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). The occurrence of impact craters can be generally illustrated by describing an entry of extraterrestrial object into the earth’s atmosphere. When this object was not completely burned out by the Earth’s atmosphere it will eventually reach the surface of the earth. As it strikes the targeted surface or rocks compression will occur and quickly there will be a formation of circular depression commonly known as Impact craters (Nelson, 2007).
The shape and size of the crater is largely dependent upon the size and velocity of the impacting object. For some massive objects some of its fragments may be found in the formed crater however, this happens rarely since the high 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).
During these events, the shock wave will then cause an irreversible chemical and physical change to the target rocks in a phenomenon known as shock metamorphism. This is made possible as melting of the impacting object or target rocks unavoidably take place due to high pressure and temperature that accompanied the impact. This leads to the formation of shatter cones which are “conical fractures generally produced in fine-grained rocks”. They may form singly or in clusters when shock wave passes through them.
Scientist and researchers had revealed that the apex of the cone originally points towards the impact but as huge landslides may follow after the impact their orientation is changed. It is important to note that shatter cones are only produced in a high velocity impact (like cause by meteorites and the like) or by nuclear explosions. Other shock effects are multiple planar deformation features of silicate materials notably Stishovite. High mineral phases of feldspar are also formed.
The presence of these high-pressure forms of minerals is indicative that a crater is formed by impacts from objects from outer space (aside from those created by nuclear explosion). In ordinary volcanic processes such minerals cannot be found since its temperature and pressure is considerably lower (Roy, 2007). There are two shapes of impact craters, simple and complex. Simple craters are relatively small with smooth bowl shape while complex craters are large ones that both have a peak ring and a shallower depth.
Complex craters are large that due to its immensity the material in the initial walls splashes downward and inward. However, in both structure formations, a raised rim surrounds the crater (Hamilton, 2001). The rim is formed when materials at the lip of the crater fold itself. Ejecta blankets surround the crater as ejecta fall back to the earth while the floor is covered with breccia. In some instances faulting may also occur in the rocks around the crater, causing the crater to become enlarged, and producing concentric set of rings (Wiggins, 2006).
“Impact Cratering on Earth” Earth impact database, 2007 <http://www. unb. ca/passc/ImpactDatabase/>(Accessed: 23 August 2007) from http://www. unb. ca/passc/ImpactDatabase/essay. html Hamilton, Calvin “Terrestrial Impact Craters” Solar System. 2001, Accessed 23 August 2004, Available from http://www. solarviews. com/eng/tercrate. htm Nelson, Stephen, ”Meteorites, Impacts and Mass Extinction” Natural Disasters, 21 April 2006, Accessed 22 August 2007 available from http://www. tulane. edu/~sanelson/geol204/impacts. htm