billion years ago, Earth could have transitioned from a modern-looking field, having a "strong" magnetic field with two opposite poles in the north and south of the planet, to having a "weak" magnetic field that fluctuated wildly in terms of intensity and direction and originated from several poles. Then, shortly after the predicted timing of the core solidification event, Driscoll's dynamo simulations predict that Earth's magnetic field transitioned back to a "strong," two-pole one.

他的結果顯示大約在10億年前，地球磁場從跟現今磁場十分相似，也就是磁場強度十分強且兩個磁極相對位於南北極，轉變成強度微弱的磁場，其強度和方向變動的十分劇烈，且源自於好幾個磁極。接著，在地核凝固事件推估發生的時間點不久之後，Driscoll的發電機模型預測地球磁場會再度變強且具備雙極性。

"These findings could offer an explanation for the bizarre fluctuations in magnetic field direction seen in the geologic record around 600 to 700 million years ago," Driscoll added. "And there are widespread implications for such dramatic field changes."

「這項發現可以解釋約莫在6億至7億年前的地質紀錄中觀察到磁場方向出現難以理解的波動。」Driscoll補充。「磁場出現如此劇烈的變化還具有相當多的意涵。」

Overall, the findings have major implications for Earth's thermal and magnetic history, particularly when it comes to how magnetic measurements are used to reconstruct continental motions and ancient climates. Driscoll's modeling and simulations will have to be compared with future data gleaned from high quality magnetized rocks to assess the viability of the new hypothesis.

總結來說，這些發現對於地球熱學及地磁史來說都有相當重要的意義，對於經常用到地球磁場測量結果的陸塊運動重建以及古氣候研究來說更是如此。 Driscoll的模型和模擬結果未來勢必要跟從磁化品質相當良好的岩石中蒐集到的資料互相比較，才能評估這項新假說的可行性。

引用自：Carnegie Institution for Science. "What did Earth's ancient magnetic field look like?." ScienceDaily. ScienceDaily, 24 June 2016. 

火山爆發前的寧靜

原文網址：www.sciencedaily.com/releases/2016/06/160623150201.htm

Volcanoes get quiet before they erupt

火山爆發前的寧靜

When dormant volcanoes are about to erupt, they show some predictive characteristics--seismic activity beneath the volcano starts to increase, gas escapes through the vent, or the surrounding ground starts to deform. However, until now, there has not been a way to forecast eruptions of more restless volcanoes because of the constant seismic activity and gas and steam emissions. Carnegie volcanologist Diana Roman, working with a team of scientists from Penn State, Oxford University, the University of Iceland, and INETER has shown that periods of seismic quiet occur immediately before eruptions and can thus be used to forecast an impending eruption for restless volcanoes. The duration of the silence can indicate the level of energy that will be released when eruption occurs. Longer quiet periods mean a bigger bang.

當處於休眠狀態的火山即將噴發時，它們會先展現出一些前兆，像是火山之下的地震開始增加、噴氣孔冒出氣體，或者周遭的地面逐漸變形。然而到目前為止，對於那些靜不下來的火山，我們尚未有方法可以預測它們是否即將噴發，這是因為它們附近總是不斷地發生地震，且持續冒出蒸氣和各種氣體。卡內基科學研究所的火山學家 Diana Roman與來自賓州州立大學、牛津大學、冰島大學以及尼加拉瓜地球科學研究所(INETER)的科學家團隊通力合作，發現緊接在噴發之前那些活躍的火山會出現一段地震活動停止期，因此可以利用此特性來預測即將到來的火山爆發，而緘默的時間長短可以指示之後噴發時釋放出的能量多寡。沉寂的越久，規模就越是浩大。

The research is published in Earth and Planetary Science Letters.

這項研究發表在期刊《地球與行星科學學報》(Earth and Planetary Science Letters)之上。

The team monitored a sequence of eruptions at the Telica Volcano in Nicaragua in 2011. It is a so-called stratovolcano, with a classic-looking cone built up by many layers of lava and ash. They started monitoring Telica in 2009 with various instruments and by 2011 they had a comprehensive network within 2.5 miles (4 kilometers) of the volcano's summit.

研究團隊監測了尼加拉瓜的特利卡(Telica)火山於2011年發生的一系列噴發事件。特利卡火山是座「層狀火山」，由火山灰和熔岩層層堆疊而成並呈現出典型的錐狀外觀。他們從2009年開始便利用各種儀器來監測特利卡火山，到2011時他們已經在火山頂端2.5哩的範圍中架設好一座十分全面的觀測網。

The 2011 eruptive event was a month-long series of small to moderate ash explosions. Prior to the eruption, there was a lack of deep seismicity or deformation, and small changes in sulfur dioxide gas emissions, indicating that the eruption was not driven by fresh magma. Instead, the eruption likely resulted from the vents being sealed off so that gas could not escape. This resulted in an increase in the pressure that eventually caused the explosions.

2011年的噴發事件是一連串小至中規模的火山灰噴發，約莫持續了一個月之久。在噴發之前，深層地震活動及變形曾一度停止，二氧化硫的噴氣情形也發生了微幅改變，顯示這起噴發事件並非由新鮮的岩漿驅使。反之，噴發可能是因為火山內部的管道被阻塞造成氣體無法逸散。這使得壓力持續累積最終導致火山爆發。

Of the 50 explosions that occurred, 35 had preceding quiet periods lasting 30 minutes or longer. Thirteen explosions were preceded by quiet intervals of at least five minutes. Only two of the 50 did not have any quiet period preceding the explosion.

在發生的50次噴發中，35次在之前有持續超過30分鐘或更長的靜止期，另外13次先行發生的靜止期持續了至少5分鐘，只有2次在之前從未有靜止期出現。

"It is the proverbial calm before the storm," remarked Roman. "The icing on the cake is that we could also use these quiet periods to forecast the amount of energy released."

「這就像俗話說的『暴風雨前的寧靜。』」 Roman如此評論他們的研究。「更妙的是我們還可以用靜止期來預測釋放出的能量多寡。」

The researchers did a "hindsight" analysis of the energy released. They found that the longer the quiet phase preceding an explosion, the more energy was released in the ensuing explosion. The quiet periods ranged from 6 minutes before an explosion to over 10 hours (619 minutes) for the largest explosion.

研究人員事後分析了釋放出的能量，他們發現在噴發之前的靜止期持續越久，隨後發生的噴發就會釋放出越多能量。靜止期的持續時間從某次噴發之前的六分鐘，至最劇烈噴發之前的10小時以上(619分鐘)不等。

The researchers were also able to forecast a minimum energy for impending explosions based on the data from the previous quiet/explosion pairs and the duration of the particular quiet period being analyzed. The correlation between duration of quiet periods and amount of energy released is tied to the duration of the gas pathways being blocked. The longer the blockage, the more pressure builds up resulting in more energy released. Sealing might be occurring due to mineral precipitation in cracks that previously acted as gas pathways, or due to the settling of the rock near the volcano's surface.

根據過往數據中的靜止期和噴發能量之間的成對關係，研究人員可以分析特定靜止期的持續時間來推算即將發生的噴發事件釋放出的最小能量。靜止期的持續時間會和釋放出的能量多寡之間成正比跟氣體通道被阻塞了多久有關。通道被堵住越久，就會累積越高的壓力而釋放出更多能量。堵塞可能是因為以往作為氣體通路的裂縫中有礦物沉澱出來，或者是火山表面附近的岩石滾入裂縫而導致。

"What is clear is that this method of careful monitoring of Telica or other similar volcanoes in real time could be used for short-term forecasts of eruptions," Roman said. "Similar observations of this phenomenon have been noted anecdotally elsewhere. Our work has now quantified that quiet periods can be used for eruption forecasts and that longer quiet periods at recently active volcanoes could indicate a higher risk of energetic eruptions."

「明顯地，詳細監測特利卡或者其他類似火山即時活動的方法可以用來當作火山爆發的短期預測。」 Roman說。「之前在其他地方觀察到的類似現象多半被當作奇聞軼事處理。現在我們的研究利用定量化的方式顯現靜止期確實可以用來預測噴發，且近期處於活躍狀態的火山平息的時間越久，代表劇烈噴發的風險也越高。」

The paper's other authors are Mel Rodgers of Oxford University, Peter LaFemina of Penn State University, Halldor Geirsson of the University of Iceland, and Virginia Tenorio of the Instituto Nicaraguense de Estudios Territoriales.

此篇論文的其他作者尚有牛津大學的Mel Rodgers、賓州大學的Peter LaFemina、冰島大學的Peter LaFemina以及尼加拉瓜地球科學研究所的 Virginia Tenorio。

This work was supported by the National Science Foundation and the Nicaraguan Institute of Earth Sciences (INETER).

這項研究由美國國家科學基金會與尼加拉瓜地球科學研究所資助。

引用自：Carnegie Institution for Science. "Volcanoes get quiet before they erupt." ScienceDaily. ScienceDaily, 23 June 2016. 

在理論上確認鹽水於地震帶的導電度

原始網址：www.sciencedaily.com/releases/2016/06/160613131100.htm

Electrical conductivity of salt water in seismogenic zones theoretically determined

在理論上確認鹽水於地震帶的導電度

A joint research team consisting of Hiroshi Sakuma, senior researcher, Functional Geomaterials Group, Environment and Energy Materials Division, National Institute for Materials Science (NIMS), Japan, and Masahiro Ichiki, assistant professor, Graduate School of Science, Tohoku University, Japan, succeeded in theoretically determining the electrical conductivity of NaCl solution (salt water) in a high-temperature and high-pressure environment at ground depths ranging from 10 to 70 km. By comparison with electrical conductivity data collected underground, the theoretical approach indicated the presence of salt water deep underground. This discovery may reinforce the theory that underground salt water influences the occurrence of earthquakes and volcanic eruptions.

包括日本國家物質材料研究機構環境與能源部，基礎地質材料研究團隊的高級研究員 Hiroshi Sakuma，以及日本東北大學理學院的助理教授 Masahiro Ichiki在內的聯合研究團隊，成功在學理上確立 NaCl溶液(鹽水)在地下10至70公里處高溫高壓環境下的導電度。藉由比對從地下蒐集到的導電度資料，這項理論可以指出在地底深處是否有鹽水的存在。此發現或許能更加鞏固地下鹽水會影響地震及火山爆發形成的學說。

It is commonly said that the presence of salt water in bedrock makes a fault prone to slide, influencing the occurrence of earthquakes, or decreases the melting points of rocks, influencing volcanic eruptions.

一般認為存在於岩床中的鹽水會造成斷層更容易滑動而影響地震發生，或者降低岩石的熔點而影響火山爆發。

However, it is difficult to directly verify the presence of salt water through drilling surveys deep underground. Since liquids including salt water have electrical conductivity about six orders of magnitude higher than that of solids, surveys involving the measurement of electrical conductivity are often carried out to detect the presence of salt water. However, because the electrical conductivity of salt water under the high-temperature and high-pressure conditions occurring in such environments as crustal seismogenic zones is unknown, it had been impossible to associate electrical conductivity measurements with the presence of salt water.

然而，要透過鑽井探測直接證實地底深處是否有鹽水存在是相當困難的。既然包括鹽水在內的流體導電度一般都比固體高出大約百萬倍，因此經常會施行包含導電度測量在內的探勘來偵測是否有鹽水存在。然而，由於在像地殼內的地震生成帶這種高溫高壓環境下的鹽水導電度會是多少仍未知曉，因此要直接將導電度測量結果跟鹽水的存在與否畫上等號是不可行的。

The research team developed a molecular model to reproduce the supercritical state of water. Using the model, the team successfully calculated electrical conductivity of salt water with NaCl concentrations ranging from one-sixth to triple that in seawater at high temperature and high pressure (temperature: 673-2,000 K, pressure: 0.2-2 GPa), conditions that are difficult to simulate in experiments. These electrical conductivity data indicated that high electrical conductivity measured under the ground in the Tohoku region may be explained by the presence of salt water with salt concentrations equivalent to seawater.

研究團隊發展出一個分子模型可以重現超臨界狀態下的水。團隊利用這個模型而成功計算出在實驗難以模擬的高溫高壓環境下(溫度：絕對溫度673至2000 K，壓力：0.2-2百萬帕)，NaCl濃度為海水濃度六分之一至三倍的鹽水的導電度。計算出的導電度資料指出在東北區域地下測量到的高導電度，或許可用有濃度等同海水的鹽水存在來解釋。

In future studies, we will combine these results with the electromagnetic crustal observations across Japan to identify the presence of salt water deep underground where seismic and volcanic activities are high, as in subduction zones, and conduct research in order to understand the mechanism of the outbreak of earthquakes and volcanic eruption.

在未來的研究中，我們會將這些結果跟日本各處地殼電磁的觀測數值結合起來，以確認隱沒帶這類地震及火山活動頻繁的區域地下深處是否有鹽水存在，並進行相關研究來瞭解地震和火山爆發的發生機制。

This research was conducted as a part of the projects "Geofluids: nature and dynamics of fluids in subduction zones," (Grant-in-Aid for Scientific Research on New Academic Related Areas) and "Research on understanding supercritical fluid properties in crust through molecular dynamics calculation and its influence on earthquake occurrence," (Grant-in-Aid for Challenging Exploratory Research) supported by the Ministry of Education, Culture, Sports, Science and Technology.

這項研究為以下兩計畫的一部分，分別是「地質流體：流體在隱沒帶的性質和力學」(由Scientific Research on New Academic Related Areas 補助)以及「透過分子動力計算模型以瞭解超臨界流體在地殼中的性質，及對地震活動影響的研究」(由Challenging Exploratory Research補助)，並在文部科學省的支持下進行。

This research was published in the online version of Journal of Geophysical Research: Solid Earth, on January 20, 2016.

此篇研究於2016年1月20日刊登在線上版的《地球物理研究期刊：固體地球》(Journal of Geophysical Research: Solid Earth)。

引用自：National Institute for Materials Science. "Electrical conductivity of salt water in seismogenic zones theoretically determined." ScienceDaily. ScienceDaily, 13 June 2016. 

島弧火山會釋放出地殼與地函的混合物

原始網址：www.sciencedaily.com/releases/2016/06/160613153408.htm

Arc volcano releases mix of material from Earth's mantle and crust

島弧火山會釋放出地殼與地函的混合物

Volcanoes are an explosive and mysterious process by which molten rock from Earth's interior escapes back into the atmosphere. Why the volcano erupts -- and where it draws its lava from -- could help trace the lifecycle of materials that make up our planet.

火山噴發這種極具爆炸性且神秘的作用會讓地球內部的熔岩重見天日。瞭解火山為何噴發，以及岩漿來自何處，有助於我們追蹤地球組成物質的循環過程。

New University of Washington research shows that a common type of volcano is not just spewing molten rock from the mantle, but contains elements that suggest something more complicated is drawing material out of the descending plate of Earth's crust.

華盛頓大學的新研究顯示一種常見火山類型噴發出的熔岩不只來自地函，它含有的元素指出還有某些更加複雜的作用從隱沒板塊的地殼中抽取出別的物質。

Geologists have long believed that solidified volcanic lava, or basalt, originates in the mantle, the molten rock just below the crust. But the new study uses detailed chemical analysis to find that the basalt's magnesium -- a shiny gray element that makes up about 40 percent of the mantle but is rare in the crust -- does not look like that of the mantle, and shows a surprisingly large contribution from the crust. The paper was published the week of June 13 in the Proceedings of the National Academy of Sciences.

玄武岩是一種由火山岩漿凝固而成的岩石，地質學家長久以來認為其來源為地殼正下方的地函產生的熔岩。但此篇新研究利用詳盡的化學分析發現玄武岩的鎂含量跟地函並不相似，反之，他們驚訝地發現鎂大部分是由地殼提供。鎂是一種亮灰色的金屬，占地函組成約40%，但在地殼當中卻十分稀少。此篇論文刊登於6月13日這周的《美國國家科學院院刊》(Proceedings of the National Academy of Sciences)。

"Although the volcanic basalt was produced from the mantle, its magnesium signature is very similar to the crustal material," said lead author Fang-Zhen Teng, a UW associate professor of Earth and space sciences. "The ocean-floor basalts are uniform in the type of magnesium they contain, and other geologists agree that on a global scale the mantle is uniform," he said. "But now we found one type of the mantle is not."

「雖然火山玄武岩的來源為地函，然而它的鎂含量呈現出來的訊號卻跟地殼物質十分雷同。」第一作者，華盛頓大學地球與太空科學的副教授 Fang-Zhen Teng說。「洋底玄武岩的鎂含量在類型上相當一致，因此其他地質學家同意以全球尺度來看，地函成分也是相當均一的。」他說。「但現在我們找到一種地函其鎂含量的類型並不相同。」

The study used rock samples from an inactive volcano on the Caribbean island of Martinique, a region where an ocean plate is slowly plunging, or subducting, beneath a continental plate. This situation creates an arc volcano, a common type of volcano that includes those along the Pacific Ocean's "Ring of Fire."

研究使用的岩石樣品來自於加勒比海天堂島上的休火山，該區域的海洋板塊緩緩俯衝(隱沒)至大陸板塊之下。此種環境會創造出島弧火山這種常見的火山類型，沿著太平洋「火環」分布的火山多隸屬之。

Researchers chose to study a volcano in the Caribbean partly because the Amazon River carries so much sediment from the rainforest to the seabed. One reason scientists want to pin down the makeup of volcanic material is to learn how much of the carbon-rich sediment from the surface gets carried deep in the Earth, and how much gets scraped off from the descending plate and reemerges into the planet's atmosphere.

研究人員會選擇研究加勒比海的火山有部分是因為亞馬遜河會攜帶大量雨林來的沉積物至海床上。科學家想定出火山物質成分的其中一個原因，便是想要得知有多少富含碳的的沉積物會從地表被帶至地球深處，又有多少會從隱沒板塊中脫離而重回地球大氣層。

Analyzing the weight of magnesium atoms in the erupted basalt shows that they came not from the mantle, nor from the organic sediment scraped off during the slide, but directly from the descending oceanic crust. Yet the volcanic basalt lacks other components of the crust.

分析噴發行成的玄武岩中鎂原子的重量顯示鎂並非來自地函，也不是隱沒過程中剝落的有機沉積物，而是直接來自下沉的海洋地殼。但火山玄武岩中卻缺乏其他地殼應有的成分。

"The majority of the other ingredients are still like the mantle; the only difference is the magnesium. The question is: Why?" Teng said.

「其他成分大體上還是跟地函十分相似，唯一有差異的就只有鎂。問題便是：為什麼會有這種現象?」Teng說。

The authors hypothesize that at great depths, magnesium-rich water is squeezed from the rock that makes up Earth's crust. As the fluid travels, the surrounding rock acts like a Brita filter that picks up the magnesium, transferring magnesium particles from the crust to the mantle just below the subduction zone.

作者假設在地球相當深處富含鎂的水分會從組成地殼的岩石內部擠壓出來。隨著這些水分在岩石當中行進，周圍的岩石會像Brita濾水器般將鎂萃取出來，造成隱沒帶下方的鎂原子從地殼轉移到地函當中。

"This is what we think is very exciting," Teng said. "Most people think you add either crustal or mantle materials as a solid. Here we think the magnesium was added by a fluid."

「我們的想法相當振奮人心。」Teng說。「大多數的人會想像地殼和地函的物質是以固體形式彼此參和。但我們在這裡卻認為鎂是以液體的形式混和至另一方。」

Fluids seem to play a role in seismic activity at subduction zones, Teng said, and having more clues to how those fluids travel deep in the Earth could help better understand processes such as volcanism and deep earthquakes.

Teng說液體在隱沒帶的地震活動中可能是相當重要的要素，瞭解液體如何在地球深處流動的更多資訊，有助於讓我們愈加理解火山噴發以及深層地震的形成過程。

He and co-author Yan Hu, a UW doctoral student in Earth and space sciences, plan to do follow-up studies on basalt rocks from the Cascade Mountains and other arc volcanoes to analyze their magnesium composition and see if this effect is widespread.

他和共同作者，華盛頓大學地球與太空科學的博士生Yan Hu，計畫在之後的研究當中探討喀斯開山脈及其他島弧火山的玄武岩，分析這些岩石的鎂含量以觀察這種效應是否十分普遍。

The other co-author is Catherine Chauvel at the University of Grenoble in France. The research was funded by the U.S. National Science Foundation and the French National Research Agency.

此篇研究的另一位共同作者是法國科諾伯勒第一大學的Catherine Chauvel，並由美國國家科學基金會和法國國家研究總署資助。

引用自：University of Washington. "Arc volcano releases mix of material from Earth's mantle and crust." ScienceDaily. ScienceDaily, 13 June 2016. 

過往地質事件留下的深層「傷疤」會對今日的地震造成重大影響

原文網址：www.sciencedaily.com/releases/2016/06/160610094437.htm

Deep 'scars' from ancient geological events play role in current earthquakes

過往地質事件留下的深層「傷疤」會對今日的地震造成重大影響

Super-computer modelling of Earth's crust and upper-mantle suggests that ancient geologic events may have left deep 'scars' that can come to life to play a role in earthquakes, mountain formation, and other ongoing processes on our planet.

以超級電腦對地球地殼以及上部地函做的模擬顯示，過往的地質事件可能會在這些地帶留下深遠的「傷疤」。當這些創傷再次發作就會對現今地球上發生的地震、造山運動及其他作用造成重大影響。

This changes the widespread view that only interactions at the boundaries between continent-sized tectonic plates could be responsible for such events.

這改變了廣為接受的看法中認為只有大陸尺寸的板塊在彼此之間的交界相互作用時，才會產生這些事件。

A team of researchers from the University of Toronto and the University of Aberdeen have created models indicating that former plate boundaries may stay hidden deep beneath the Earth's surface. These multi-million-year-old structures, situated at sites away from existing plate boundaries, may trigger changes in the structure and properties at the surface in the interior regions of continents.

由多倫多大學及亞伯丁大學組成的研究團隊建立的模型，指出先前的板塊邊界可以在地表深處潛伏許久。這些可以遠離現存板塊邊界長達數百萬年的構造，或許能造成陸塊內部地區的地表發生構造以及性質方面的變化。

"This is a potentially major revision to the fundamental idea of plate tectonics," says lead author Philip Heron, a postdoctoral fellow in Russell Pysklywec's research group in U of T's Department of Earth Sciences. Their paper, "Lasting mantle scars lead to perennial plate tectonics," appears in the June 10, 2016 edition of Nature Communications.

「這項研究可能使我們必須大幅重新審視板塊構造學說的中心思想。」第一作者 Philip Heron說。他是多倫多大學地球科學系 Russell Pysklywec研究團隊中的博士後研究員。他們這篇題名為「Lasting mantle scars lead to perennial plate tectonics」的論文刊登於2016年6月10日這期的《自然通訊》之上。

Heron and Pysklywec, together with University of Aberdeen geologist Randell Stephenson have even proposed a 'perennial plate tectonic map' of the Earth to help illustrate how ancient processes may have present-day implications.

Heron、Pysklywec以及亞伯丁大學的地質學家Randell Stephenson之前就已經共同提出一幅地球上「重複活動的板塊構造地圖」，幫助他們闡述過往的地質作用對現今的地質現象有何意涵。

"It's based on the familiar global tectonic map that is taught starting in elementary school," says Pysklywec, who is also chair of U of T's Department of Earth Sciences. "What our models redefine and show on the map are dormant, hidden, ancient plate boundaries that could also be enduring or "perennial" sites of past and active plate tectonic activity."

「這份地圖建立在大家都很熟悉的全球板塊構造地圖，小學入門課程中都會教到。」同時也是多倫多大學地球科學系主任的Pysklywec說。「我們的模型重新界定出那些休止、潛伏的古代板塊邊界並將它們標示在這份地圖上。它們也是過往及現今持續或『間歇』發生板塊運動的地區。」

To demonstrate the dominating effects that anomalies below the Earth's crust can have on shallow geological features, the researchers used U of T's SciNet -- home to Canada's most powerful computer and one of the most powerful in the world- to make numerical models of the crust and upper-mantle into which they could introduce these scar-like anomalies.

為了顯示地殼之下的異常地帶對淺部地質構造的主要影響，研究人員藉助了多倫多大學的SciNet，他們在此進行的數值模擬將這些傷痕似的異常構造引入地殼以及上部地函之內。SciNet是存放加拿大最強力，同時也是世上首屈一指的超級電腦的場所。

The team essentially created an evolving "virtual Earth" to explore how such geodynamic models develop under different conditions.

基本上，研究團隊創造了一顆「虛擬地球」來探討在不同的情況下這類地質動力學模型會如何發展。

"For these sorts of simulations, you need to go to a pretty high-resolution to understand what's going on beneath the surface," says Heron. "We modeled 1,500 kilometres across and 600 kilometres deep, but some parts of these structures could be just two or three kilometres wide. It is important to accurately resolve the smaller-scale stresses and strains."

「對於這類型的模擬來說，你必須要達到相當高的解析度才能了解在地表之下究竟發生了何種作用。」 Heron說。「我們可以模擬範圍寬達1500公里，深達600公里的構造，然而，這些構造當中可能有部分僅約2至3公里寬。因此要能準確解析尺度比這還要小的應力及應變是相當重要的。」

Using these models, the team found that different parts of the mantle below the Earth's crust may control the folding, breaking, or flowing of the Earth's crust within plates -- in the form of mountain-building and seismic activity -- when under compression.

利用這些模擬結果，研究團隊發現當受到壓力時，位於地殼之下的地函異常部分，可能會控制板塊內部的地殼產生褶皺、破裂與移動，並形成造山運動和地震。

In this way, the mantle structures dominate over shallower structures in the crust that had previously been seen as the main cause of such deformation within plates.

如此說來，過往認為是板塊內變形主因的淺層構造，實際上是受到地函中的構造控制。

"The mantle is like the thermal engine of the planet and the crust is an eggshell above," says Pysklywec. "We're looking at the enigmatic and largely unexplored realm in the Earth where these two regions meet."

「地函就像是地球的熱機，而地殼則是覆於其上的蛋殼。」Pysklywec說。「我們目前正在研究的是這兩個區域相交之處，是地球上充滿謎團且大多仍未被好好探討的區域。」

"Most of the really big plate tectonic activity happens on the plate boundaries, like when India rammed into Asia to create the Himalayas or how the Atlantic opened to split North America from Europe," says Heron. "But there are lots of things we couldn't explain, like seismic activity and mountain-building away from plate boundaries in continent interiors."

「真正大型的板塊構造運動多半是發生在板塊邊界，像是印度衝撞亞洲而創造喜馬拉雅山脈，或是大西洋打開使得北美從歐洲分裂出來。」Heron說。「但有很多事物是我們無法解釋的，比如在遠離板塊邊界處的大陸內部為何會有地震活動以及造山運動發生。」

The research team believes their simulations show that these mantle anomalies are generated through ancient plate tectonic processes, such as the closing of ancient oceans, and can remain hidden at sites away from normal plate boundaries until reactivation generates tectonic folding, breaking, or flowing in plate interiors.

研究團隊認為古代的板塊構造作用，像是海洋閉合，會在地函產生這些異常帶。它們可以隱藏在遠離一般板塊邊界的地方許久，直到再次活動而在板塊內部產生構造皺褶、破裂或移動作用。

"Future exploration of what lies in the mantle beneath the crust may lead to further such discoveries on how our planet works, generating a greater understanding of how the past may affect our geologic future," says Heron.

「未來對地殼之下的地函有何構造進行的探索，或許可以讓我們對地球的運作方式上有更多類似發現，並更加了解過去發生的事物會如何影響未來的地質活動。」 Heron說。

The research carries on the legacy of J. Tuzo Wilson, also a U of T scientist, and a legendary figure in geosciences who pioneered the idea of plate tectonics in the 1960's.

此篇研究傳承了 J. Tuzo Wilson。這位地質科學界的傳奇人物同樣是多倫多大學的科學家，他在1960年代便已率先倡導板塊構造學說。

"Plate tectonics is really the cornerstone of all geoscience," says Pysklywec. "Ultimately, this information could even lead to ways to help better predict how and when earthquakes happen. It's a key building block."

「板塊構造學說確實是地質科學各領域中的基石。」 Pysklywec說。「最終這些資訊甚至能讓我們更準確的預測地震會在何時及如何發生。這絕對會是至關重要的一塊拼圖。」

引用自：University of Toronto. "Deep 'scars' from ancient geological events play role in current earthquakes." ScienceDaily. ScienceDaily, 10 June 2016. 

南極出土的化石顯示恐龍滅絕時，就連南方大陸上的生物都無法倖免於難

原文網址：www.sciencedaily.com/releases/2016/05/160526091956.htm

Antarctic fossils reveal creatures weren't safer in the south during dinosaur extinction

南極出土的化石顯示恐龍滅絕時，就連南方大陸上的生物都無法倖免於難

A study of more than 6,000 marine fossils from the Antarctic shows that the mass extinction event that killed the dinosaurs was sudden and just as deadly to life in the polar regions.
一則對超過6000具從南極出土的海洋生物化石進行的研究顯示，突如其來消滅恐龍的大滅絕事件對極區生物而言同樣地致命。

Previously, scientists had thought that creatures living in the southernmost regions of the planet would have been in a less perilous position during the mass extinction event than those elsewhere on Earth.

之前科學家認為在大滅絕事件發生時，相較於地球上其他地方，居住在地球最南方的生物的處境應該沒那麼岌岌可危。

The research, published today in the journal Nature Communications, involved a six-year process of identifying more than 6,000 marine fossils ranging in age from 69- to 65-million-years-old that were excavated by scientists from the University of Leeds and the British Antarctic Survey on Seymour Island in the Antarctic Peninsula.

此篇刊登於今日《自然通訊》(Nature Communications)期刊的研究由里茲大學及英國南極調查所的科學家進行。在為期六年的研究過程中，他們從南極半島中的西摩島挖掘並辨識出超過6000具海洋生物的化石，它們的年代介於6900萬至6500萬年前。

This is one of the largest collections of marine fossils of this age anywhere in the world. It includes a wide range of species, from small snails and clams that lived on the sea floor, to large and unusual creatures that swam in the surface waters of the ocean. These include the ammonite Diplomoceras, a distant relative of modern squid and octopus, with a paperclip-shaped shell that could grow as large as 2 metres, and giant marine reptiles such as Mosasaurus, as featured in the film Jurassic World.

這是世界上對此年代的海洋化石進行的最大規模採集作業之一。他們蒐羅的物種種類可謂包羅萬象，從在海床上生活的小型海螺和蚌殼，到在海水表層泅游的罕見大型生物。後者包括Diplomoceras菊石，牠們是現代烏賊及章魚的古代遠親，身上迴紋針狀的甲殼可以長到2公尺之長；還有巨型海洋爬蟲類，像是電影「侏儸紀世界」中大放異彩的中龍。

With the marine fossils grouped by age, the collection shows a dramatic 65-70% reduction in the number of species living in the Antarctic 66 million years ago -- coinciding exactly with the time when the dinosaurs and many other groups of organisms worldwide became extinct at the end of the Cretaceous Period.

將這些海洋生物化石以年代進行分類後，這些樣品顯現出生活在南極的物種數目在6600萬年前急遽下降了65%至70%。這個時間點跟在白堊紀末期時，全世界恐龍以及許多種類的生物滅絕的時間點不謀而合。

James Witts, a PhD student in the University's School of Earth and Environment and lead author of the new research paper said: "Our research essentially shows that one day everything was fine -- the Antarctic had a thriving and diverse marine community -- and the next, it wasn't. Clearly, a very sudden and catastrophic event had occurred on Earth.

此篇新研究論文的第一作者，里茲大學地球與環境學院的博士生James Witts說：「大體而言，我們的研究成果描繪出的圖像顯示某天一切仍然如昔，南極擁有相當生氣蓬勃且多樣的海洋生物族群，但在下一刻一切都變了調。無庸置疑地，地球上發生了一件極具破壞力的突發事件。」

"This is the strongest evidence from fossils that the main driver of this extinction event was the after-effects of a huge asteroid impact, rather than a slower decline caused by natural changes to the climate or by severe volcanism stressing global environments."

「從化石得到的證據強烈指出這起滅絕事件的主因是大型小行星撞擊造成的後續效應，而非自然氣候變遷，或者是劇烈火山爆發導致全球環境惡化造成的緩慢衰亡。」

The study is the first to suggest that the mass extinction event was just as rapid and severe in the polar regions as elsewhere in the world.

這項研究首度提出如同世上其他地區，大滅絕事件在極區發生的速度也是相當地迅速且致命。

Previously, scientists had thought that organisms living near the Poles were far enough away from the cause of the extinction to be badly affected -- whether this was an asteroid impact in the Gulf of Mexico, where a giant buried impact crater is found today, or extreme volcanism in the Deccan volcanic province in India. Furthermore, it had been proposed that animals and plants in the polar regions would have been more resilient to global climatic changes associated with an asteroid impact as a result of living in environments that were always strongly seasonal. For example, life near the Poles has to adapt to living in darkness for half of the year and to an irregular food supply.

大滅絕事件的成因可能為小行星撞擊墨西哥灣，深埋於此的撞擊坑直到今日才被發現；另一個可能則是發生於印度德干火成岩省的超級火山噴發事件。不管為何者，先前科學家認為生活在極區的生物離這些災難夠遠，因此所受的影響不會那麼嚴重。此外，也有研究提出棲息在極區的動植物因為生活環境季節變化十分激烈，比方說半年的黑夜生活以及糧食供應不規則，因此能夠適應於此的生物比較能抵抗小行星撞擊引發的全球氣候變遷。

Professor Jane Francis from the British Antarctic Survey, a co-author of the study, said: "These Antarctic rocks contain a truly exceptional assemblage of fossils that have yielded new and surprising information about the evolution of life 66 million years ago. Even the animals that lived at the ends of the Earth close to the South Pole were not safe from the devastating effects of the mass extinction at the end of the Cretaceous Period."

英國南極調查所的共同作者Jane Francis教授說：「這些南極岩石中的化石組成真的相當特殊，我們從中獲得了6600萬年前令人驚訝的生物演化新資訊。在白堊紀末期，即便是生活在地球邊境，靠近南極的動物依然無法從大滅絕造成的毀滅效應中安全脫身。」

While some previous studies have suggested that the demise of the dinosaurs and other groups was gradual, many scientists argue that the dinosaur fossil record in particular is patchy, and cannot compete with marine fossils in terms of quantity and biodiversity.

雖然部分前人研究中認為恐龍以及其他種類的生物滅亡過程是漸進式的，許多科學家卻爭論說恐龍的化石紀錄特別零散，不論是生物多樣性以及保存品質都難以跟海洋生物化石相提並論。

James Witts said: "Most fossils are formed in marine environments, where it is easy for sediment to accumulate rapidly and bury parts of animals, such as bones, or bodies of creatures with a hard shell. For a dinosaur or other land animal to become fossilised, a series of favourable events are needed, such as for bones to fall into stagnant water and be buried rapidly to prevent decomposition, or be washed out to sea by rivers.

James Witts說：「大部分化石都形成在海洋環境當中。這是因為海洋中的沉積物很容易便迅速累積而埋藏生物遺骸的一部分，比方說骨頭或是有硬殼的部位。對於恐龍或是其他陸生生物而言，要成為化石必須發生一連串的有利事件，像是骨骸必須要落入缺氧水體中，接著被迅速埋藏而免於分解，或是被河水沖入海洋當中。」

"This means that marine fossils are generally much more abundant. They can give us a much larger data set for studying how ecosystems and biodiversity change over time in the geological past, and enable us to draw robust conclusions about events during periods of rapid environmental change, like mass extinctions."

「這代表海洋生物的化石數量一般來說是比較豐富的。它們可以給出更多資訊讓我們研究在地質史的過去，生態系和生物多樣性是如何隨著時間變化，而對像大滅絕之類的急遽環境變化期間發生了何種事件，我們也能依此得出更加穩健的結論。」

引用自：University of Leeds. "Antarctic fossils reveal creatures weren't safer in the south during dinosaur extinction." ScienceDaily. ScienceDaily, 26 May 2016.