Impact crater formation at the Earth's formation
© E.P. Gurov
About 155 - 160 impact structures are known on the Earth surface to recent. Discovery of 4-5 new craters every year is an evidence of a great reserve of still unknown impact structures. The high activity of endogenic and exogenous processes on the Earth surface determines the fast erosion and disappearance of impact structures, while the oldest ones have been preserved on the surface of the Moon and some other bodies of the Solar System for more than 4 billion years. The age of the oldest terrestrial impact structures is up to 2 billion years, but most terrestrial craters are younger than 200 Ma. The diameters of terrestrial impact craters vary from tens of meters up to 200-300 kilometers. The crater structure and morphology depends of their size. The smallest simple craters of bawl-shaped structure form transition to the complex craters with a central uplift at the rise of diameters to 3-5 km, and the last ones turn into the ring and multiring impact structures with further increasing of the diameters to 30-50 km and more.
When the asteroid strikes the Earth surface, its kinetic energy bursts out as an explosion. The shock wave generates the profound changes in the target rocks and minerals those have named shock metamorphism and shock melting. High shock pressures and post-shock temperatures differ from those in the conditions of endogenic processes, that enables the determination of impact rocks and diagnosis of the impact structures.
The main sequences of impact are formation of impact structure and deposition of eject around it, but the most important process at the gigantic crater formation is ejection of the high-temperature dust-gaseous plume into the stratosphere that spreads upon some part or the whole Earth surface and causes catastrophic sequences of the environment. Formation of the Chixulub impact crater, 240 km in diameter, in Mexica 65 Ma years ago caused one of the most catastrophic mass extinctions in the Earth history. The probable connection of some main mass extinctions with gigantic impacts in the Earth history is suggested.