Extraterrestrial impacts: When rock flows like water

28.08.2016

News from Climate Science: Once a month, climate researchers report on their latest findings in the newspaper "Hamburger Abendblatt". Ulrich Riller is a geologist and explains how an impact shockwave can turn the rock into liquid.

Aerial view of Yucatán Peninsula from space.
Ulrich Riller is a structural geologist at CEN and an expert on extraterrestrial impacts
Off-shore drilling ship Myrtle
Rock lab on the Myrtle analysing a quartz crystal after the drilling.

Experts are agreed: It was a meteorite that struck the Earth 65 million years ago that killed the dinosaurs. It also left behind a gigantic crater, known by the Aztec name Chicxulub (pronounced tschick-schulub), off Mexico’s Yucatan peninsular. Whereas initially the sea temperature rose dramatically, the resulting rock dust blocked the sun for years causing a prolonged cold period. Three-quarters of the world’s animal species were eradicated as a result of this climate change.

Such an impact not only affects climate and life forms, it is also a geological revolution. Depending on its size and composition, a projectile can penetrate up to 15 kilometers deep into the Earth’s crust within one to two seconds. The meteorite itself usually vaporizes completely in the process. The impact creates a shockwave in the rock – in the case of Chicxulub this caused the depth of the crater to increase by a further 15 kilometers. To start with the crater walls are steep, but these collapse immediately, flattening and extending the diameter of the crater to up to 200 kilometers. Thanks to computer simulations, we now know that the crater was completely formed in the space of just ten minutes.

As a geologist at the Center for Earth System Research and Sustainability, I’m interested in how such highly dynamic deformations can arise in such a short time. A meteorite impacts at about 20 kilometers per second – cosmic velocities that produce heat and tremendous pressure in the rock in the Earth’s crust: a situation that no laboratory can reproduce, which means we can only surmise which physical processes occur in these few seconds.

Many researchers believe that under such extreme conditions, the rock is not only shattered, compacted and displaced, but that for a short time it also behaves like a liquid. There’s no other explanation, for example, for craters of this size being flat bottomed. But as yet there has been no proof.

We’ve now made considerable progress toward clarifying the process: Using samples taken from similar craters in Canada and South Africa, for the first time we’ve been able to identify structures that offer clues as to what happens during crater formation. We were able to find solidified molten rock at a microscopic scale in the cavities between individual crystals – a sure sign that it was once fluid.

On a practical level, our research can help in the search for raw materials, since large impact craters are known for their valuable copper, nickel and platinum deposits: These metals are normally only found in the Earth’s crust in particles the size of a grain of sand, making them unattractive for industrial mining. However, under the effects of a meteorite these grains melt together to form giant deposits. Understanding exactly what occurs in the first few minutes and seconds after an impact provides vital clues to where these metals can be found.

A costly scientific drilling project at Chicxulub – to a depth of 1300 meters – has just been completed. The bore samples are now on their way to Bremen, which is home to one of the world’s three storage facilities for such samples. In September, I and a team of 30 international researchers will investigate the core samples to unravel further mysteries surrounding meteorite craters.

Contact Ulrich Riller