Meteorite Falls have been a part of our history as living organisms of this planet - Metamorphism project introduction. It may have been that our existence on this planet has been brought about by a major Meteor strike in the sense that it was hypothesized that the extinction of the once predominant species known as the Dinosaurs was caused by a major meteor strike (Courtillot, 2003). The said meteor strike caused climate changes whch proved fatal to the said species. Although meteorite falls has been reported for thousands of years, until this century no one has ever identified a crater created by such a fall. The reason being given is simple, Even a large meteorite may be slowed so much by the Earth’s atmosphere that it lands without making a significant hole. However, despite this unique protection, the Earth still experienced the devastating effect of a meteor impact. Scientists have now identified over 150 proven impact sites. Evidence suggests that there have been many thousands of other impacts over the course of the earth’s history. Meteorites weighing a quarter of a pound or more hit the earth thousands of times a year (Paine, M. 1999) . The Earth, without its protective layer called atmosphere, would have been more heavily scarred than the Moon, craters are continually erased by erosion and redeposition as well as by volcanic resurfacing and tectonic activity. Spacecraft orbital imagery has helped to identify structures in more remote locations for further investigation. Meteorites are classified as stony, stony iron, and iron meteorites.
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The stony meteorites are further divided into those with chondrules (chondrites) and those without chondrules (achondrites), which are roundish articles that show that their fillings crystallized from a melt. Chondrites are further subdivided into several classes according to their mineralogical and chemical composition. Metamorphism can be defined as the solid state re-crystallisation of pre-existing rocks due to changes in heat and/or pressure and/or introduction of fluids without melting. It is the alteration and transformation in pre-existing rock masses effected by temperature and pressure, but excluding changes produced by weathering and sedimentation. The changes may include the production of new minerals, structures, or textures, or all three. They give a distinctive new character to the rock as a whole, but they do not involve the loss of individuality of a rock mass, such as changes brought about by fusion. Quantitatively, the metamorphic rocks, including gneisses and migmatites, are the most important group of rocks in the crust of the continents. There will be mineralogical, chemical and crystallographic changes (World of Earth Science Metamorphism, 2005)
Some scientists argue that changes in atmospheric pressures are not metamorphic, but some types of metamorphism can occur at extremely low pressures (Kring D, 2000). The temperature lower limit of metamorphism is considered to be between 100 – 150°C, to exclude diagenetic changes, due to compaction, which result in sedimentary rocks. The upper boundary of metamorphic conditions is related to the onset of melting processes in the rock. The temperature interval is between 700 – 900°C, with pressures that depend on the composition of the rock. Migmatites are rocks formed on this borderline. They present both melting and solid-state features.
II. The Geological consequences of major meteorite impacts.
It is a known fact substantiated by science, that the entry of a large asteroid or meteorite into Earth’s atmosphere may occur without any warning. The casualties may be quite astronomical in terms of both human lives and properties destroyed, if too large a chunk of this substance remains solidly intact despite its friction with the earth’s atmosphere and collides with the Earth’s surface (Courtillot, V 2000).. The said collision course may take place within a blink of an eye, which means that people may have too little time to take any course of action that would permit survival for those close to the impact site. All organisms that survived the air blast would most probably be rocked by a massive earthquake which may be hundreds or thousands of times greater in magnitude than the largest one ever recorded. The impact might create a deep fissure which may range from about 10 to 20 times the size of the colliding meteorite/asteroid. Pseudotachylites are formed upon the said impact. A pseudotachylites is a fault rock that has the appearance of the basaltic glass, tachylyte. It is generally found either along fault surfaces, often as the matrix to a breccia, or as veins injected into the walls of the fault.
It may also happen that when such a massive meteorite makes contact with the earth, another event that occurs would be the forming of impact ejecta. Craters often contain smashed rocks known as breccia and may be surrounded by a blanket of ejecta, displaced particles thrown outward by force of the impact. Ejecta dispersal from a meteorite impact occurs in the form of gas as well as rocks. “Upon impact, the asteroid essentially digs a hole in the atmosphere, and then in the earth’s crust.” The gasses which are produced as a result of such an event include sulfur and nitrogen. Heat from the impact can cause melting of rocks on the crater floor (Hamilton, C. 2002). The impacting body is typically pulverized by the force of the collision although some small fragments may occasionally be preserved. The atomic structures of minerals in the rocks of the impact site will be altered by the extreme force of the collision to form a suite of features that are unique to impact events.
After the collision between the meteorite and the earth’s surface has taken place, a great cloud of dust and rock would be distributed across the earth. Massive amounts of dust and small pieces of rock are sent up into the atmosphere as the extraterrestrial object strikes the earth. The said dust and rock fragments will block sunlight, and keep it from getting through the atmospheric layers. Hence because of this event, day would become as dark as night for several months at a time, replicating the climate in the Arctic. The consequences of this event would be extremely detrimental to the existing life forms in the planet. Freezing conditions may occur in the oceans, away from the coastlines. Without sunlight, the life-supporting process of photosynthesis ceases in plants and algae (Paine, 1999). “The cold and darkness would cause the collapse of the food pyramid” (Dott & Prothero, 1994). “The disappearance of plants would break the food chains and the carnage would begin” (Courtillot, 1999). Conditions would probably be more deploring than the extreme winter occurring in the Arctic Circle. The Earth, in this case, would probably remain dark and cold for over a year.
Another major effect of a meteorite impact on the land, would be the occurrence of global wildfires or firestorms. Firestorms are perpetuated by massive amounts of methane gas that are released from the Earth by the collision. Methane is an extremely flammable substance. Lightning can ignite the released methane gas. When the meteorite hits, it shakes up the earth, rupturing pockets of methane that are trapped in gas hydrates. The fire burns close to the ground and quite high into the atmosphere. Such fires would not be simple forest fires that only burn vegetation (Bland, P. 2000). Due to the fact that this fire is fueled by extraordinary levels of methane gas, the atmosphere itself would also be on fire. The fires would incinerate global flora and fauna. The blaze would also decrease oxygen supplies and increase levels of carbon dioxide instigating a run-away greenhouse effect, a major, overall heating of the planet (Paine, 1999).
It is plain that, in the aftermath of a massive meteorite impact the Earth would not be a pleasant place. A large percentage of life would cease to exist. Another thing which occurs upon meteorite impact would be the formation of a substance which we call shatter cones. Shatter cones are distinct conical fractures which are thought to be produced by extremely high pressure from the impact these substances are among the physical markers found in or around an impact site (Fineberg, J. 2002). According to Mark Cintala, a space scientist at the Experimental Impact laboratory at Johnson Space center “We don’t know exactly how they form, It is one of these things that is taken as an act of geological faith that we know they are associated with craters, therefore, when you find shatter cones you find craters”. Origin of shatter cones by tension during the upheaval of the central uplift accounts for the centripetal orientation of shatter cone axes and their tendency to be most abundant in central uplifts, where rarefaction would presumably be greatest..
III. CASE ANALYSIS: THE VREDEFORT DOME
The Vredefort Dome, with a radius of 190km, is also the largest and definitely the most deeply eroded.It provides critical evidence of the earth’s geological history and is crucial to our understanding of the evolution of the planet. The said impact occurred approximately 2,023 million years ago, making it the oldest astrobleme found on earth so far (Gibson R. 2001). Vredefort Dome bears witness to the world’s greatest known single energy release event, which caused devastating global changes, including, according to some scientists, major evolutionary changes. The Crater provides the only structurally intact exposure of the basement which is below the crater floor of the large astrobleme. This geological feature is quite unique for the planet since it shows a geological section that reaches from the rocks which once covered the crater floor, through the floor, and down into the basement of the structure. The central cone of the crater rose (rebound) by approximately 38 km to provide a surface outcrop equivalent of mantle rocks. These mantle rocks also shows a type of metamorphism found only in conditions of very high energy release. It was discovered that the said structure was caused by an extreme impact event with an asteroid which approximately measures around 10 kilometres in diameter. The original crater is now eroded away, and it was estimated to have measured around 250 – 300 kilometres in diameter. It may be that some 70 cubic kilometres of rock may have disintegrated from the said impact. It does not however include evidences of impact melts. Shock deformation features and pseudotachylitic breccias in the Vredefort Dome This has been attributed to the combined effects of an increase in the intensity of shock heating towards the center of the impact structure and the exhumation of progressively deeper crustal levels towards the center of the central uplift (Gibson, R 2001) . In conclusion, it has high quality exposures of a complex meteorite impact event that are readily accessible.
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Courtillot, Vincent. Evolutionary Catastrophes: The Science of Mass Extinction. Trans. Joe McClinton. Cambridge University Press. New York. 1999. (OBIS)
Kring, David, A. Impact Events and Their Effect on the Origin, Evolution, and Distribution of Life. GSA Today, v. 10, no. 8, August 2000. http://www.geosociety.org/pubs/gsatoday/gsat0008.htm (GSA)
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W.U. Reimold, Meteorites and Meteorite Craters (2002), Department of Geology, University of the Witwatersrand . http://www.saao.ac.za/~wgssa/as2/reimold.html
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UNITED NATIONS ENVIRONMENT PROGRAM. July 2005 – December 2005 http://www.unep-wcmc.org/sites/wh/vredefort.html
J. Fineberg, (2002) SHATTER CONES—RAPID FRACTURES BY IMPACT. Racah Institute of Physics, Hebrew University of Jerusalem, http://www.whoi.edu/science/GG/geodynamics/2005/images2005/sagy04_JGR.pdf
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Dynamic Metamorphism, Definition and much more from answers.com.,
Chondrites, Definition and much more from answers.com.,
Sedimentation, Definition and much more from answers.com.,
Weathering Definition and much more from answers.com.,
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Atmosphere, Definition and much more from answers.com.,
Migmatite, Definition and much more from answers.com
Dawn Sinclair., EARTHLY IMPRESSIONS: Crater Chasers. http://www.msnbc.com/onair/nbc/dateline/m-crater.asp?cp1=1
 The Atmosphere is the protective layer which surrounds the earth, its main purpose is to shield the earth against ultraviolet rays and against meteorites or any space debris from collision against the earth’s surface.
 The moon does not have any atmospheric protection against space debris, hence it is susceptible to collision.
 Chondrites are stony meteorites that have not been modified due to melting or differentiation of the parent body. They formed when various types of dust and small grains that were present in the early solar system accreted to form primitive asteroids.
 Weathering is the process of breaking down rocks, soils and their minerals through direct contact with the atmosphere.
 Sedimentation is the motion of molecules in solutions or particles in suspensions in response to an external force such as gravity, centrifugal force or electric force.
 Gneisses is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from preexisting formations that were originally either igneous or sedimentary rocks.
 Migmatite is a rock at the frontier between igneous and metamorphic rocks. It forms under extreme temperature conditions during prograde metamorphism, where partial melting occurs in pre-existing rocks.
 Diagenetic Change is any chemical, physical, or biological change undergone by a sediment after its initial deposition and during and after its lithification, exclusive of surface alteration (weathering) and metamorphism.
 Asteroids are rocky and metallic objects that orbit the Sun but are too small to be considered planets. They are known as minor planets.
 A meteorite is a stony or metallic mass of matter that has fallen to the earth’s surface from outer space.
Breccia is a lithified sedimentary rock consisting of angular or subangular fragments larger than 2 millimetres (0.08 inch).
 Ejecta refers to the debris that is ejected during the formation of an impact crater
 Photosynthesis is a process which concerns synthesizing of the sun’s rays and changing it into stored energy. This is the process used by plant life on this planet to make their `food’.