科学美国人60秒 SSS 巨大冲击引发火山喷发

Large Impacts May Cause Volcanic 1 Eruptions 2 巨大冲击引发火山喷发

Other rocky planets in our solar system show a common feature: within giant craters 4 caused by impacts there’s evidence of volcanic activity. Which made scientists wonder—can big impacts actually cause volcanic eruptions? And has that scenario 5 ever happened here? To find out, scientists went to one of the few massive craters on Earth not erased 6 by plate tectonics: the Sudbury crater 3 in Canada.

太阳系中其他岩质行星拥有一个共同特征：由冲击形成的巨大陨石坑内都存有火山活动的证据。因而科学家们不禁怀疑——巨大的冲击真的会导致火山爆发吗？地球上曾经发生过这种情况吗？为找到答案，科学家们来到地球上为数不多的、没被板块构造消除的巨大陨石坑之一——加拿大的萨德伯里陨石坑。

“Sudbury is a 1.85-billion-year-old impact structure.”

“萨德伯里是一个拥有18.5亿年历史的冲击构造。”

Teresa Ubide, a geochemist at the University of Queensland in Australia.

特里萨·乌维德是澳大利亚昆士兰大学的一名地球化学家。

\“[Sudbury] was generated when a bolide of 10 to 15 kilometers in diameter hit Earth…and what happened was it obviously generated a large basin and also it melted the…crust on top of the Earth at that time. And generated a massive melt pool, 2.5 kilometers in depth.”

“萨德伯里是由一个直径为10到15千米的火流星撞击地球而形成的……很显然当时形成了一个巨大的盆地，而且地表顶层的地壳也被溶解了，还生成了一个深2.5千米的巨大的融池。

But Ubide and her colleagues found that the impact did more than that: it also seems to have triggered eruptions of magma that came from deep in the mantle 7. The evidence lies in the fact that the chemistry of the lava 8 that erupted at Sudbury changed over time. At first, it matched the surface rocks, suggesting it was just from local melting. But as the eruptions continued, the lava appeared to come from deep in the mantle, suggesting the impact stirred things up inside the Earth.

然而乌维德和同事们发现，冲击导致的后果并不止于此：它似乎还引发了来自地幔的岩浆喷发。证据就是：萨德伯里喷发的熔岩的化学反应随着时间推移发生了变化。首先，它与地表岩石相匹配，这表明它来自于局部熔融。但随着火山喷发的持续，熔岩似乎来自地下深处的地幔，这就表明陨石撞击触动了地球内部的物质。

No one knows yet exactly how the impact could have sparked a prolonged episode of volcanism. One possible explanation is that after the object smashed into the surface, the crust would have rebounded 9 upward, temporarily decompressing the mostly solid mantle and causing it to melt and produce magma. The results are in the Journal of Geophysical Research: Planets.

目前尚没有人清楚冲击为何会引发长期的火山活动。一种可能的解释是，陨石撞进地球表面后，地壳会向上反弹并暂时性地解压缩最坚硬的地幔，使其融化并产生岩浆。这项研究结果发表于《地球物理学研究杂志：行星》。

It’s hard to know if the same thing happened on other planets, Ubide says. In many spots in the solar system it looks like volcanism happened much longer after the impact than what she saw at Sudbury. But without material to examine directly, we can’t be sure.

乌维德表示，很难知道同样的事情是否会发生在其他星球上。与她在萨德伯里看到的相比，太阳系许多地方的火山活动要在陨石撞击发生更久之后才会发生。但是如果没有可直接查验的材料，我们无法确定。

Nevertheless, these results do help explain a mysterious chapter in Earth’s own history. Most major impacts on our planet happened about four billion years ago, when the solar system was still settling down. But there’s no crust around from that time, suggesting the entire planet got a makeover soon after.

尽管如此，这些结果仍然有助于解释地球自身历史中的一个神秘章节。地球上大多数主要的陨石撞击大都发生在约40亿年前，那时太阳系还正在稳定中。但那时候周围没有地壳，这表明整个地球很快就发生了大转变。

“We’re suggesting that impacts are able to not only generate the crater, but also, generate melting deeper, say, in the mantle, and bring to the surface material from depth. So actually, recycle and resurface.”

“我们认为，冲击不仅能够产生陨石坑，而且还能熔化更深处的物质，比如地幔，并将深处物质带回地表。因而这实际上就是回收和重塑。”

It seems the pummeling that Earth endured in the beginning may have triggered volcanic activity that helped wipe away the evidence of those very impacts. In other words, our planet took a beating that may have accelerated its own recovery.

看起来，地球早期经受的冲击可能引发了火山活动，而它们恰好能帮它抹去那些遭受过冲击的证据。换言之，我们的星球可能遭受了能使其加速自身复原的重创。