Impact Craters of the Earth
By Eugene P. Gurov

The surface of the Earth is constantly exposed to meteorite impacts. Small meteorites are braked in the atmosphere, whereas the bigger meteorites strike the surface with cosmic velocity. These meteorite impacts cause the instantaneous discharge of their energy and formation of the meteorite craters.

About 150 impact craters have been formed on the Earth's surface. From 3 to 4 craters are discovered every year on all the continents and the sea-floor. The impact craters are quickly eroded from the surface and the best preservation of these is in the most stable regions on the shields and cratons (oldest, stable parts of continents). Twenty-five impact structures are known on the Canadian Shield, 19 craters are spread in Australia, and 7 craters are situated on the Ukrainian Shield over a territory of about 250,000 km2. A big reserve of unknown craters may lie in South America, Asia and Africa which have been insufficiently studied in the search for craters.

Study of the Moon, Mars and some other bodies of the Solar System bears witness to the intense meteorite bombardment of their surfaces in the early stage of their history. The traces of the earliest bombardment of the Earth are not known. The most ancient terrestrial impact craters are presented by two gigantic astroblemes: the Vredefort crater in South Africa, about 2 billion years old and 140 km in diameter, and the Sudbury astrobleme in Canada, 1.85 billion years old.

The youngest impact craters were formed in the Quaternary period. The famous Barringer crater in Arizona (USA), 1200 m in diameter and about 180 m deep, was formed some 50,000 years ago. The last crater forming impact was on May 17, 1990 near the town of Sterlitamak in the Urals foreland. An iron meteorite fell in a plowed field and formed a crater 9 m in diameter and 4 m deep. Russian scientists have studied the crater in the days after the event and found fragments of meteoritic iron in it.

Terrestrial craters vary in size from about ten to tens of meters to 100 - 200 km in diameter. The structure and morphology of impact craters depends on the craters size. The smallest craters up to 3 - 5 km in diameter are presented by the simple bowl-shaped structures very similar to the craters of bombs and nuclear explosions. These simple craters are known in Europe, Asia, America, and Africa. The Wolf Creek, Dalgaranga craters, and the group of Henbury craters in Australia are examples of such impact craters. We analyzed a group of five craters called Macha in the taiga of the Western Yakutia. The two biggest craters in this group, 300 and 180 m in diameter, formed the double crater occupied by a lake (Fig. 1).

The impact structures with the diameter from 3-5 km to about 20 km have a complex structure. The complex crater is presented by a circular depression complicated with the uplift or peak in the central part of the crater's floor. This is the most disseminated type of terrestrial impact crater. The height of the uplift is about half of the crater's depth. The deep trough is filled with impact melt rocks, brecciated and metamorphosed rocks of the crater basement. In the Boltysh crater in Ukraine the annular trough is filled with an impact melt sheet 11 km in diameter and up to 240 m thick. This sheet was formed by cooling of the lake of high temperature silicate melt that surrounded the central uplift immediately after the crater formation.

Many complex craters are widespread in America, Europe, Asia, etc. The space images of the complex Spider crater in Australia reminds one of a spider's figure by the radial system of ridges spread from the center of the uplift. But the central uplift is not visible in many complex craters that are filled with sediments. The El'gygytgyn in Chukotka, 18 km in diameter is occupied by a lake, and its central peak is hidden under the water and lakustrine sediments. Impact glassy bombs (Fig. 2) of drop-like and cake-like form are spread on the crater rim.

The terrestrial craters with a central uplift form a transition to the ring and multi-ring impact structures as their diameters increase to some tens of kilometers. The West Clearwater crater in Canada is a classic example of a ring impact structure. The annular uplift of the basement rock forms a ring of islands in the crater lake. Ring and multi-ring impact structures are widespread on many bodies of the Solar System. Very interesting are the space images of the Mare Orientale basin, 950 km in diameter, on the other side of the Moon. Three systems of concentric annular ridges are the main morphological feature of this impact structure.

The great crater formation is a catastrophic process, the scale of which is dependent on the meteoritic energy and the crater diameter. Estimations made by some scientists show, that the formation of a crater from 150 - 200 km in diameter and more, causes global climatic changes on the Earth that leads to ecological catastrophe. One such catastrophic event took place about 65 million years ago at the end of Cretaceous. The formation of the Chicxulub impact crater about 200 km in diameter in the Yucatán peninsula in Mexico was one of the most dramatic events on Earth in Mezo-Cenozoic time. The material ejected from the crater consisted of rocks dust, melt, silicate, sulfur, and water vapor. One of the consequences of this event was the disappearance of the dinosaur fauna. Traces of this catastrophe are presented in a thin layer of the sediment that fell on the surface from the fireball. This layer in the Cretaceous/Tertiary boundary sediments has been diagnosed and investigated in North and Central America, Asia, Europe and even in New Zealand.

Crater investigations give the best understanding of the Earth's history and the development of its biosphere. Investigations of terrestrial craters help in the study of the meteorite bombardment of the planets and their satellites as an important process in the Solar System.

Institute of Geological Sciences

Ukrainian Academy of Sciences

Kiev, Ukraine