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Craters of asteroids and meteorites in South Africa

4 November 2014

www.leopard.tv

An asteroid is a smallish planet and a meteorite is a solid fragment of a planet. Millions of meteorites drift in space and they sometimes collide with planets. Although the Earth is not located in the middle of the Universe such collisions are not unusual but they can be catastrophic for life and will change entire landscapes. That meteorites collided with the Earth was unknown to man until members of the French Academy of Science witnessed a meteorite falling on the Earth towards the end of the 18th century. Numerous huge asteroids collided with the earth some 3 to 4 billion years ago. These collisions probably created fissures in the crust of the Earth and stimulated continental drift on tectonic plates.

It has been calculated that at least 200 large meteorites have collided with the Earth in the past 3 billion years. However, later geological activities such as volcanic eruptions have subsequently filled in or covered many of the resultant craters. Because the Earth’s Moon is much more geologically stable than the Earth itself, numerous craters that are older than 3.8 billion years are still visible on the surface of the Moon. The chances of new asteroids and meteorites colliding with the Earth are relatively low, but such an impact will cause large- scale destruction on the heavily populated Earth.

www.leopard.tvThe largest known asteroid with a diameter of 37 to 58 km collided with the Earth in the Barberton greenstone belt near Swaziland some 3.26 billion years ago. The impact caused the water of the oceans to boil, filled the atmosphere with dust, shook the earth for 30 minutes and left a crater with a diameter of 478 km. In comparison the Waterberg Mountain Range is some 170 km long and 80 km wide. The impact was also the equivalent of an earthquake that measures 10.8 on the Richter Scale, it hit the Earth with a speed of 14 400 km per hour and released millions of times more energy than the atom bomb that was dropped on Hiroshima.

The remnants of the Hoba meteorite that collided with the Earth at Grootfontein in Namibia are the largest known such fragments on Earth. The Earth’s Moon is thought to have been born when a huge proto-planet as big as Mars hit the primordial Earth at a time when its crust was still pliable. It sent a huge plume of dust into space where it developed into a solid heavenly body with time.

Earth’s largest known meteorite with a diameter of 10 km, larger than Table Mountain, collided with the Earth near Vredefort in the Free State some 2.02 billion years ago and left a crater with a diameter of 300 km. The explosion that measured some 1000 megatonnes may be one reason why the atmosphere of the earth suddenly became rich in oxygen at that time. The meteorite also fell in a huge lake in the Witwatersrand Basin and liquefied rocks that contained gold and uranium and formed the current mineral deposits there. The inner of three concentric impact circles is still visible as the hills near Vredefort and Parys and is known as the Vredefort Dome.

www.leopard.tvOne of South Africa ’s more well-known meteorite craters is the Tswaing Crater at Soutpan some 40 km north of Pretoria. It was formed when a meteorite as large as half a rugby field collided with the Earth some 220 000 years ago at a calculated speed of 4000 km per hour and an impact that was equivalent to 100 atom bombs. It left an almost perfectly round crater with a diameter of some 1.4 km and 200 m deep, was ringed by a crater wall of 60 m and is one of the craters that has been preserved best in the world. The name Tswaing is Setswana for the place of salt because a salt lake developed in the bottom of the crater with time through the inflow and evaporation of water. The crater is surrounded by a large wetland which is home to some 240 types of bird and is fed by the Soutpan Creek. When this meteorite collided with the Earth it would have decimated all life within a radius of 3 km around the site of the impact, while everything in a radius of 1000 km around it would have been flattened by a wind of some 1000 km per hour. The salt and soda-ash in the crater’s bed was mined for 44 years up to 1956 but the pioneer Voortrekkers had used it to obtain salt for many years before that. The origin and age ot this crater were established in 1989 by drilling a core of 200 m in the bottom of the crater. Fossils show that giant wildebeest, hartebeest, zebras, buffaloes with long horns and early humans lived there at the time of the impact.

The second largest known crater on the earth at Sudbury in the Ontario province of Canada has a diameter of 250 km and the impact occurred 1.85 billion years ago. The third largest known crater was created 65 million years ago when a meteorite with a diameter of some 10 km hit Mexico at Chicxulub and left a crater with a diameter of 170 km that was half under the Gulf of Mexico. The centre of the impact was in a shallow sea and is the reason why the crater was only discovered relatively recently. Because this region contains geological sediments rich in sulphur, burning rock fragments and an intense acid rain were distributed widely into the atmosphere. This caused worldwide fires which combined with several years of darkness due to the smoke and dust in the atmosphere to kill the food plants of the herbivorous dinosaurs and the ultimate extinction of all the dinosaurs followed.

Currently there are some 150 impact craters known that were caused by meteorites that collided with the Earth 1 to 2 billion years ago. Their diameter varies from a few to 250 km and two to four new craters are being discovered every year. Recently a crater with a diameter of 70 km that was formed 145 million years ago was found at Morokweng near Severn in the Kalahari.

Meteorites primarily consist of rock, rock-like iron ore, nickel and iron. Those that contain iron can be distinguished by their metallic appearance and others contain rare minerals such as titanium and iridium. The composition of a meteorite is determined by its origin.

 

References:

Alexander, M 2012. The world’s biggest meteor crater. South Africa info, 8 March. http://www.southafrica.info/about/geography/vredefort-080605.htm

Anon 2012. File: Yucatan chix crater.jpg. http://en.wikipedia.org/wiki/File:Yucatan_chix_crater.jpg

Davie, L 2012. The Tswaing meteorite crater. South Africa info, 8 March. http://www.southafrica.info/travel/cultural/tswaingcrater.htm

Melosh, H J 2012. Origin of the moon. Tucson: Lunar and Planetary Laboratory, Department of Planetary Sciences, University of Arizona. http://www.lpl.arizona.edu/outreach/origin/of/the/moon

Prigg, M 2014. Scientists reconstruct asteroid impact six times bigger than the blast that wiped out the dinosaurs which boiled the oceans, burned the sky, and shook the earth for thirty minutes. Daily Mail, 10 April.

Reimold, W U 1993. Tales of dinosaurs, gold and meteorite craters. Archimedes, May: 13 - 19.

Reimold, W U 2012. Meteorites and meteorite craters. file:///c:DOCUME~1/Admin/LOCALS~1/Temp/reimold.html

Scott, L 1990. Die Pretoria-Soutpan. Spectrum 28(2): 37 - 39.

 

Article by Prof J du P Bothma

 

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