The shocking truth about impact metamorphism

In the history of the planetary systems , there is a considerable evidence of interplanetary collisions as seen on the impact deformation in the surfaces of almost all planets (at least those seen by the naked eye with the aid of telescopes . In fact , scientist considered impacts as part of geologic process of the early solar system . Common impacting agents are the fast moving asteroids , comets and large meteors ( Impact Catering , 2007 . Such impact causes geological changes in the structure of the planets

The proof that collision or bombardment had taken place is when

impact craters (circular depression ) are present . They are formed when a large impacting agents (meteors , comets or asteroids ) crashes into another larger solid planetary body . The surfaces of other planets , such as the Moon and Mars , are able to preserved all its impact history since the surfaces of these bodies had remained unchanged for a million years unlike that of the earth where it is geologically active (constant activity of erosion , volcanism , tectonic activity and infilling that leads to destruction or burial of impact structures ) inspite of the fact that it more frequently bombarded due to its stronger gravitational pull . About 160 impact craters are discovered and they can be found mostly in North America , Europe , Southern Africa and Australia (Koeberl , 2007

pressure and temperature caused by the impact may completely melt and vaporize the object (Koeberl , 2007 . The impact will cause the rocks to break into pieces and send them flying into the air . This fragmented pieces is called ejecta (Nelson , 2007 . Some of the rocks that are hurled into the air melt and partially remelted back forming small particles called tektites which are characteristically identified as opaque to translucent , green , brown , grey , yellow-grey or black glass ( Tektites , 2004 . At the same time , the impact will also send a shock wave into the rocks below , breaking the rocks beneath into small particles forming a breccia (Nelson , 2007 . In impact structures , the formation of breccia dikes contributes to the understanding of the processes involved in impact cratering . Scientists had declared that for the most part the breccia dikes are formed in the excavation stage by injection of brecciated material into the walls and the floor of the expanding excavation cavity (Kord 2007 . Another by-product of this event is the formation of pseudotachylite vein which is a dike-like breccia which are formed by frictional melting in the basement of impact structures and which may contain unshocked and shocked mineral and lithic clasts in a fine-grained aphanatic matrix . It resembled to a tachylite (black volcanic glass formed as basaltic magmas cooled found in volcanic surfaces (Kord 2007 . A good example of this can be seen in the Vdredefort Dome . To date The Vredefort Dome is believed to be the oldest and largest clearly visible meteor impact structure in the world calculated to have formed about two billion years ago . Located in South Africa , approximately 100 kilometers southwest of Johannesburg its diameter is approximately 250 kilometers ( The Vredefort ' 2006

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