對火山管道系統的最新觀點

原文網址：www.sciencedaily.com/releases/2017/07/170710122935.htm

對火山管道系統的最新觀點

將上升岩漿困在地殼中的「能階反轉」(reverse energy cascade)作用中的一部份，形成了像是優勝美地酋長岩之類的壯觀景色。

火山爆發顯示了岩漿在地殼中移動時具有的爆裂性質，比方說1980年的聖海倫火山爆發。現在地質學家則對優勝美地酋長岩這類的隆起花崗岩體十分感興趣，它們訴說了在噴發到地表之前，就在地底凝固的岩漿的故事。

這些稱作岩漿侵入體的花崗岩構造是在地下5到20公里處，少量岩漿一次又一次的冷卻並結晶，經年累月之下堆積而成。奧勒岡州立大學的Leif Karlstrom表示，它們同樣含有數千萬至數億年前岩漿貫入至地下的歷史。

在7月10日發表在期刊《自然―地質科學》(Nature Geoscience)線上版的論文中，由Karlstrom領導的三人研究團隊呈現出一種新的理論框架，可以用來瞭解在地表上看到的火山型態如何反映出地下岩漿管道系統的結構。據Karlstrom表示，有將近50%至90%的岩漿無法穿越地殼來到地表。

他說：「加州優勝美地谷和北喀斯開山脈的花崗岩地景具有由裸露岩石形成壯觀且高聳的峭壁。如果你看仔細一點，就會發現這類地景呈現出各式各樣的入侵體，記錄了不同次岩漿進入地殼的過程。希望我們的發現能讓你注視著酋長岩時以某些嶄新的觀點去理解它。」

在這篇由國家科學基金會支持的研究中，Karlstrom、南加州大學的Scott R. Paterson和英屬哥倫比亞大學的A. Mark Jellinek檢視了北美科迪勒拉山脈的火成岩侵入體大小如何分佈，長達10年以上的測量結果。

Karlstrom表示在活火山地區，像是喀斯開山脈、夏威夷和冰島，岩漿上升時通常呈現出薄層的席狀入侵體，一般稱作岩床和岩脈。它們的形成是因為岩漿在脆性的地殼中造成了破裂作用。然而，經過很長一段時間之後，此過程會產生變化。

研究人員表示這些變化是「能階反轉」過程中的一部份。「能階反轉」是上升的岩漿貫入體被困在地殼並喪失能量的過程。

當岩漿冷卻結晶時會跟周遭的岩石混和或結合在一起。岩漿反覆注入至地殼內部而喪失的熱量會持續加熱地殼岩石，造成之後使花崗岩入侵複合體堆積擴張的岩漿冷卻作用進行時，行為比較偏向黏性而非脆性。我們可以從今日露出地表的岩層中，看到最終形成的結構。

Karlstrom表示：「將熱能挹注至地殼的行為長期下來改變了當岩漿注入時地殼產生的力學反應特性。地殼就像是一座可以過濾上升岩漿的系統。岩漿是在地球深處產生，透過某些方式它們可以攜帶熱量和像是二氧化碳的揮發性物質來到地表，而此作用會如何進行則取決於地殼內部的岩漿運輸系統。」

據他表示，研究岩漿長期注入至地殼的背後過程，有助於科學家對火山、它們對全球氣候的影響和大型火山噴發可能發生的地點建構出更多的認識。

Karlstrom表示：「這篇論文切入了當今火山學研究中的一項主要問題。藉此我們可以做出相當有力的論述顯示深部的岩漿侵入活動和地表呈現的火山活動之間的關聯。我認為我們的發現提供了一個理論框架，可以用來瞭解地球或別顆星球上有關岩漿作用的其他問題。」

A fresh view on volcanic plumbing systems

Massive features such as Yosemite's El Capitan form as part of a 'reverse energy cascade' that traps rising magma in the crust

Volcanic eruptions such as Mount St. Helens' in 1980 show the explosiveness of magma moving through Earth's crust. Now geologists are excited about what uplifted granite bodies such as Yosemite's El Capitan say about magma that freezes before it can erupt on the surface.

These granite structures, called magmatic intrusions, form incrementally over time by small pulses of magma that cool and crystalize 5 to 20 kilometers -- 3 to 12 miles -- underground. Many are uplifted and exposed by erosion. They also contain the history of magma injections that occurred tens to hundreds of millions of years ago, says Leif Karlstrom of the University of Oregon.

In a paper placed online July 10 by the journal Nature Geoscience, a three-member research team led by Karlstrom unveiled a new framework for understanding what the pattern of volcanoes seen at the surface implies about the structure of subsurface magma plumbing systems. Some 50 to 90 percent of magma, Karlstrom said, doesn't get through the crust.

"Granitic landscapes in the Yosemite Valley in California and in the North Cascades are iconic, huge cliffs with exposed rock," he said. "If you look closer, the structure of that landscape shows all kinds of intrusive bodies that record different pulses of magma coming into the crust. Our findings hopefully allow you to stare up at El Capitan and make sense of it in some new way."

In the National Science Foundation-funded research, Karlstrom, Scott R. Paterson of the University of Southern California and A. Mark Jellinek of the University of British Columbia examined more than a decade of measurements of size distributions of igneous rock intrusions in the North American Cordillera.

Magma rising in active volcanic regions in places such as the Cascades, Hawaii and Iceland, Karlstrom said, often occurs as narrow, sheet-like intrusions commonly called dikes and sills. This occurs as a cracking process in brittle crustal rocks. Over long timescales, however, the process changes.

These changes are part of a transition into a "reverse energy cascade," in which rising magma injections become trapped and lose energy, the researchers say.

Magma mixes and merges with surrounding rocks as it cools and crystalizes. Heat lost from repeated injections of magma continues to heat crustal rocks, building and the expanding granitic intrusive complexes formed by frozen magma in a viscous, rather than brittle, manner. The resulting structures are seen today where the formations are exposed.

"That act of dumping heat into the crust over time changes the nature of the mechanical response to injections of magma," Karlstrom said. "Earth's crust is a filter for rising melts. You have magma that is generated deep in Earth, and somehow it gets to the surface carrying heat and volatiles, such as carbon dioxide. How that happens is through the crustal magma transport system."

Studying the processes behind magma injections over long timescales, he said, helps build better understanding of volcanoes, their impacts on global climate and where large volcanoes are likely to occur.

"This paper hits on one of the primary current research problems in volcanology," Karlstrom said. "We are able to make a strong statement about the connection of deep intrusive magmatism to the surface expression of volcanism. We think that what we found provides a framework for understanding other kinds of problems related to magmatism on Earth and other planets."

原始論文：Leif Karlstrom, Scott R. Paterson, A. Mark Jellinek. A reverse energy cascade for crustal magma transport. Nature Geoscience, 2017; DOI: 10.1038/ngeo2982

引用自：University of Oregon. "A fresh view on volcanic plumbing systems: Massive features such as Yosemite's El Capitan form as part of a 'reverse energy cascade' that traps rising magma in the crust." ScienceDaily. ScienceDaily, 10 July 2017.