Impact craters

Impact craters on Earth

With space exploration we have seen that the majority of planetary surfaces are scattered by numerous craters more or less large.
Earth like other objects in the solar system was bombarded at the same rate by celestial bodies more or less big. Every day, tens of tons of cosmic dust fall on Earth, but an asteroid 350 meters in diameter that strikes the Earth every 16 000 years, an asteroid with a diameter of 75 meters once every 1000 years. These collisions with celestial objects create impact craters on the surface of terrestrial planets like Mercury, Venus and Mars.
If we do not see very well impact craters on Earth is because they have been eroded by time, but they have not so far disappeared, it is still found today. Five new sites are discovered every year. Richard Grieve Centre Geological Survey of Canada has ranked with his team impact craters by age.
The number of impact crater is 169 and it estimate to 5000 undiscovered craters. The giant crater Vredefort in South Africa is the largest impact crater with a diameter of 300 km. Many gold mines were found on the site of the crater, which alone accounts for half of the world production of gold.
The giant crater in Sudbury in Canada hides one of the largest nickel deposit among the most important in the world's.
Crater Popigai in Russia is rich in diamonds. By fracturing the rock impacts promote the storage of the most diverse fields such as oil or gas accumulate in the gaps caused by the huge shock. Over time huge reservoirs were formed in fractures in the earth's surface. If meteorite impacts often destroys life, they also stored resources now essential to the evolution of humanity.

Image: Impact craters listed in the 2000s. Space adventure documentary credit, the cradle of life, NHK.

Vredefort crater

Vredefort Dome or Vredefort crater is located about 120 km south-west of Johannesburg.
Impact structure in multiple crowns is the trace of a gigantic collision that took place there are 2 billion years. It is in 2010, the oldest meteorite crater discovered on Earth. It is very difficult to measure the diameter of a crater as old as it still suffered the erosion of time, but geologists give a diameter of 300 km, is the largest and deepest in the world.
These trace of the past shows us the power that can have an impact and we can imagine the huge release of energy that ensued. Certainly that the shock was devastating causing global change irreversible with major changes in terms of evolution. The Vredefort Dome is an important testimony of the geological history of the Earth because it exposes geological strata accessible of high quality, ranging from central areas to upper regions of the earth's crust, and that over 2 billion years. Covering an area of ​​30,111 hectares, the site is protected against any prospecting or mining exploitation from development pressures, in order to preserve the visual integrity of the development and the visit of the World Heritage property. The center of the Vredefort structure include an agricultural plain in which lies the town of Parys.

The reliefs in arc to the north, are highly fractured sediments, together form a dome. Vredefort Dome itself is only a small part of the impact structure, which extends nearly 300 km in diameter. The Vredefort Dome is one of the rare multi-ring craters, while they are common on other planets or the moon because the geological evolution of the Earth, erosion and plate tectonics, erasing such crater. In 2005, the Vredefort Dome was inscribed on the World Heritage List of UNESCO for its visual and scientific interest.

NB: Large craters often have a central bulge surrounded by the crater itself. The bulge forms itself by "kickback" of the soil due to the impact and its diameter is generally equal to 20% of the total diameter. The edges of the crater are often collapsed, with shifts along fractures circular.

Image: Vredefort Dome or Vredefort crater has a diameter of 300 km. In this picture we see that the center of the structure. The asteroid that hit Earth to Vredefort had an estimated diameter between 10 and 15 km.
Credit: Earth Sciences and Image Analysis Laboratory, NASA.