與羅馬混凝土相似的火山岩解釋了為何義大利的火山臼能以破紀錄的速率抬升

原文網址：www.sciencedaily.com/releases/2015/07/150709144850.htm

Volcanic rocks resembling Roman concrete explain record uplift in Italian caldera

與羅馬混凝土相似的火山岩解釋了為何義大利的火山臼能以破紀錄的速率抬升

The discovery of a fiber-reinforced, concrete-like rock formed in the depths of a dormant supervolcano could help explain the unusual ground swelling that led to the evacuation of an Italian port city and inspire durable building materials in the future, Stanford scientists say.

史丹佛大學的科學家認為在已經休眠的超級火山深處形成的被纖維強化(fiber-reinforced)、類似混凝土的岩石，可以解釋造成義大利港都必須緊急撤離的不尋常地面隆起，並且對未來更耐久的建築材料有所啟發。

The "natural concrete" at the Campi Flegrei volcano is similar to Roman concrete, a legendary compound invented by the Romans and used to construct the Pantheon, the Coliseum, and ancient shipping ports throughout the Mediterranean.

發現於坎皮佛萊格瑞(Campi Flegrei)火山地區的「天然混凝土」與羅馬混凝土十分相似。由羅馬人發明的這種偉大材料被用來興建萬神殿、羅馬競技場與遍佈地中海地區的古代港口。

"This implies the existence of a natural process in the subsurface of Campi Flegrei that is similar to the one that is used to produce concrete," said Tiziana Vanorio, an experimental geophysicist at Stanford's School of Earth, Energy & Environmental Sciences.

「這代表說發生在坎皮佛萊格瑞地下的自然作用，跟一種混凝土的製程相當類似。」史丹佛大學地球、能源與環境科學學院(Stanford's School of Earth, Energy & Environmental Sciences)的實驗地球物理學家，Tiziana Vanorio說。

Campi Flegrei lies at the center of a large depression, or caldera, that is pockmarked by craters formed during past eruptions, the last of which occurred nearly 500 years ago. Nestled within this caldera is the colorful port city of Pozzuoli, which was founded in 600 B.C. by the Greeks and called "Puteoli" by the Romans.

坎皮佛萊格瑞位處一稱作火山臼(caldera)的大型低地中央。此地形內密佈著過去火山噴發時造成的火山口，最近的一次噴發發生在500年前。坐落於其中的波佐利(Pozzuoli)是一座繽紛的港都，在西元前600年由希臘人建造，被羅馬人稱為「Puteoli」。

Beginning in 1982, the ground beneath Pozzuoli began rising at an alarming rate. Within a two-year span, the uplift exceeded six feet-an amount unprecedented anywhere in the world. "The rising sea bottom rendered the Bay of Pozzuoli too shallow for large craft," Vanorio said.

1982年起波佐利的地面開始以令人擔憂的速率上升，僅僅兩年之間就抬升了超過六呎。在世上其他地方從未觀測到如此大的抬升量。「海底隆升導致波佐利灣變淺而無法讓大型船隻停泊。」Vanorio說。

Making matters worse, the ground swelling was accompanied by swarms of micro-earthquakes. Many of the tremors were too small to be felt, but when a magnitude 4 quake juddered Pozzuoli, officials evacuated the city's historic downtown. Pozzuoli became a ghost town overnight.

更糟的是，地表隆升還伴隨著許多微震(micro-earthquake)的發生。雖然大多數的震動都微弱的難以察覺，但一次規模4的地震撼動波佐利時，官方決定疏散歷史悠久的市中心。波佐利在一夜之間人去樓空。

A teenager at the time, Vanorio was among the approximately 40,000 residents forced to flee Pozzuoli and settle in towns scattered between Naples and Rome. The event made an impression on the young Vanorio, and inspired her interests in the geosciences. Now an assistant professor at Stanford, Vanorio decided to apply her knowledge about how rocks in the deep Earth respond to mechanical and chemical changes to investigate how the ground beneath Pozzuoli was able to withstand so much warping before cracking and setting off micro-earthquakes.

Vanorio在青少年時為被迫撤離波佐利，安頓在那布勒斯至羅馬之間許多小鎮的40,000名左右波佐利居民中的其中之一。這起事件深深烙印在Vanorio的腦海之中，啟發了她對地質科學的興趣。現為史丹佛大學助理教授的Vanorio，下定決心要以她對地球深處的岩石會如何因應力學與化學變化的知識，來探討為何波佐利的土地能夠在破裂並產生微震之前，承受這麼大幅度的彎曲。

"Ground swelling occurs at other calderas such as Yellowstone or Long Valley in the United States, but never to this degree, and it usually requires far less uplift to trigger earthquakes at other places," Vanorio said. "At Campi Flegrei, the micro-earthquakes were delayed by months despite really large ground deformations."

「在其他的火山臼，像是美國黃石公園或長谷(Long Valley)，地表也會有隆升現象，但從來不會到這種程度。其他地方通常抬升幅度遠小於它時就會開始發生地震了。」Vanorio說，「但在坎皮佛萊格瑞，儘管地表變形程度已經如此劇烈，微震卻還是晚了數個月才發生。」

To understand why the surface of the caldera was able to accommodate incredible strain without suddenly cracking, Vanorio and a post-doctoral associate, Waruntorn Kanitpanyacharoen, studied rock cores from the region. In the early 1980s, a deep drilling program probed the active geothermal system of Campi Flegrei to a depth of about 2 miles. When the pair analyzed the rock samples, they discovered that Campi Flegrei's caprock-a hard rock layer located near the caldera's surface-is rich in pozzolana, or volcanic ash from the region.

為了瞭解此火山臼的表面為何能夠處於那麼大的應變卻不會突然破裂，Vanorio與博士後研究員Waruntorn Kanitpanyacharoen研究了採集於此地的岩心。在1980年代早期，為了探查坎皮佛萊格瑞的活躍地熱系統，所進行的深層鑽探計畫曾鑽至地下約2哩深。這對科學家分析岩石樣品後，發現靠近火山臼地表的堅硬岩層，即坎皮佛萊格瑞的蓋岩(caprock)，富含在這個地區稱為pozzolana的火山灰。

The scientists also noticed that the caprock contained tobermorite and ettringite-fibrous minerals that are also found in humanmade concrete. These minerals would have made Campi Flegrei's caprock more ductile, and their presence explains why the ground beneath Pozzuoli was able to withstand significant bending before breaking and shearing. But how did tobermorite and ettringite come to form in the caprock?

科學家同時發現蓋岩含有雪矽鈣石(tobermorite)與鈣礬石(ettringite)，為可在人類製造的混凝土中發現的纖維狀礦物。這些礦物造成坎皮佛萊格瑞的蓋岩更具塑性，解釋了為何波佐利的地面能在破裂與剪動之前承受這麼大的彎曲。然而雪矽鈣石與鈣礬石要如何形成於蓋岩之中？

Once again, the drill cores provided the crucial clue. The samples showed that the deep basement of the caldera-the "wall" of the bowl-like depression-consisted of carbonate-bearing rocks similar to limestone, and that interspersed within the carbonate rocks was a needle-shaped mineral called actinolite.

再一次地，岩心提供了重要的線索。樣品顯示火山臼的深部基岩-即這個碗形陷落的碗壁-含有類似石灰岩的含碳酸鹽岩石，且有一種稱作陽起石(actinolite)的針狀礦物散佈在這些碳酸鹽岩石中。

"The actinolite was the key to understanding all of the other chemical reactions that had to take place to form the natural cement at Campi Flegrei," said Kanitpanyacharoen, who is now at Chulalongkorn University in Thailand.

「瞭解坎皮佛萊格瑞形成天然混凝土的所有必要化學反應的關鍵在於陽起石。」Kanitpanyacharoen說，她現在任職於泰國的朱拉隆功大學(Chulalongkorn University)。

From the actinolite and graphite, the scientists deduced that a chemical reaction called decarbonation was occurring beneath Campi Flegrei. They believe that the combination of heat and circulating mineral-rich waters decarbonates the deep basement, prompting the formation of actinolite as well as carbon dioxide gas. As the CO₂ mixes with calcium-carbonate and hydrogen in the basement rocks, it triggers a chemical cascade that produces several compounds, one of which is calcium hydroxide. Calcium hydroxide, also known as portlandite or hydrated lime, is one of the two key ingredients in humanmade concrete, including Roman concrete. Circulating geothermal fluids transport this naturally occurring lime up to shallower depths, where it combines with the pozzolana ash in the caprock to form an impenetrable, concrete-like rock capable of withstanding very strong forces.

由岩石中發現的陽起石及石墨，科學家推論坎皮佛萊格瑞下方發生了一種稱作脫碳酸鹽化(decarbonation)的化學反應。她們認為高熱與富含礦物的流水的共同作用使深部基岩脫碳酸鹽化，促進了陽起石及二氧化碳氣體的形成。當二氧化碳與基岩中的碳酸鈣及氫氣混合，發生的一連串化學反應會形成數種化合物，其中之一為氫氧化鈣。氫氧化鈣又稱作氫氧鈣石(portlandite)或熟石灰(hydrated lime)，為製造包含羅馬混凝土在內的人造混凝土的兩種關鍵材料之一。流動的地熱流體會將這些天然石灰往上運至淺部，在那裡它們會與蓋岩中的火山灰結合，而形成可以承受相當高應力、類似混凝土的堅固岩石。

"This is the same chemical reaction that the ancient Romans unwittingly exploited to create their famous concrete, but in Campi Flegrei it happens naturally," Vanorio said.

「羅馬人在不自覺的情況下利用了同樣的化學反應製造出他們舉世聞名的混凝土，但在坎皮佛萊格瑞這完全是自然發生的。」Vanorio說。

In fact, Vanorio suspects that the inspiration for Roman concrete came from observing interactions between the volcanic ash at Pozzuoli and seawater in the region. The Roman philosopher Seneca, for example, noted that the "dust at Puteoli becomes stone if it touches water."

事實上，Vanorio猜測啟發羅馬混凝土的靈感，便是來自觀查此區海水跟波佐利火山灰之間的反應。舉例來說，羅馬哲學家西尼加(Seneca)就曾經注意到「波佐利的沙石若跟海水接觸就會變成石頭。」

"The Romans were keen observers of the natural world and fine empiricists," Vanorio said. "Seneca, and before him Vitruvius, understood that there was something special about the ash at Pozzuoli, and the Romans used the pozzolana to create their own concrete, albeit with a different source of lime."

「羅馬人不但是對自然界相當敏銳的觀察者，同時也是優秀的經驗主義者。」Vanorio說。「西尼加與在他之前的維特魯威(Vitruvius)都瞭解到波佐利的火山灰必定有特殊之處，而羅馬人利用火山灰調製出他們自己的混凝土，儘管他們用的是另外一種不同的石灰來源。」

Pozzuoli was the main commercial and military port for the Roman Empire, and it was common for ships to use pozzolana as ballast while trading grain from the eastern Mediterranean. As a result of this practice, volcanic ash from Campi Flegrei-and the use of Roman concrete-spread across the ancient world. Archeologists have recently found that piers in Alexandria, Caesarea, and Cyprus are all made from Roman concrete and have pozzolana as a primary ingredient.

波佐利在羅馬帝國時期為主要的商業及軍事用港，他們在跟東地中海地區交易榖物時，常常用火山灰當作壓艙物。由於這種習慣，來自坎皮佛萊格瑞的火山灰，以及羅馬混凝土的使用傳遍了整個古代世界。考古學家最近發現亞力山卓(Alexandria)、凱撒利亞(Caesarea)與賽普勒斯(Cyprus)的港口皆是以羅馬混凝土建成，而其中的主要原料即為火山灰。

Interestingly, the same chemical reaction that is responsible for the unique properties of the Campi Flegrei's caprock can also trigger its downfall. If too much decarbonation occurs-as might happen if a large amount of saltwater, or brine, gets injected into the system-an excess of carbon dioxide, methane and steam is produced. As these gases rise toward the surface, they bump up against the natural cement layer, warping the caprock. This is what lifted Pozzuoli in the 1980s. When strain from the pressure buildup exceeded the strength of the caprock, the rock sheared and cracked, setting off swarms of micro-earthquakes. As pent-up gases and fluids vent into the atmosphere, the ground swelling subsided. Vanorio and Kanitpanyacharoen suspect that as more calcium hydroxide was produced at depth and transported to the surface, the damaged caprock was slowly repaired, its cracks "healed" as more natural cement was produced.

有趣的是，造成坎皮佛萊格瑞蓋岩獨特性質的化學反應，卻也能破壞它。如果脫碳酸鹽化反應太多，這可能發生於當有太多鹽水，或者鹵水灌入此系統時，就會產生過量的二氧化碳、甲烷與水蒸汽。隨著這些氣體往上遷移，它們會抬高這塊自然水泥層，造成蓋岩拱起。這即為1980年代造成波佐利隆升的原因。當壓力累積造成的應力超過蓋岩的強度，蓋岩便會破裂並剪動，引發許多微震。隨著封閉在地下的氣體與液體逐漸逸散至大氣，地面又會開始下沉。Vanorio與 Kanitpanyacharoen推測當深處產生的氫氧化鈣被運至表層時，會逐漸修復被破壞的蓋岩。也就是隨著天然混凝土的形成，蓋岩中的裂痕也會慢慢「癒合」。

Vanorio believes the conditions and processes responsible for the exceptional rock properties at Campi Flegrei could be present at other calderas around the world. A better understanding of the conditions and processes that formed Campi Flegrei's caprock could also allow scientists to recreate it in the lab, and perhaps even improve upon it to engineer more durable and resilient concretes that are better able to withstand large stresses and shaking, or to heal themselves after damage.

Vanorio認為造成坎皮佛萊格瑞岩石獨特性質的環境條件與過程，也會出現在其他世界各地的火山臼之下。對坎皮佛萊格瑞蓋岩的環境條件與形成過程瞭解得更加深入，有助於科學家在實驗室中複製整個條件及過程，甚至能讓工程師改良出更耐久、更有彈性的混凝土，其能夠承受更大的應力與震動，或在受損後能自我修復。

"There is a need for eco-friendly materials and concretes that can accommodate stresses more easily," Vanorio said. "For example, extracting natural gas by hydraulic fracturing can cause rapid stress changes that cause concrete well casings to fail and lead to gas leaks and water contamination."

「現在需要的是能更輕易承受應力的對生態友善材料與混凝土。」Vanorio說，「舉例來說，利用水力壓裂法抽取天然氣時，造成的應力迅速變化可能會破壞混凝土製的氣井套管，導致氣體洩漏及地下水汙染。」

引用自：Stanford's School of Earth, Energy & Environmental Sciences. "Volcanic rocks resembling Roman concrete explain record uplift in Italian caldera." ScienceDaily. ScienceDaily, 9 July 2015.