火山學：海水減少，岩漿增多

原文網址：https://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo3040.html

火山學：海水減少，岩漿增多

減壓作用是地函發生部分熔融的主要方式之一。一般來說，在中洋脊下方或者隱沒帶後方，地函岩石上湧的時候便會發生此種熔融現象。對此作用一項合理的推論是，岩石感受到的壓力變化或許可以無關於穩定而緩慢的地函對流，而是施加於地球表面的壓力發生迅速變化。過往科學家曾特別提出冰層後退或是海平面變動導致的壓力變化，可以調節大範圍火山系統產生與噴發的岩漿多寡。然而，它在不同地體構造的影響規模有多大，以及在地質紀錄中會留下什麼樣的痕跡，卻都還是爭論中的議題。Pietro Sternai和其同僚在《自然―地質科學》撰寫的論文中，呈現出的證據指出大約600萬年前的墨西拿鹽度危機(Messinian salinity crisis)期間，一部分地中海蒸發的同時火山活動也跟著增加。他們提出當時海平面降低數公里造成地函受到減壓作用影響，是最能解釋岩漿活動為何激增的原因。

科學家長久以來認為全球水圈變化連帶使地表承受的荷重發生改變，可以從上往下擾動地函受到的壓力，使得減壓熔融發生。舉例來說，厚達2公里的冰層在2000年內消失可以迅速減輕地函受到的壓力，其效率是地函持續上湧有關的減壓作用的十倍以上。此效應能解釋冰島在10000至8000年前記錄到的火山活動增加現象。類似地，科學家現在認為冰河期―間冰期循環中，跟海平面動盪有關的壓力變化，可以對中洋脊產生的熔岩量有10%左右的影響。

Sternai等人探討墨西拿鹽度危機對岩漿活動造成的作用，而把全新觀點帶入這項議題之中。墨西拿鹽度危機事件標記了一段時期，當時地中海大量蒸發並在盆地內部堆積大量鹽分。究其原因為直布羅陀海峽關閉，但海平面大幅下降也可能牽涉其中。地中海區域的地體構造相當複雜，原因為非洲板塊和歐亞板塊在此聚合而產生了許多小型隱沒系統，提供底下的高溫地函許多可以上湧、熔融，然後在地表點燃火山活動的空間。不過，彙整火山活動發生時間的現有定年數據之後，研究人員指出在560萬年前墨西拿鹽度危機期間，這些火山系統發生岩漿噴發和入侵事件的數目變得異常地多。

這段火山活動高峰期大多數發生在地中海盆地邊緣的火山。Sternai等人提出其為下方地函因為海平面變化導致的減壓作用而產生的反應。為了建立兩者之間關聯的可信度，他們首先計算以公里為單位的海平面下降對地表荷重造成的改變有多大，接著確認這些異常火山活動的岩漿庫和來源所處的地函和地殼，其中是否有部分能感受到此荷重變化。雖然跟全球海平面變化對中洋脊造成的效應比起來，此荷重變化顯得微小許多，但是發生在地中海中央附近的減壓作用仍然可以透過間接方式影響遠處位在盆地邊緣的火山系統。地質動力學的模擬結果顯示出如果應力變動的幅度夠大且速度夠快，甚至還能透過黏彈性的岩石圈和上部軟流圈傳遞到距地中海海岸數百公里遠的內陸地區。

圖一：墨西拿鹽度危機期間造成地中海火山活動增加的機制

(i) 地中海盆地在大約570至550萬年前發生的大規模蒸發現象和海平面迅速下降，可以減少地殼和下方地函受到的壓力。(ii) 地函減壓導致部分熔融。(iii) 另一方面，地殼減壓的同時產生拉伸，讓更多岩漿以岩脈的形式往地表移動。(iv) 最終導致地表火山活動增加。Sternai和其同僚利用數值模型顯示這一連串的事件，可以解釋地中海盆地的火山活動為何在大約560萬年前出現一段異常高峰期。

模型也測試了不同的負載路徑，也就是岩石圈因為鹽分沉澱而受到的壓力增加，以及蒸發導致水位下降而造成的壓力減輕，兩者的綜合效應。模型顯示唯有快速且大規模的海平面下降可以對岩漿來源地區和岩漿庫受到的壓力產生夠大的影響，這和墨西拿鹽度危機與海平面劇烈下降有密切關係的說法一致。他們提出發生在地表的減壓作用不只能提升地函的熔融作用，同時也減少地殼受到的應力，有利於岩漿發生岩脈入侵作用。遺憾的是，可以同時處理剛性地殼變形和岩漿在其中如何移動的地質動力學自洽(self-consistent)模型還很稀少，且額外產生的熔融物質到達地表需要多少時間也還沒有確定下來。若要精確定量岩漿傳輸和入侵對瞬變作用力的敏感度，還需要發展出更先進的模型基礎。

要辨識出從地表傳遞至下方的力量對岩漿活動的調節作用還有其他難處，像是岩漿活動的時間序列有所缺失或是取樣可能出現偏差。由於我們只能研究我們能取得的地質紀錄，因此要降低這些爭議並不容易。另外，要在有許多內部變因的複雜系統當中辨識出特定來源的外在變因——在此例中為減壓作用——也格外具有挑戰性。研究墨西拿鹽度危機的優勢之一是，它就像是一起突發事件，發生在其他對岩漿活動有影響的地質作用力大都可視為平穩狀態的時期。Sternai和同僚採用的方法為彙整大範圍地區的數據，以辨識出整個泛地中海地區不同火山事件的共時性，這讓他們可以宣稱其發生原因並非個別岩漿系統本身具有的波動。要進一步證實這些說法需要對其背景特性有通盤瞭解；找出地中海整區未受影響下的岩漿產量；並納入減壓熔融以外的作用來解釋不同的熔岩產生模式。

Sternai和共同研究人員顯示墨西拿鹽度危機期間迅速且大規模的海平面下降可能會減少地函受到的壓力，而促進岩漿生成並在地表引發更頻繁的火山活動。他們提出的關係可以激勵研究人員蒐集高解析度的野外數據來更加精確地定年地中海的火山活動，並且發展新方法建構岩石圈—岩漿的耦合動力學模型。地表荷重對岩漿活動的影響在地質動力學中仍然是倍受爭議的問題。認識此交互作用背後運行的物理機制的重要性不只是能讓我們更加瞭解岩漿系統，也可以利於我們評估當地表發生變化火山活動會有何反應，這或許能讓我們找出全球氣候系統當中尚未發現的反饋作用。

Jean-Arthur Olive

Volcanology: When less water means more fire

Decompression is the primary way to partially melt Earth's mantle. Such melting occurs routinely as mantle rocks upwell beneath a mid-ocean ridge or behind a subduction zone. A corollary of this process is that rocks may also feel changes in pressure that have nothing to do with slow and steady mantle convection, but instead relate to rapid shifts in the loads that weigh on Earth's surface. In particular, pressure changes due to ice-sheet retreat or sea-level variability have been proposed to modulate the amount of magma produced and erupted in a wide range of volcanic systems^{1, 2, 3, 4}. The magnitude of this modulation across different geodynamic settings, and its signature in the geological record, however, remain a matter of debate^{5, 6}. Writing in Nature Geoscience, Pietro Sternai and colleagues⁷ present evidence for enhanced volcanism coincident with partial evaporation of the Mediterranean Sea during the Messinian salinity crisis, about six million years ago. They argue that this boost in magmatism is best explained by decompression of the mantle caused by a kilometre-scale drop in sea level at that time.

It has long been thought that rapid shifts in surface loads due to changes in the global hydrological cycle could perturb mantle pressure from the top down, causing decompression melting. For example, the removal of an ice sheet approximately 2-km thick within about 2,000 years would decompress the mantle ten times faster than decompression associated with steady mantle upwelling. This effect can account for the increase in volcanic productivity documented in Iceland between 10,000 and 8,000 years ago^{1, 2}. Similarly, pressure changes associated with glacial/interglacial sea-level fluctuations are now thought to modulate the magma supply at mid-ocean ridges by about 10% (refs 4,5).

Sternai et al.⁷ bring an original perspective into this discussion by considering the magmatic consequences of the Messinian salinity crisis. This crisis marked an episode of massive evaporation and salt deposition in the Mediterranean basin that was triggered by the closure of the Gibraltar Straight, and was potentially associated with spectacular sea-level drop⁸. The Mediterranean region is tectonically complex because the convergence between the African and Eurasian plates has created several small-scale subduction systems⁹ that provide numerous opportunities for hot mantle to ascend, melt and fuel volcanism at the surface. However, using a compilation of existing age data that constrain the timing of volcanism, the researchers show that an anomalously high number of eruptive and intrusive events occurred in these systems during the Messinian salinity crisis, around 5.6 million years ago.

Sternai et al.⁷ propose that this pulse in volcanism, which occurred mostly along the edges of the Mediterranean basin, was a response to sea-level-driven decompression of the underlying mantle. To convincingly establish this connection, they first calculate the magnitude of the change in surface load associated with a kilometre-scale sea-level drop, and then determine whether this change in load would have been felt in parts of the mantle and crust that sourced and stored the anomalous volcanism. Although the latter may seem trivial when considering the effect of global sea-level change on a mid-ocean ridge, it is not straightforward that unloading near the centre of the Mediterranean Sea would have consequences for volcanic systems located far away from the centre, along the edge of the basin (Fig. 1). Geodynamic modelling, however, reveals that if the stress fluctuations were sufficiently large and rapid, they could have been transmitted through the visco-elastic lithosphere and upper asthenosphere, hundreds of kilometres inland beyond the Mediterranean shores.

Figure 1: Mechanisms for enhanced volcanism in the Mediterranean during the Messinian salinity crisis.

Large-scale evaporation and rapid sea-level fall in the Mediterranean basin about 5.7 to 5.5 million years ago would have unloaded the crust and underlying mantle (i). Mantle decompression leads to partial melting (ii), whereas crustal unloading and extension allows more magma to migrate towards the surface via dykes (iii), generating increased volcanism at the surface (iv). Sternai and colleagues⁷ use a numerical model to show that this sequence of events could explain an anomalous pulse of volcanism in the Mediterranean basin about 5.6 million years ago.

Various loading paths, representing scenarios of salt precipitation that loads the lithosphere and evaporative drawdown that unloads the lithosphere, were tested in the models. Only a rapid and large magnitude sea-level drop can exert a sizeable pressure modulation on the magma production and storage areas, which is consistent with the idea that the Messinian salinity crisis was associated with significant sea-level change. Unloading at the surface is proposed to not only generate additional melt in the mantle, but also to promote volcanic eruptions because reduced stress in Earth's crust creates conditions that are favourable for initiating magma-filled dyke intrusions. Unfortunately, geodynamic models that self-consistently handle deformation and melt transport through the brittle lithosphere are rare¹⁰, and the time necessary for any extra melt to reach the surface is debated. To accurately quantify the sensitivity of magma transport and extrusion to transient forcings warrants the development of novel modelling frameworks.

Additional difficulties in identifying top-down controls on magmatism include incomplete preservation and potential sampling bias in the time-series of magmatic activity. Mitigating these issues is difficult to do because we can only work with the geological record that is available to us. Identifying a particular source of external variability — in this case surface unloading — in a complex system with multiple sources of internal variability is especially challenging. One advantage of studying the Messinian salinity crisis is that this was a sudden pulse-like event during which other geodynamic forcings on magmatic activity can be considered mostly steady. Sternai and colleagues adopt a large-scale data collection approach to identify synchronicity in volcanic events across the entire pan-Mediterranean region, which argues against intrinsic fluctuations specific to each magmatic system. To further test these ideas will require a thorough characterization of the background, unperturbed magma output of the Mediterranean domain, and to account for the diversity of melt-generation modes beyond just decompression melting.

Sternai and co-workers⁷ show that rapid and large-scale sea-level fall during the Messinian salinity crisis could have decompressed the mantle, boosted magma production and increased surface volcanism. This proposed link will motivate the collection of high-resolution field data that better constrain the timing of volcanism in the Mediterranean, along with the development of novel approaches for coupled lithosphere–magma dynamics. The influence of surficial loading on magmatic activity remains a contentious question in geodynamics. Knowledge of the physical processes at play during such interactions is not only critical for our understanding of magmatic systems, but also because appreciation of the volcanic response to changes at Earth's surface may allow the identification of previously unrecognized retro-actions in the global climate system³.

原始文章：Jean-Arthur Olive. Volcanology: When less water means more fire, Nature Geoscience, 2017. doi:10.1038/ngeo3040