Impact craters are referred to the circular depression caused on the surface of the earth, planetary body, or other solid bodies in the solar system, by a hypervelocity impact of a smaller body or a meteorite. The energy released during this impact is usually very enormous as the velocity ranges between 11 and 72 Kilometers per seconds. It is worthy of note that the formation of an impact crater on the surface of the earth is almost instantaneous.
Characteristics of an Impact crater
Most impact craters have raised rims and floors that are usually lower in elevation than its surrounding environment.
Impact craters could be small, bowl-shaped, simple, large, complex, and multi-ringed impact basin.
According to the Earth’s impact database, the Earth has a total of 190 confirmed craters. All of the impact craters on the Earth were caused by either of the two processes:
- The collision of meteors, consisting of large asteroids fragments, with the Earth. (This is the case of our impact crater of focus).
- The collision of comets (consisting of dust particles, ice, and rocky fragments) with the Earth.
Earth’s impact craters can be classified as either of the following;
- Young craters: less than 10 thousand years old and a diameter of 330 ft or more. Examples are the Rio Cuarto crater in Argentina, The Macha carter in Russia, Morasko crater in Poland, and also the Kaali crater in Estonia.
- Large Craters: between 10 thousand to 1 million years old and diameter of 0.62 miles. Examples include the Tswaing crater in South Africa, the Zhamanshin crater in Kazakhstan, the Lonar crater in India, and also the Meteor crater in the US.
- Larger craters: 1 to 10 million years old with 3.1 miles or more. Examples include the Bosumtwi crater in Ghana, and also the Bigach crater in Kazakhstan.
- Largest craters: More than 10 million years old and over 12 miles in diameter. One of the most astonishing example and most likely the oldest is the Vredefort crater in South Africa.
Regarded as the largest verified impact crater so far on Earth, the Vredefort crater is more than 190 miles (300 Kilometers) in diameter. It is located in the Free State province of South Africa and derives its name from the town Vredefort located near the center.
Vredefort crater (mostly called the Vredefort Dome because it has been eroded), stands as the oldest crater in the whole of the world’s history. It also is the most deeply eroded and most complex meteorite structure on Earth. In terms of age, the Vredefort crater is estimated to be over 2.023 billion years. The Vredefort dome was listed in the UNESCO world heritage sites because of its geological properties, and also the fact that it is the only crater on Earth that provides us with a full geographical profile of an astrobleme below the floor.
Structure of the Vredefort Crater (geographical features)
Vredefort was probably hit by one of the greatest asteroids in the history of the Earth thought to have been between 10 to 15 kilometers in diameter. The original Vredefort carter had an estimated 300 kilometers of diameter and has since over time been eroded. The structures that remain, usually called the “Vredefort Dome,” now consist of a concentric ring system that surrounds the central cater.
Contrary to previous claims that the dome was of volcanic origin, the Vredefort dome was formed out a huge bolide impact. This discovery was uncovered around the mid-1990s when shatter cones were seen around the bed of the nearby Vaal River. The bolide impact distorted the Witwatersrand which was laid down before the Vredefort impact over a period of 250 million years. These rocks then form partial concentric rings or semicircle around the crater center as we see in the Witwatersrand rocks (having a semi-circle of 25 kilometers from the center of the crater. The Witwatersrand rocks from the very visible arc of hills seen to the North-west of the crater center. This is followed by the Ventersdorp lavas, the narrow band of the Ghaap Dolomite rocks, and finally the Pretoria Subgroup which all form a 30-kilometer wide band.
From research description, we see that the Vredefort structure consists of a central core which is made up of mainly of granitic gneiss terrain holding diameter of about 45 Kilometers. Three-quarters of the core is seen to be surrounded by a collar that is formed by series of overturned and upturned strata which includes 2.7-3.0Ga Witwatersrand metasediments and Ventersdrop lavas, 3.07Ga Dominion group metavolcanic, and 2.25-2.5Ga rocks from the Transvaal Sequence. There is also the presence of a semi-annular magnetic anomaly in the inner part of the Vredefort structure with a positive concentric Bouguer gravity anomaly at the central area of the Dome. This is an indicator that there are denser materials below the Dome.
The Vredefort Event
According to Glikson, the impact event on Vredefort took place close to the end of the great accretionary period of the planets of the solar system. This period is the accepted date range for the formation of planets by the accretion of dust, rubble and ice all revolving around the sun. It was the coming together of these materials by collisions that formed the planets. There is still evidence of craters which is visible on most planets that are located close to the sun, when those planets are not covered either by frozen gases or water.
Most craters that formed on the surface of the earth had a different situation entirely. This is due to the fact that the earth’s surface is partly covered by water and therefore the craters were mostly eliminated by erosion or covered by subsequent deposition of sediments.
Though most accretionary events of ancient astroblemes, such as Vredefort, have only been recognized of recent, there are speculations that an estimated 200 impact basins of over 200 kilometers in diameter could have been created during the beginning stages of the world’s creation. Despite the numerous events of accretion on Earth, the Vredefort still stands as the astrobleme with the largest recorded episode of the release of explosive energy experienced on the earth’s surface. This has made the Vredefort a source of continued research owing to the fact that all effects caused by the event are not entirely known. Scientists hope to research more and hopefully acquire more knowledge on the events surrounding the formation of the Vredefort crater.
The vredefort’s unique ecology
The vredefort Dome is flooded with uniqueness throughout its dome. This geological structure has birthed a diversity of microhabitats that ensure plant communities that are unique and exceptional to South Africa. There are at least 99 species of plant recorded in the area while there exist valleys between hills that are well wooded and unspoiled with springs and streams in virtually all the valleys.
Some species found there include the typical Bushveld trees, such as Bushwillow species, also found there are the Red Ivory, tree fern, and also wild olive trees. Vegetation in the dome consists majorly of grassveld, Bankenveld, Riverine bush and some specific vegetation that occurs only in valleys and slope areas.
The vredefort dome is unique for its bird life solely because of the diversity of habitat in the dome. The dome is home to more than 200 species of bird, about 50 species of small mammals, and more species of butterfly than in the whole of the British Isles.
There have been records of different animal species in the dome, including the “big five”: the African Elephant, Lion, Leopard, African Buffalo, and the white Rhinoceros, but many games and animal species have disappeared basically because of farming activities in the area.
The area also holds treasures of remnants from settlements around the 19th century; this is also prompting more research by researchers. The site holds evidence relating to early white settlers in South Africa and also the Anglo Boer War which was a war against Britain for independence.
Around the 1880s, gold prospecting was high and later reduced around the 1920s due to the low gold content of the ores.
The Vredefort dome houses four towns; the Parys, Koppies, Venterskroon, and off course Vredefort. Amongst these four towns, Parys is the largest and is the heart of tourism in the area, Koppies and Vredefort are solely dependent on agriculture for their economic growth.
Conservation of Vredefort Dome
No doubt the Vredefort Dome is rich and unique in the whole of South Africa and the world at large owing to the presence of bird, insect, plant, and animal life. This was actually made possible by unique and sensitive ecosystems and microhabitats in the area. Furthermore, the area is also blessed with archeological and anthropological heritage.
Despite all these, the Vredefort dome is presently subject to property development and local owners are concerned about the sewage dumping in the Vaal River and crater site. This has posed a threat of destructive mining in the area. Various instruments have been put in place to ensure the integrity of the site. These include the Environmental Conservation Act No. 73 of 1989, the physical planning act No. 88 of 1967 amongst others.
The Vredefort dome is a site of significant interest in the history of our planet and worth seeing. Vredefort crater (mostly called the Vredefort Dome because it has been eroded), stands as the oldest crater in the whole of the world’s history. It also is the most deeply eroded and most complex meteorite structure on Earth. In terms of age, the Vredefort crater is estimated to be over 2.023 billion years. The Vredefort dome was listed in the UNESCO world heritage sites because of its geological properties, and also the fact that it is the only crater on Earth that provides us with a full geographical profile of an astrobleme below the floor.