地質學家提出地球早期的陸地是由地函柱形成，而非板塊碰撞

 原文網址：https://phys.org/news/2025-07-geologists-early-continents-mantle-plumes.html

近日香港大學的地質學家對於25億多年前稱為太古宙的遠古時期當中，地球的早期陸地是如何形成的這道問題有了新的進展。他們發表在《科學前緣》(Science Advances)的發現提出早期的大陸地殼可能是由發生在地球深處，稱為地函柱的作用所形成，而非今日塑造陸地的板塊構造作用。

堅若磐石：研究得出新的機制來解釋大陸如何穩定下來

 原文網址：https://www.psu.edu/news/research/story/rock-steady-study-reveals-new-mechanism-explain-how-continents-stabilized/

by Matthew Carroll

一類古老、遼闊、稱為穩定地塊的大陸地殼幫助地球的陸地數十億年來，即便經歷了陸塊漂移、山脈隆起與海洋形成等作用，仍然處在相當穩定的狀態。距今大約30億年前穩定地塊是透過什麼作用形成，在地球歷史的相關研究中是個歷時已久的問題。最近賓州州立大學的科學家提出的新機制或許能夠加以解釋。

這些稱作片麻岩的古老變質岩發現於北極海海岸。現今出露在地表的這些岩石，過去曾是陸地的根部。科學家表示穿插在這類岩石當中的沉積岩，就像熱機一樣具有讓陸地穩定下來的功能。圖片來源：Jesse Reimink

研究人員揭露了第一塊真正的陸地來自哪種岩石

 原文網址：https://science.ubc.ca/news/researchers-uncover-source-rocks-first-real-continents

地球科學家揭露了陸地形成謎團中缺失的環節，他們修改後的起源故事不需要板塊構造運動開始運作，或是其他外來因素來解釋陸地的形成。反之，上周發表於《自然通訊》(Nature Communications)的發現，只需要在地球歷史最初數億年形成的海底高原，內部發生的地質內營力就可以解釋陸地的形成。

三種花崗岩類：英雲閃長岩(tonalite)、奧長花崗岩(trondhjemite)和花崗閃長岩(granodiorite)，合稱TTG。

40億年前的板塊構造運動或許協助了生命誕生

 原文網址：https://phys.org/news/2023-09-plate-tectonics-billion-years-life.html

by Hannah Bird , Phys.org

地球表層形成陸地的部分，或稱作地殼，最初大約是在40億年前形成，成分為25-50公里厚的玄武岩(一種火山岩)。科學家原先認為當時是由一塊完整的岩石圈地殼將整個地球包覆起來；相較之下我們今日所見各自分離的板塊，一般相信還要再過10億年才開始形成。然而，現在有越來越多科學家對此假說抱持質疑的態度。

史太古代的大陸地殼可能的形成機制：(a)重力沉降(重力造成密度較高的地殼沉入地函)；(b) 大陸地殼在聚合性板塊邊界層層堆疊，但密度仍低於下方的地函。(c)在聚合性板塊邊界發生的淺層隱沒作用。此研究支持模型b與c。圖片來源：Hastie et al., 2023與Nutman, 2023。

研究對於地球陸塊的出現原因提供了新的線索

 原文網址：https://www.si.edu/newsdesk/releases/study-presents-new-clues-about-rise-earths-continents

造成陸地出現的性質源自何方？新實驗顯示一種廣為接受的解釋其實不太可能

地球在太陽系的行星中是唯一適合居住的，原因之一是地球擁有陸塊。但驚人的是，對於地球表層這些巨大的裂片以及它們的特殊性質是如何產生這方面，我們卻所知不多。相較於海洋地殼，大陸地殼的鐵含量較低而氧化程度較高的問題，由Elizabeth Cottrell(史密森尼美國國立自然史博物館的岩石研究員暨地質學家)及主要作者Megan Holycross(之前為該博物館彼得•巴克獎學金與美國國家自然基金會獎學金補助的研究員，現為康乃爾大學的助理教授)進行的新研究測試了一項廣為接受的假說之後將其否決，也讓我們對於地殼有更深的理解。大陸地殼的成分中鐵含量稀少是主要的原因，造成地球表面許多地方可以高出海平面而形成陸地，使得今日所見的陸生生物得以出現。

本研究中一次實驗中的顯微鏡照片，裡面包含了玻璃質(橘色)、石榴子石(粉紅色)以及其他微小的礦物晶體。視野大小為410微米寬，跟一粒糖晶差不多。圖片來源：G. Macpherson and E. Cottrell, Smithsonian

國際研究對於陸地的形成過程有了新的見解

 原文網址：https://www.monash.edu/science/news/current/international-study-reveals-new-insight-into-how-continents-were-formed

由蒙納許大學的地質學家領導的國際研究，對於地球第一座陸地如何形成提出了新的見解。

地球或許比之前認為的還早就擁有讓大陸地殼、甚至是生命出現的條件

http://news.uchicago.edu/story/earth-could-have-supported-crust-life-earlier-thought

地球或許比之前認為的還早就擁有讓大陸地殼、甚至是生命出現的條件

科學家表示地殼可能在地球僅有3.5億歲的時候就形成了

By Louise Lerner

根據芝加哥大學領導的科學家團隊進行的研究顯示，早期地球開始適合生命居住的時間或許比過去認為的還要早上許多。

地球化學：地殼改變造成大氣氧化

地球化學：地殼改變造成大氣氧化

J. Elis Hoffmann

大氣中的氧對於複雜生命的存活與生長來說至關重要。在地球歷史中有將近一半的時光，地球大氣處於缺氧狀態。接著，於23至24億年前左右發生的大氧化事件中(Great Oxygenation Event ,GOE)氧氣含量急遽上升，開啟了大氣氧濃度逐步上升的趨勢。是什麼原因造成了大氧化事件時氧氣濃度飆升，至今仍未明瞭。Smit和Mezger在《自然―地質科學》(Nature Geoscience )撰寫的文章中，提出約莫在大氧化事件發生的10億年之前，上部大陸地殼總成分產生了改變，造成地表化學反應消耗的氧氣減少，使得大氣中的氧氣可以累積起來。

地球大氣中的氧氣大部分是經由植物(包括浮游植物)或藍綠菌進行的光合作用產生。因此大氧化事件或許跟藍綠菌族群增加有關。然而，在大氧化事件時藍綠菌已經能形成稱作「氧氣綠洲」的小規模有氧環境，所以它們演化出來的時間很可能比事件早上許多。較早之前的大氣氧氣增加可能會被海水和大陸的成分抵銷：像是從太古宙海底熱泉流入海洋的還原鐵，以及年輕地球特有鐵含量較豐富的基性地殼。任何出現在大氣中的自由氧都會跟這類還原物質反應，造成氧被吸收而阻止它們在大氣中累積。因此，只有地殼成分出現變化之後，氧氣才能在大氣中聚積。不過，妨礙大氣氧化的吸收過程其確切性質仍然眾說紛紜。

Smit和Mezger彙整出全球碎屑沉積物的化學成分資料庫，年代涵蓋了地球歷史大部分時間。它們利用稀有元素鉻和鈾的比例(以太古宙之後來自大陸地殼的沉積物當作現今平均值進行標準化)，重建出地殼礦物組成隨時間如何變化。具體來說，研究人員是利用了難熔礦物(refractory mineral)碎屑中鉻和鈾的分異作用彼此不同的特性。除非受到有氧風化作用而影響到溶解度，不然這兩種元素在岩石裡都是屬於不活動元素。有氧風化進行時，鉻會跟鉻鐵礦之類的礦物結合，鈾則會跟鋯石之類的礦物結合。但是，因為鋯石幾乎不會存在於基性或超基性地殼中，所以沉積物的鉻/鈾比可以用來指示源區成分為何。研究人員的觀察結果跟過去研究一致：在大約33至24億年前地殼成分發生了改變，上部大陸地殼的性質從基性―超基性岩為主轉變成酸性岩為主；礦物組成也從橄欖石和輝石為主，變成含有石英和長石之類的礦物。

Smit和Mezger根據他們建立的鉻/鈾資料庫，提出基性―超基性地殼中的礦物在熱水置換過程中會因為水合作用形成蛇紋石類礦物。此類礦物在進行脫氧作用時會把OH原子團納入自己的結晶構造並釋放出氫氣。這種反應造成母岩轉變成典型太古宙綠岩帶中出露的蛇紋岩(圖一)。由於蛇紋岩化會促成還原性物質產生，因此有蛇紋岩在的地表水鹼度可能會變得相當高並影響藍綠菌的生長環境，使其可以吸收細菌產生的氧氣。

圖一：西格陵蘭南方伊蘇阿綠岩帶的蛇紋岩，其年代超過37億年。

Smit and Mezger現在提出這種太古宙上部大陸地殼的常見成分或許能吸收氧氣，因而抑制早期地球大氣中的氧含量。(Source: Elis Hoffmann)

地殼從基性為主轉變成較為酸性可能跟全球地質作用的動力機制改變有關，從靜止蓋層(stagnant lid)的構造作用型態變成開始跟今日運行的板塊構造作用較為類似。隨著隱沒作用開始進行，基性上部地殼受到侵蝕並循環回地函，造成地殼中層演化程度較高的酸性地殼露出地表。基性地殼減少造成露出地表的蛇紋岩變少，使得蛇紋岩化作用再也無法成為大量消耗氧氣的機制，因此讓大氣的氧含量得以增加。

氧氣主要受到蛇紋岩化作用而消耗的模型跟過往提出的假說有所區隔。之前的研究提出藍綠菌族群擴大之後過一段時間大氣氧氣才上升的主因，是地表有許多類型的還原鐵可供氧氣進行還原作用。實際上，Smit和Mezger顯示在某些種類的岩石中鉻/鈾會跟成分有類似的變化，但它們的鐵含量卻都相同，意謂鐵含量並非控制大氣自由氧有多少的主要因素。此外，蛇紋岩化模型也允許太古宙晚期的上部大陸地殼成分存有某種程度的變異範圍。太古宙結束之際蛇紋岩對陸地逕流化學性質的影響逐漸淡去，或許造成海水上層部分處於有氧狀態，而海床之下發生的海洋地殼蛇紋岩化作用則讓太古宙之後的海水底層仍然處在缺氧狀態。

雖然蛇紋岩化作用確實是在大氧化事件發生前將還原作用的反應物帶到環境中的重要作用，但它並非唯一一種。其他可以把氧氣從大氣移除的重要消耗作用包括火山玻璃，基性與超基性岩中含硫礦物的風化；火山氣體變化、水下火山活動增加、以及海中熱泉噴出的鐵發生的沉澱反應。經過30億年之後，較為酸性的海床沉積物進入隱沒帶後可能會改變此處的氧化還原狀態，造成島弧跟陸弧火山噴出更多氧化氣體，而讓大氣進一步氧化。

Smit和Mezger顯示蛇紋岩化作用或許能緩衝太古宙地球的氧化還原狀態，減緩大氣氧氣的累積速率。研究人員現在應該要用數值模型來定量此種過程的重要性，並且跟硫同位素的紀錄比對――其為世上唯一涵蓋此年代的氧氣定量紀錄。如果太古宙時蛇紋岩化作用確實是相當重要的地質作用，則蛇紋岩的低密度可能也會對其他地質作用有深遠影響，像是早期地球的地殼循環過程和大陸地殼的穩定性。

Geochemistry: Oxygenation by a changing crust

Atmospheric oxygen is vital for the development and habitability of complex life. Earth's atmosphere lacked oxygen for nearly half of the planet's history. Oxygen levels then rapidly increased about 2.3 to 2.4 billion years ago, during the Great Oxygenation Event (GOE)^1, 2, 3, which initiated a stepwise rise in atmospheric oxygen concentrations¹. The reason for this increase during the GOE is unclear. Writing in Nature Geoscience Smit and Mezger⁴ propose that a change in the bulk composition of the upper continental crust about one billion years before the GOE would have caused a decrease in oxygen-consuming reactions at Earth's surface, allowing oxygen to build up in the atmosphere.

Most of Earth's atmospheric oxygen is produced via photosynthesis by plants (including phytoplankton) and cyanobacteria. The GOE is thus probably related to an increasing population of cyanobacteria. However, cyanobacteria are likely to have evolved much earlier than the GOE⁵, when they formed oxygen oases — small-scale oxidized environments. An earlier increase in atmospheric oxygen could have been mitigated by the composition of the oceans and continents^1,6, 7, 8: reducing agents such as reduced iron introduced by hydrothermal fluids into the Archaean oceans, as well as the more iron-rich mafic crust that characterized the younger Earth, may have reacted with any free oxygen, creating oxygen sinks that prevented its accumulation in the atmosphere. Only with a change in crustal composition could oxygen concentrate in the atmosphere. However, the precise nature of the sinks that hampered oxygenation of the atmosphere is ambiguous.

Smit and Mezger⁴ compile a global database of clastic sediment chemical compositions that covers most of Earth's history. Using the ratio of the trace elements Cr and U (normalized to the modern average value for post-Archaean sediment derived from continental crust), they reconstruct changes in crustal mineral composition through time. Specifically, the researchers take advantage of the different partitioning behaviours of Cr and U in refractory detrital minerals — both elements are immobile unless oxidative weathering influences the solubility. During oxidative weathering, Cr combines with minerals such as chromites, while U combines with minerals such as zircon. However, zircon is largely absent in mafic and ultramafic crust, so the Cr/U ratio can be used as a fingerprint of the composition of the source area for the sediments. In line with past work^6, 7, 8, the researchers observe a change in crustal composition about 3.3 to 2.4 billion years ago from a dominantly mafic–ultramafic upper continental crust that consisted mainly of minerals such as olivine and pyroxene, to a dominantly felsic upper crustal composition, comprising minerals such as quartz and feldspar.

Based on their Cr/U database, Smit and Mezger propose that minerals within the mafic–ultramafic crust were hydrated during hydrothermal alteration, forming serpentine minerals. Serpentines are a class of minerals that incorporate OH groups into their crystal lattice and release H₂ in oxygen-scavenging reactions⁴. These reactions transform the host rock into a serpentinite, which is found exposed in typical Archaean greenstone belts (Fig. 1). Since serpentinization triggers the production of reducing species⁴, surface waters in the presence of serpentinites would have been highly alkaline and could have influenced cyanobacterial habitats, acting as a sink for the bacteria-produced oxygen.

Figure 1: Serpentinites from the Isua Greenstone Belt of southern West Greenland, which is more than 3.7 billion years old.

Smit and Mezger⁴ now argue that these typical components of Archaean upper continental crust may have acted as an oxygen sink, suppressing oxygen levels in Earth's early atmosphere.   Source: Elis Hoffmann

The transition from a dominantly mafic to a more felsic crust was probably related to a changing style of global geodynamics, from a stagnant-lid tectonic regime towards the onset of plate tectonic processes similar to those active today⁹. With the onset of subduction, the upper mafic crust was eroded and recycled into the mantle, leaving the more evolved felsic crust from mid-crustal levels exposed at the surface of the continents. Removal of the mafic crust would reduce the amount of exposed serpentinites, minimizing serpentinization as an important sink for oxygen thereafter, and allowing oxygenation of the atmosphere.

The model of serpentinization as a primary oxygen sink stands independent from previous suggestions that the higher abundance of reduced iron species available for oxygen reduction was responsible for the delay in the rise of atmospheric oxygen following the expansion in cynobacterial populations. Indeed, Smit and Mezger show that the Cr/U ratios are similar in a number of rock types with variable compositions, yet the same iron abundance, implying that iron content was not the primary control on free oxygen availability. The serpentinization model also leaves some freedom for compositional heterogeneities within late Archaean upper continental crust. The dilution of the influence of serpentinites on the chemistry of continental runoff by the end of the Archaean may have led to partial oxidation of the upper ocean water column⁴, while sub-seafloor serpentinization in the oceanic crust kept the lower water column anoxic beyond the Archaean¹.

Although serpentinization is certainly an important process in introducing reduced reagents into the environment before the GOE, it is not the only process. Other key sinks for extracting oxygen from the atmosphere include weathering of volcanic glasses⁸, sulfur-bearing minerals in mafic and ultramafic rocks⁷, a change in volcanic gas composition¹⁰, increased subaerial volcanism¹¹, and hydrothermal Fe precipitation in seawater³. After three billion years, the subduction of more felsic ocean floor sediments may have changed the redox conditions in subduction zones, potentially leading to more oxidized gases being emitted at arc volcanoes, triggering further oxygenation⁷.

Smit and Mezger⁴ show that serpentinization may have buffered the redox conditions on the Archaean Earth, mitigating the accumulation of atmospheric oxygen. The importance of this process should now be quantified using numerical models and compared with the sulfur isotope record — the only quantitative record of oxygen on Earth across this time period². If serpentinization was indeed a critical process during the Archaean, the low density of serpentine may also have had a profound impact on geodynamic processes such as crustal recycling and stabilization of continental crust on the early Earth.

原始論文：Matthijs A. Smit and Klaus Mezger. Earth’s early O₂ cycle suppressed by primitive continents. Nature Geoscience, 2017.  doi:10.1038/ngeo3030.

引用自：J. Elis Hoffmann. Geochemistry: Oxygenation by a changing crust. Nature Geoscience, 2017. doi:10.1038/ngeo3038

https://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo3038.html

板塊構造：地殼循環的演化

原文網址：https://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo3015.html

板塊構造：地殼循環的演化

Valentina Magni

大陸地殼形成和循環的過程會隨著時間經過產生變化。數值模型顯示此過程從早期地球經由下部地殼剝離導致的大範圍循環，演變成今日藉著隱沒板塊斷裂形成的局部循環。

適當的混和方式才能調出大陸地殼

原文網址：http://www.geologypage.com/2017/05/continental-crust-thanks-right-mixture.html

適當的混和方式才能調出大陸地殼

蘇黎世聯邦理工學院的科學家首度利用電腦模擬，來成功重現出太古代大陸地殼的形成過程。這次模擬有助於科學家更加了解30至40億年前發生的地質作用。