Impact craters

Category includes outstanding impact craters – detectable scars on the surface of Earth left by a body coming from outer space. Category includes also meteorites – natural objects from the outer space.

Bombardment from the space

There are many pieces of solid matter flying around in space. And VERY frequently they fall on the surface of Earth. There are estimates that every year on Earth fall 18,000 – 84,000 meteorites larger than 10 grams: e.g. one meteorite every 6 – 30 minutes. Every 500 years meteorites form a crater with a diameter of 100 m. It is rather unusual that no giant meteorite has devastated Earth over the last two million years – but it is also calculated that just a few percent or all small craters are found thus far.

The speed of meteorites is dizzying: up to 15 – 20 kilometers per second! Theoretically, they might be even 70 km/s fast (252 000 km/h…). Now try to imagine what happens if a meteorite has a diameter of one kilometer and hits the surface of Earth 300 times faster than jet aircraft! It burns a profound hole in the atmosphere and kicks the crust of Earth with a terrible force.

Earth has experienced such terrible bombardment. It has been calculated that meteorites with a diameter of 1 km hit Earth every 500,000 years and sometimes there come true behemoths, with a diameter more than 10 km. They leave scars on the surface of Earth and traces in the history of climate and life.

The scars – impact craters

As the meteorite hits the ground, it is suddenly stopped by the solid mass of Earth’s crust. Energy can not disappear without a trace – and the immense kinetic energy of the meteorite is transformed into thermal energy: an incredible explosion takes place in the impact site. If the meteorite is smaller, it may explode before hitting the Earth’s surface, but if it is large – it penetrates the ground and explodes under it, creating a circular crater.

The size of such craters on Earth might be amazing. Vredefort Crater in contemporary South Africa was created by a 5 – 10 km large asteroid some 2 billion years ago and has a diameter of 160 km. It seems that this blow triggered the creation of new volcanoes around the impact crater. Nowadays it would erase most life forms on Earth.

The giant impact craters have left much more profound changes and can be detected even billions of years later. Such catastrophic events happen very rarely – once in ten or a hundred million years. As a result, these craters still exist but have lost their "visual appeal" – they are just circular structures in the Earth’s crust, very often covered with later sediments.

Smaller impact craters disappear much sooner (if a few million years go together with a term "sooner") but they are more frequent. If they are recent, they still look like true craters and have a visual appeal.

How many impact craters are on Earth?

The existence of impact craters on Earth has been proved recently – in the 1920s. Since then there have been found some 190 impact craters. They are diverse. Some are small, distinctive, and fairly new craters. Some are very old, large, and eroded. And – some are covered with sediments and invisible.

Theoretically the density of impact craters on Earth should be uniform – there is no reason to believe that some parts of Earth are hit more often than others. The density of known craters though isn’t uniform. There are known more craters in:

• Places where the geological exploration has been more intense – United States, Europe, South Africa, etc.;
• places, where are exposed old rocks – Canada, Scandinavia, Sahara, Australia.

Very few impact craters are known in oceans – the ocean floor is constantly renewed by geological processes and can not preserve impact craters that are hundreds of millions of years old.

It is expected that many more impact craters will be discovered in the future, but most of them will not be visually impressive structures – they are buried under thick layers of sediments. But there still are discovered visually very interesting impact craters as well – such as the Kamil Crater (Egypt), which was discovered through Google Earth application in 2008.

And, don’t forget, new craters will be formed as well. Every minute there can fall a large meteorite which can cause a devastating explosion and leave a fresh crater.

There are the following articles about the impact craters: