Dino-killing asteroid flipped a patch of Earth's crust inside out

Rock, tens of kilometres below the surface, was pushed down and pulled up to burst onto the surface in a ring of mountains. Belinda Smith reports.

An artist's impression of the Chicxulub crater soon after it formed, 66 million years ago. Geologists drilling into the inner ring of rock – called the peak ring – suggest it's rock from tens of kilometres below, brought to the surface as the asteroid pulse rebounded.
D Van Ravenswaay / SPL / Getty Images

The ring of mountains in the giant crater left behind by the collision that killed the non-avian dinosaurs was formed when masses of rock originating deep within the Earth blasted out and crashed back onto the surface.

Joanna Morgan from the UK’s Imperial College London and colleagues drilled more than 1.3 kilometres into the Chicxulub crater – the 200-kilometre basin off Mexico’s Yucatan Peninsula – and found shattered rocks consistent with the so-called “dynamic collapse theory”.

The work, which confirms one of two ring-formation theories, was published in Science.

Earth and other bodies in the solar system are constantly peppered with meteors. If they hit the ground as meteorites, they can leave telltale craters.

Sometimes, a particularly big collision will leave a crater with a peak in its centre, like a miniature mountain, where material rushes in the fill the space left by the impact. You see the same effect if you plop an ice cube into a glass of water.

In some cases, the peak evolves into a peak ring – an uneven circle of hills or mountains within the crater. But the exact process from peak to peak ring has been unclear.

As the peak is lifted from the underlying rock, does part of it melt and disperse into the ring formation? Or does the peak simply crumble to become the ring?

Finding out is tricky. Large impact craters are common on the moon, Mercury and Venus, but they’re far away. Analysing them in detail is nearly impossible.

Conversely, Earth harbours very few impact craters, let alone those large enough to have a peak ring formation. Our dynamic atmosphere means craters are constantly eroded away.

The best preserved big crater in Earth is the Chicxulub, which formed around 66 million years ago. But the reason for its pristine nature is also a massive inconvenience: it lies beneath a kilometre or so of limestone.

Geologists know the crater’s size and shape, thanks in part to seismic studies, which measure how waves travel through the Earth’s layers to give an idea of its composition. Morgan was part of a team who, in 2000, reported Chicxulub contains a peak ring.

So in April and May this year, she and her colleagues drilled into the peak ring and pulled up a cylindrical core of rock that reached 1,335 metres below sea level.

The peak ring, they found, started around 618 metres below sea level. Its uppermost layer comprised around 130 metres of breccia – rock made of broken fragments held together by fine-grained cement. Below the breccia layer was granite-rich rock.

Slices of the core were cut and upon closer inspection, the researchers found faults and shattered patches throughout – signs of “shock metamorphism”.

Part of the core drilled from the Chicxulub impact crater. The right-most section is granite interspersed with potassium feldspar, and is crosscut with fractures and shear lines.

They calculated these rock layers were subjected to pressures between 10 and 35 gigapascals. This is in line with the peak crumble theory rather than the melt theory, which should see shock pressures much lower or higher than that range.

The rocks also retained the coarse crystalline structure of deeper material. If it had completely melted, those crystal patterns would have disappeared.

When the asteroid or comet hit Earth, it pushed rocks tens of kilometres below to surface down and outwards. As the surface bounced back, those rocks shifted direction and headed towards the impact point, burst onto the surface and rained down to become the peak ring.

In other words, the collision turned that patch of the Earth inside out.

Penny Barton from the University of Cambridge in the UK writes in a Perspectives piece that the work appears to “validate the collapse models” but poses “many new questions for further work on these exciting samples”.

And given dynamic collapse retains material from deep below a crater without melting it, analysis of peak rings can tell planetary scientists about an object’s inner composition and layering.

For instance, Morgan and colleagues write, peak ring material could help researchers verify how the moon was formed.

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Belinda Smith is a science and technology journalist in Melbourne, Australia.
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