A mysterious meteorite crater is on the menu for stage 8 of the Tour de France. The riders and viewers will just see the peaceful, rolling fields of the Limousin region. However, it wasn’t exactly peaceful 200 million years ago on this exact location. At that moment, a space rock with a diameter of about one kilometer crashed into the place where the picturesque village of Rochechouart is now located. That’s not far from today’s route.
The result? The only proven meteorite impact crater in France. The crater is about 20 to 30 kilometers wide. Remnants of this catastrophic event are not easily recognizable in the landscape. That’s why we go on a search for extraterrestrial evidence in the French countryside.
Impact craters: an extraterrestrial treasure trove
About 40,000 tons of extraterrestrial material crashes into planet Earth every year. This is mainly in the form of minuscule extraterrestrial particles of barely 1 micrometer to 2 millimeters in size. We call them micrometeorites. You can find these everywhere, even in your garden or in your gutter. They are easy to collect with just a bucket, magnet and microscope. It might be a nice activity for the first rest day.
Once in a while, the scenario as outlined in the recent film ‘Don’t Look up‘ can actually become a reality, namely if our planet falls victim to a collision with a larger meteorite. A recent example is the Chelyabinsk meteorite. It was about 20 meters in diameter and struck ten years ago near the Russian city of the same name. The shock wave injured 1,500 people and damaged over 7,000 buildings.
When meteorites are larger than 50 meters they often leave a scar on the Earth’s surface in the form of impact craters. The most famous example is Meteor Crater in Arizona. It’s a very well-preserved simple (crescent-shaped) impact crater of 1.2 kilometers in diameter.
Another very infamous crater is the Chicxulub impact crater. This is a complex crater of 200 kilometers wide that is today buried under Mexico’s Yucatán Peninsula. The impact of the Chicxulub asteroid, yielding a diameter of the entire Brussels-Capital Region, is responsible for causing the extinction of the non-avian dinosaurs, among others, 66 million years ago. Also see the blog of stage 3 on the Cretaceous-Paleogene mass extinction, visible at Zumaia on the Basque coast).
We need proof
Not every pit in the landscape is a mysterious meteorite crater. So, how do we prove such catastrophic events? Meteorite impacts are one of a kind, as they are instantaneous geological events. They happen in seconds. Meteorites fall to Earth at a speed between 10 and 70 kilometers per second. In other words: tens to hundreds of times faster than firing a bullet from a pistol.
They reach temperatures of more than 10,000 degrees Celsius and more than 100 Giga Pascal at the impact site in less than a second. This is far beyond the reach of geological processes such as the formation of mountains and even volcanic eruptions. The shock wave created by an impact event can cause permanent changes in the crystal structure of minerals such as quartz. We call these features then shocked quartz.
Shocked quartz was first observed when astrogeologist Eugene Shoemaker studied the sand grains in craters formed by nuclear tests in the Nevada desert during the Cold War in the early 1960s. Afterwards, he and his colleagues found the same shock lamellae in quartz within Meteor Crater in Arizona. The field of ‘planetary geology’ was born.
Astronauts on the Apollo missions were personally trained by Shoemaker in Meteor Crater in subsequent years. This way they could be optimally prepared for the heavily cratered terrain of the Moon.
Today, nearly 200 meteorite craters have been proven on our planet. New ones are discovered almost every year. You can contribute to these discoveries yourself by looking for new impact craters via satellite images on the citizen science platform Vigie Cratère. Maybe you discover a mysterious meteorite crater on Monday’s rest day?
There is possible competition for our mysterious meteorite crater in Rouchechouart though. In March this year a proposal was put forward for a new meteorite crater in France in – how could it be otherwise – a vineyard in the French countryside! Whether this 200-meter-wide impact crater, aptly named ‘Domaine du Météore‘, near Montpellier will indeed be accepted by the scientific community and thereby becoming the second French impact crater: time will tell.
Rochechouart: the clues
How do we know that a meteorite has hit Rochechouart? How do we uncover the story of this mysterious meteorite crater? First of all, shocked quartz was discovered in rocks from this region (Figure 4A). Besides this microscopic evidence, there are rocks and structures that can be seen by the naked eye that provide more clues. An example are shatter cones (Figure 4B). These are striped cones that form within rocks that were impacted and are only found within or close to meteorite craters. The tip of these cones point in the direction from which the shock wave came.
Furthermore, the romantic Chateau de Rochechouart is built on rocks that we call suevites. These are rocks consisting of angular fragments including molten material (Figure 4C). Suevites were also mined in the region of Rochechouart and were used as building blocks for the castle.
Our final evidence for the mysterious meteorite crater are rocks found that melted completely due to the heat of the impact and then solidified (Figure 4D). Enrichments in the element nickel have also been found in these impact melt rocks. Meteorites are rich in this metal so an extraterrestrial link was quickly established.
Despite these clues, there are still many unanswered questions for Rochechouart. One question is the size of the original crater. Estimates vary between 10 and even 50 (!) kilometers in diameter. This uncertainty is mainly related to a deep erosion of more than one kilometer that has broken off large parts of the original crater since its formation more than 200 million years ago. Below the castle of Rochechouart you can now – as it were – walk on the crater floor and envision that there was once more than hundreds of meters of material above you, which was deposited in no-time.
In 2017, a drilling campaign was carried out by the local CIRIR institute to learn more about the structure of the crater below the surface. Our AMGC research group at the Vrije Universiteit Brussel is currently studying these drill cores in detail to reconstruct the processes that took place in the minutes, hours, days and years after this event 200 million years ago. How much material was melted? How strong was the earthquake produced by the impact? What was the landscape like at the time of the Rochechouart impact? Was there water? And if so, how high were the tsunami waves?
No more future meteorite craters?
The first results indicate that there are enough reasons to make sure that such disastrous large meteorite impact events no longer take place on Earth. NASA’s Double Asteroid Redirection Test (DART) mission last year gave hope for the future by redirecting an asteroid, but our planet continues to be threatened by debris from space…
Mysterious meteorites are things that the Tour de France riders don’t have to worry about today. While the breakaway has a few minutes and all is under control they can chat between the Limousin cows, along the Rochechouart crater and on the banks of the Vienne river. However, today’s finish in Limoges is still viciously steep. Will the sprinter teams shoot forward like meteorites or will a surprise winner cause a shock wave in the peloton?