Only in South Africa — The Vredefort Crater
X Marks the Smash
Around 2,023 million years ago, a rock in space tumbled towards Earth. It was mountain-sized, measuring approximately 10 kilometers (6.21 miles) across. When it hit baby Africa, the meteorite was ironically also traveling at around 10 kilometers a second. The impact was the world's biggest known energy-release event and gauged a footprint 300 kilometers (185 miles) long and ten times deeper than the Grand Canyon. Today, it can be found in the Free State province and is anything but dead. Apart from a lively tourist trade, the crater has its own locals. It was even named for them — near the epicenter stands the town of Vredefort, home to around 15,000 people.
What Makes it Unique?
Compared to the other planets and even the solar system's moons, Earth is remarkably free of craters. Only around 130, give or take a few, have been confirmed. Undoubtedly, some remain undiscovered but most were erased by geological processes such as erosion, volcanic activity and tectonics. The Vredefort site is special for several reasons.
Vredefort was Once Huge
1. It Remains the Biggest and Oldest
Earth's biggest energy burst also left behind the largest and most ancient crater found so far. The area didn't escape erosion, which shrunk the modern-day Vredefort down to a radius of 190 kilometers (118 miles). Despite this, the crater remains the oldest and biggest impact site. Interestingly, the erosion also made it the deepest.
2. The Zircon Debate
After the meteorite hit, the immense heat melted the Earth's crust. A sea of magma, or melted rock, swelled to fill up the new crater. Scientists have always believed that all the signs of this “ocean” have long since disappeared. In the 1990s, a geochronologist named Desmond Moser was inside the crater, just doing what geochronologists do — trying to put a finer detail on the age of geological things. In this case, Moser wanted to narrow down Vredefort's age when he accidentally found minerals called zircon.
The green-black rocks set off a debate that rages to this day. Moser and like-minded researchers believed that the minerals are the only remains of the lost magma sea. They were found inside a rock called gabbronorite, which allegedly crystallized from local rock melted inside the magma and not the actual impact. However, not everyone’s on board with the idea. Though the zircon minerals are both a lucky and rare find, some researchers feel that there isn't enough evidence to prove they didn't crystallize during the impact.
End of the Debate in Sight?
Science may, however, favour Camp Moser. A follow-up investigation yielded three positive clues that they indeed found traces of the sea.
- The zircons' distribution was random and interconnected with other kinds of minerals, an unlikely scene had they formed later than other types of minerals which were present before the impact
- The zircons also crystallized at 725-928 Celsius (1,337-1,702 Fahrenheit), the same temperature range identified at the Sudbury impact melt, a crater slightly younger and smaller than Vredefort
- The presence of the element hafnium was a strong indication that the magma came from surface rocks and didn't well up from inside the Earth's crust, as some believe
3. The Only Astrobleme Profile
When it comes to Vredefort, most people prefer the countless tourist activities and sightseeing to dry scientific tidbits. However, few know that the crater is unique in another way. In geological terms, it's also an astrobleme — the eroded remnant of an impact. Thus far, Vredefort remains the only impact site with the complete geological profile of an astrobleme below the crater floor. These layers can one day help researchers understand how craters are formed during, and immediately after, the collision that shaped it.
4. The Russian Connection
When the meteorite violently connected with the surface, not all of its consequences drove down into the earth. Some of them went up. Forced into the air by a powerful blast, particles traveled 2,500 kilometers (1,550 miles) before they fell on future Scandinavia and northwest Russia. At first, the tiny spheres were mistaken for ooids, something that forms in tropical waters. Further investigation, however, proved they were impact debris. They contained platinum and ruthenium, both rare elements linked to space. The spheres were also found in rock roughly dating back to the Vredefort event. However, scientists are keeping an open mind - another impact could be responsible for the tiny rocks.
Did You Know?
- The site has UNESCO status and is South Africa's seventh World Heritage Site
- The asteroid behind Vredefort is believed to one of the largest to ever hit Earth
- There are around 100 different plant species inside the dome, more than 70 kinds of butterflies and 300 species of birds
- The devastating global effects of the Vredefort event is believed by some scientists to have caused major evolutionary changes
- The unusual layered appearance of the crater's impact melt could help identify older craters, especially those known only from faraway particles blasted into the air (if the Russian spheres aren't Vredefort's, this would be one of those missing craters)
Questions & Answers
© 2018 Jana Louise Smit