Morokweng
Key facts
- Country
- South Africa
- Diameter
- 70 km
- Age
- about 145 million years
- Location
- 26.47° S, 23.53° E
Almost nothing: the structure lies beneath the sands of the Kalahari, and is known mainly from drill cores and geophysical surveys.
Morokweng, in South Africa's North West Province near the border with Botswana, is a large impact structure hidden beneath the sands of the Kalahari. It is known for one extraordinary discovery: a sizeable piece of the impactor itself, recovered from deep underground.
The impact. About 145 million years ago, close to the boundary between the Jurassic and Cretaceous periods, an asteroid struck granite and gneiss in what is now southern Africa. The crater is thought to have been about 70 km wide, perhaps more. Since then the area has been eroded and then covered by younger Kalahari sediments, so the crater cannot be seen at the surface.
Discovery. In the 1990s geologists examining magnetic and gravity maps noticed a circular feature beneath the Kalahari. Drilling recovered rock that had once been completely melted, along with shocked minerals, and in the mid-1990s Morokweng was recognised as an impact structure. Its age was measured from minerals in the impact melt.
A piece of the impactor. Most large impactors are vaporised or melted almost completely, leaving only chemical traces. In 2006 scientists reported that a drill core at Morokweng contained an unmelted fragment of the impactor about 25 cm long, found within the impact melt rock. The fragment resembled a chondrite, the most common type of stony meteorite, though with unusual features. Finding such a large surviving piece from a crater this size was a surprise, and it suggested that some impactor fragments can survive even very energetic collisions, perhaps because they were caught up in the impact melt and cooled rather than vaporised.
Chemical studies of the melt rock had already pointed to a chondrite impactor, because the melt is enriched in elements such as nickel, chromium and the platinum-group metals, which are much more common in meteorites than in the Earth's crust.
A boundary impact? Because Morokweng formed close to the Jurassic–Cretaceous boundary, some researchers have asked whether it played a role in the changes in life at that time. However, there was no major mass extinction at the Jurassic–Cretaceous boundary comparable to the one at the end of the Cretaceous, and any link remains speculative.
Why it matters. Morokweng shows that large impact structures can remain completely hidden even on land, buried beneath younger sediments. It is a reminder that the list of known craters is incomplete, and that geophysical surveys and drilling still turn up new ones. Its surviving impactor fragment also gives scientists rare direct evidence of what struck the Earth.
Did you know?
A drill core here contained a fragment of the impactor about 25 cm long.
The crater was found by studying magnetic and gravity maps, not by seeing it on the ground.
It formed close to the boundary between the Jurassic and Cretaceous periods.