|Craters: when an impactor reaches the ground|
Impact craters are geological structures formed when a meteorid isn't totally burned out in the atmosphere and reaches the ground. Impact craters are found not only on Earth, but on all the terrestrial planets, on asteroids and on most of the moons of the outer planets (click here to see some beautiful craters' structures). However, around the solar system, these impact structures look quite different (click here to know why they look so different )
What happens during an impact ?
If the impactor reaches the ground, it forms a crater with a morphology that depends basically on the target surface and on the dimension of the impactor. Pieces of the impactor can sometimes be found within the crater, but in massive events, pressure and temperature reach very high values and can completely vaporize the meteorite, leaving only the crater structure. During these events, the impactor as well as the target rocks, are usually melted and mixed together, giving birth to a new chemical composition, which is characteristic of impact events, the impact melt. The shock wave generated by the impact, is responsible for the production of irreversible chemical and physical changes in target rocks: this phenomena is called shock metamorphism .
What do you see in an impact crater? Some definitions
There are several physical characteristics of craters that are somehow a signature of the impact origin of the structure. These characteristics are a good instrument for scientists to recognize the impact nature of a crater (click here to know how scientists recognize impact craters from other kind of craters).
|The floor of the crater can be bowl shaped or with a central uplift, while around it the walls form a raised rim. The crater is usually surrounded by an ejecta blanket, while its floor is covered with breccia, a course-grained rock, composed of broken rock fragments. During the impact both the target rock and the impactor are melted in what is called impact melt which is then dispersed in various impact deposits and ejecta, including some small particles called tektiles.(click here to know more about the morphology of craters)|
After Effects of impact events
Of course the geological structure of craters is the most visible consequence of an impact. But it is not the most dangerous. The climatic after effects of an impact are far more destructive than the explosion itself.
Furthermore, for what concerns the Earth, an impact is more likely to happen in the oceans, since 70% of the terrestrial surface is covered by water (even if past impacts with water or ice are very difficult to detect, leaving very little geological evidence). Climatic consequences of this kind of impacts are not less severe, as the phenomena of tsunami shows.