A Perfect Day

A two-week cruise on an icebreaker to the top of the world last summer gave scientists a look at the aftermath of an event once thought impossible: a violent volcanic eruption on the deep-sea floor.

In 1999, a global network of seismic instruments detected the largest swarm of earthquakes ever to occur along the planet’s system of mid-ocean ridges, where tectonic plates spread to form new ocean crust. Several aspects of the recorded vibrations suggested that the quakes were generated by volcanic activity, says Robert A. Reves-Sohn, a geophysicist at the Woods Hole Oceanographic Institution in Massachusetts.

However, he notes, many scientists have doubted that explosive volcanism can take place at the 4,000-plus-meter depth where these quakes occurred because the immense pressure of overlying water prevents seawater from flashing into steam, a major driving force for such eruptions.

The source of the quakes was the Gakkel Ridge, a mid-ocean ridge that runs along the bottom of the Arctic Ocean. Sonar scans at a stretch of the ridge about 500 kilometers from the North Pole revealed several distinctive volcanic features, says Reves-Sohn. The largest of these undersea features, which usually have flat tops scarred with prominent central craters, are about 2 kilometers across and a few hundred meters tall.

Images gathered by a remotely operated vehicle show that the ocean floor is blanketed by layers of loose volcanic ash up to 10 centimeters thick. This material is piled on top of rocks and other high-standing features on the ocean floor, a sign that the jagged, glassy particles of ash — each typically measuring no more than a couple of millimeters across — gently rained down upon the ocean floor rather than sweeping down the flanks of the undersea volcanoes, Reves-Sohn says.

He and his colleagues don’t know the full extent of the volcanic deposits, but they did find ash in all parts of the 5-by-10-kilometer area that they surveyed, they report in the June 23 Nature.

The size and shape of the larger particles hint that one of the area’s undersea volcanoes spewed 1-kilometer-tall fountains of lava during an explosive eruption. When that molten material hit the near-freezing seawater, it quickly chilled into golf-ball-size chunks and then fractured into tiny bits that rained to the seafloor, Reves-Sohn speculates. Many of the ash bits are jagged, thin, Christmas-ornament-like fragments of glass, a testament to the violence of the eruption and the bubbles contained in the molten material.

Because steam couldn’t have driven the eruption, the volcano must have been fueled by another volatile component of the magma, the researchers say. The most likely culprit, says Reves-Sohn, is carbon dioxide. The amount of gas needed to fuel a deep-sea eruption like the ones that occurred along the Gakkel Ridge, however, is about 100 times the amount normally found dissolved in molten rock, he notes.

The tectonic plates at most mid-ocean ridges spread apart about 30 millimeters each year, around the same rate at which a fingernail grows. However, the Gakkel Ridge is an ultra-slow spreading center where the plates diverge only half that fast. Whereas volcanic eruptions in many shallow seafloor locales may occur every 10 years or so, eruptions at deep-sea, slow-spreading centers may happen only once every 10,000 years or so, Reves-Sohn speculates. If so, sufficient reservoirs of carbon dioxide can easily build up in the magma chambers beneath the undersea peaks. 

 http://tech.big5.enorth.com.cn/system/2008/06/30/003473292.shtml

　　在6月26日出版的最新一期《自然》雜誌上，美國科學家公布他們的這項最新成果。美國科學家稱，海底火山一般隱藏於北極冰層以下4000公尺處，噴發直徑最大約為2000公尺，噴射高度甚至高達幾百米。這些火山大多位於加凱爾海底山脈及周邊區域。海底山脈俗稱『洋中脊』，著名的加凱爾海底山脈是北冰洋中綿延最長的『洋中脊』，在北極冰層下延伸約1800公里。目前，科學家們普遍認為，由於洋底殼斷裂層造成兩大地殼板塊分離運動，導致底層熔岩噴發上湧形成海底火山現象，熔岩進而冷卻即形成所謂的『洋中脊』。然而，由於巨大的海底壓力，使得火山的氣體和岩漿一般很難爆發出來。

　　由於加凱爾洋中脊是地球上擴張速度最慢的海洋山脊，此前人們對他的研究相對較少。近期的一些發現，終於引起了科學家對它的關注。美國馬薩諸塞州伍茲霍爾海洋研究所科學家羅伯特-裡弗斯-索恩近期於北冰洋底火山口附近發現許多散落的參差不齊的玻璃狀碎片。這些碎片表明，這些火山可能於1999年至2001年間曾經爆發過。羅伯特因此猜測，加凱爾洋中脊的緩慢擴張，導致洋底殼之下的岩漿中匯聚大量的氣體而無法排出。當氣壓超過一定極限時，必將以火山爆發形式釋放多餘的氣體。正如拔去瓶蓋的香檳酒一樣，氣體隨岩漿噴射而出。

　　裡弗斯-索恩稱，北極海底火山噴發是岩漿等噴出物在短時間內從火山口向地表的釋放。由於岩漿中含大量揮發分，加之上覆岩層的圍壓，使這些揮發分溶解在岩漿中無法溢出，當岩漿上昇靠近地表時，壓力減小，揮發分急劇被釋放出來，於是形成火山噴發。火山噴發是一種奇特的地質現象，是地殼運動的一種表現形式，也是地球內部熱能在地表的一種最強烈的顯示。美國的科學家早些時候曾在北極地區發現了不太深的火山灰死層，認為其形成的原因是發生在此前的一次超級火山噴發，結果是至今這裡的海底還可以見到一些漏斗形的大坑，它們都是那些毀滅性火山噴發後形成的火山口。

　　最近，又有消息稱北極冰層正在以史無前例的速度消融，甚至有科學家預言北極冰層將於今夏全部消失。北極冰面每年夏季都會融化，一般在9月份變得最薄，然後在漫長的冬季裡重新結冰，並於次年3月份達到最大厚度。受全球變暖影響，北極冰面近年來在夏季融化得越來越多，導致越來越多的陳年『老冰』流失到北極以外地區。因此，人們也許會立即聯想到海底火山，這些火山正如一個個巨大的海底煙囪，烘烤著北冰洋。但是，科學家們並未從中找到明顯的必然聯系。羅伯特認為，海底火山並不會對洋面冰層直接構成威脅。但是火山卻肯會對於北冰洋的水溫產生影響。火山噴發產生大量的二氧化碳、氦氣、痕量金屬和熱量，勢必會造成海水溫度的昇高。