埋在地下的隕石坑顯示出破碎的石頭如何像液體般流動
Kayla Zacharias
6600萬年前，一顆跟一座小城市差不多大的小行星撞上地球。這次撞擊不但終結了恐龍王朝，也在地球留下一道數公里深，超過180公里寬的傷疤。
由國際海洋鑽探計畫鑽出長達1.6公里的岩芯讓研究人員發現希克蘇魯伯隕石坑的形成過程。(圖片來源：國際海洋鑽探計畫)
位於墨西哥猶加敦半島之下的希克蘇魯伯隕石坑雖然被埋藏在10公里厚的岩石之下，但仍是地球保存最好的大型隕石坑。它也是地球唯一一個隕石坑在外緣內部還有一圈由碎石組成、如山峰般的環帶，稱作「環峰」(peak ring)的特徵。長久以來對於這樣的地形如何形成有許多討論。在《自然》(Nature )的一篇新研究認為它是撞擊後的關鍵幾分鐘內由於地面劇烈震動，使得岩石像液體般流動的產物。
當小行星撞上地球，一如預料的會留下一個碗型坑洞。但小行星不只會撞出凹痕，如果它的體積夠大，造成的隕石坑可能會超過30公里深，此時坑洞會變得不穩定而崩塌。
「破碎的岩石在短時間內可以像液體一樣。」普渡大學地球、大氣和行星科學的教授Jay Melosh表示。「之前科學家曾提出許多理論來解釋液化的發生機制，而我們認為是十分強烈的震動不斷晃動岩石，使得它們可以流動。」
此機制稱作「acoustic fluidization」，在小行星撞擊後的數分鐘之內因為此作用使得隕石坑內部堆起環型的山丘 (Melosh在1979首度提出這項理論)。在類地行星(地球、水星、金星、火星，也包括月球)上的隕石坑基本上都是相同的，但基於一個明顯的原因使它們難以從太空研究：我們無法以在地球上能達到的精細程度觀察它們。
希克蘇魯伯隕石坑經過6600萬年後已經被深埋在地底，因此也很難用標準的傳統方法來觀察。為此國際海洋鑽探計畫(國際大陸科學鑽探計劃之下的小組)進行它們唯一能做的事――鑽井。研究團隊從地底鑽出一個直徑大約15公分，長度1.6公里的岩芯，從中取得在這場消滅恐龍的撞擊事件中粉碎並部分熔化的岩石。
國際研究團隊探討岩芯裡破碎帶的分布及模式之後，它們發現震動隨著時序的演變過程使得岩屑可以流動。
「這項發現有助於我們了解撞擊坑的崩塌過程以及在其他情境中，像是山崩或地震發生時大量的岩石如何像液體一般的運動。」Melosh表示。「在夷平城鎮的巨型山崩中，本來認為自己是安全無虞的人們卻發現岩石如洪水般襲來。這是因為某些擾動讓夠多土石運動時而發生的行為。」
Melosh說隕石坑內部的崩塌跟恐龍的滅亡大概沒有直接關係，而是其他發生在隕石坑外面的影響所導致。但無論如何，瞭解地球上發生的大型小行星撞擊事件的後果相當重要。因為在類地行星上造成撞擊坑的作用是一樣的，所以這類發現也能證明太陽系各處撞擊坑的產生機制。
Underground crater reveals how broken rocks
can flow like liquid
Sixty-six million
years ago, an asteroid the size of a small city smashed into the earth. This
impact, the one that would lead to the end of the dinosaurs, left a scar
several miles underground and more than 115 miles wide.
Chicxulub, which lies underneath the Yucatán Peninsula of
Mexico, is the best-preserved large impact crater on Earth, although it’s
buried underneath a half mile of rocks. It’s also the only crater on the planet
with a mountainous ring of smashed rocks inside its outer rim, called a peak
ring. How these features form has long been debated, but a new study in Nature shows
they’re a product of extremely strong vibrations in the Earth that let rock
flow like liquid for a crucial few minutes after the impact.
When an asteroid crashes into the earth, it leaves a bowl-shaped
pit, just like you’d expect. But it doesn’t just leave a dent. If the asteroid
is big enough, the resulting crater can be more than 20 miles deep, at which point
it becomes unstable and collapses.
“For a while, the broken rock behaves as a fluid,” said Jay
Melosh, a professor of earth, atmospheric and planetary sciences at Purdue
University. “There have been a lot of theories proposed about what mechanism
allows this fluidization to happen, and now we know it’s really strong
vibrations shaking the rock constantly enough to allow it to flow.”
This mechanism, known as “acoustic fluidization,” is the process
that allows the ring of mountains in the crater’s center to rise within minutes
of the asteroid’s strike. (This idea was first proposed by Melosh in 1979).
Craters are essentially the same on all the terrestrial planets (Earth,
Mercury, Venus, Mars and our moon), but they’re hard to study in space for
obvious reasons: We can’t look at them with the same detail we can on Earth.
The Chicxulub crater isn’t easily accessible by traditional
standards either; it’s been buried throughout the last 66 million years. So,
the International Ocean Discovery Program (a group within the International
Continental Scientific Drilling Program), did the only thing they could – they
dug. The team drilled a core roughly six inches in diameter and a mile into the
Earth, collecting rock that was shattered and partly melted by the impact that
wiped out the dinosaurs.
In examining fracture zones and patterns in the core, the
international research team found an evolution in the vibration sequence that
would allow debris to flow.
“These findings help us understand how impact craters collapse
and how large masses of rock behave in a fluid-like manner in other
circumstances, such as landslides and earthquakes,” Melosh said. “Towns have
been wiped out by enormous landslides, where people thought they were safe but
then discovered that rock will flow like liquid when some disturbance sets a
big enough mass in motion.”
The extinction of the dinosaurs itself was probably not directly
affected by the crater’s internal collapse – other, external effects of the
impact did them in, Melosh said. Regardless, it’s important to understand the
consequences of a large asteroid strike on Earth. Because cratering is the same
on all the terrestrial planets, these findings also validate the mechanics of
impacts everywhere in the solar system.
原始論文：Ulrich Riller,
Michael H. Poelchau, Auriol S. P. Rae, Felix M. Schulte, Gareth S. Collins, H.
Jay Melosh, Richard A. F. Grieve, Joanna V. Morgan, Sean P. S. Gulick, Johanna
Lofi, Abdoulaye Diaw, Naoma McCall, David A. Kring. Rock fluidization
during peak-ring formation of large impact structures. Nature,
2018; 562 (7728): 511 DOI: 10.1038/s41586-018-0607-z
引用自：Purdue University. “Underground
crater reveals how broken rocks can flow like liquid.”
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