The shocking truth about impact metamorphism Essay

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Sometimes I happen to stop for a while, think and then wonder about the truth that lies beyond the things I see around me. Some questions even come to my mind.   Why does this volcano look like that?  Or how come there is a lake in that area?  What makes the tropical regions more diverse than temperate regions?  And many more questions.   Many of them are really hard to answer without further research.

But through studying and researching about certain phenomena, we were able to somehow understand the history of the things around us, the how and whys of everything. Thanks to the scientists and researchers who give so much of their time in studying and experimenting just to understand such phenomena.

Metamorphism which came from the Greek word meta meaning change and morph meaning form, can be defined as the process of changes in rocks structures, texture and composition due to high pressure, heat, new chemicals different from its origin, and stress. It has different types: local, regional, dynamic, hydrothermal, contact and impact metamorphism. (Nelson 2005)

Now, I would like to dwell more on impact metamorphism and its geological consequences.

            So what is impact metamorphism then?  This type of metamorphism which is also termed as shock metamorphism occurs when an extraterrestrial object such as meteorite and comet collides with the surface of the Earth.  It may also occur during the very violent volcanic eruption and ultrahigh pressures.  It is thus characterized by extremely high pressure and low temperatures.

            If the meteorite or comet reached the surface of a planet because it did not totally burned in the atmosphere, impact craters were formed either simple or complex depending on morphology.  Simple craters are relatively small with a smooth-bowl shape compared to complex craters. Complex craters were formed due to Earths gravity the pulls the steep crater walls making it fall inside the crater.

An asteroid or a meteor with its high speed that will collide with the Earth will naturally bring large amount of kinetic energy to the planets surface.  That kinetic energy will spread to all direction from the area where it hit producing a crater. (Doherty 2000)

The craters formed often contain breccia. These are smashed rocks resulted because of the explosion.  The craters also contain blankets of ejecta or the materials thrown out by the force of the impact.  The object that collides with the surface causing the explosion is more often pulverized totally than not after the collision. Sometimes it may leave particles or small fragments on the surface of the craters. The ejecta can travel great distances because of the impact, although most of them and other rock fragments may be deposited near the source.  Tektite is one of the examples. Tektites are beads of silica-rich glass, jet-blact to dark green or yellowish in color.

            Collision of extraterrestrial objects with the surface of the Earth happened several times.  But because of the erosion and geological activeness of the Earth, most traces were erased over time due to erosion and tectonic activities. But then, several of these impacts were still identified.  There are more than 150 impact craters shown. Lets look at some of these meteorite impact structures.

One example is the Vredefort Dome in South Africa. It is considered the biggest and the oldest visible structure of meteorite impact on Earth.  Geologists said that this extreme impact event is caused by an asteroid with an estimated ten kilometers diameter.  The original crater was estimated to have a diameter of 250 to 300 kilometers before it was eroded.

Another example of meteorite impact structure is the Sudbury Basin in Southern Ontario. It has shown unusual characteristics: extensive shattering of older basement rocks around the Basin, a thick unit of breccias within the Basin; large body of igneous materials;  copper, nickel, and iron ore deposits.  Evidence that this structure was caused by a meteorite impact it the discovery of unique shock-metamorphic effects. For example, shatter cones and planar deformation features of quartz.  These effects are visible also to the structures caused by meteorite impacts identified in our planet. (French, Bevan N.)

Chicxulub Crater in Mexico is another product of asteroid or meteorite impact on Earth.  The asteroid might have been 15 kilometers large in diameter which produced this 200-kilometer crater in diameter.  The asteroid hit a region rich in sulfur and ejecting billions of tons of sulfur and other materials present there into the atmosphere. The impact has killed almost 75% of life forms here on Earth.  Evidences show that there is almost absence of life in the immediate areas after the impact. After the collision, there is darkness that lasted for half a year.  Global temperatures plunge near freezing point. (Chief 2000)

Barringer Meteor Crater in Arizona is another impact crater. Discovered in this crater are breccia deposits that partially fill it.  There is also a range of shock metamorphic features and hummocky deposits just beyond the rim of the crater.  These deposits are said to be remnants of the ejecta blanket.  (Sharpton n.d.)

  We can also look at Deep Bay in Saskatchewan, Canada.  The region is dominated by shallow gouging of glacial erosion, thats why having this crater here is very unusual.  Deep Bay crater consists of a near-circular bay of about five kilometers wide and 220 meters deep.  It is a complex impact structure.  Drilling in the central structure showed metamorphic rocks deposits of mixed breccias. (Hamilton 1995)

             Collision of extraterrestrial objects like meteorite can happen without any warning at all. It may come in just an ordinary day for us.  And sometimes it is scary to think about or even just imagine it would happen one day.

            Geological structures of craters are the most visible consequence of meteorite impacts on Earth. However, it is not really the most dangerous.

The impact it may produced may be accompanied by atmospheric shock wave, a powerful air blast and after it reached the surface of the earth, earthquakes of highest possible magnitude ever recorded would follow. Because of its very high speed, it would look like a fireball as the components might burn. The impact would crash and pulverized rocks and other large objects which may send the particles and dust up into the atmosphere. This particles and dust may last for several months and sufficient amount of it may block the sunlight.  The blocking may then result to lower temperatures and short-term cooling trends. Molten rocks caused by the explosion and impact may be thrown out of the crater. These would create smoke that would also contribute darkening of the atmosphere.

            If the impact happens on the bodies of water, it may cause giant tsunamis that may drown the coastal areas and may even reach the inland areas.  And then, atmospheric chemistry would also be affected. It may be changed because of the gases from the ocean waters and pulverized rocks added to the atmosphere such as sulfur dioxide and carbon dioxide. Sulfur dioxide will result acid rain conditions and more solar radiation will be trapped causing a warming trend even decades after the impact.

            Impact metamorphism is not just an ordinary typhoon or volcanic eruption on earth.  It may be 100 times more dangerous compared to those typhoons and volcanic eruptions.  In fact, it could erase almost all life forms on Earth in just one blast.  But whatever happened in the past that made the dinosaurs to go extinct, still we should be thankful that new life forms come into existence again. Really, we cant stop the natural phenomena to happen.

            Well, sometimes its just good to wonder for a while and be curious about things around.  And then maybe it will lead you to deep thinking about the consequences of these things.  These may either made you feel disappointed or thankful.




References:




Doherty, Paul. 2000. Cratering. http://www.exo.net/~pauld/activities/

astronomy/Cratering.html (accessed August 15, 2007).

French, Bevan M. Catastrophe and Controversy: The Sudbury Meteorite Impact

            Structure, Canada. Smithsonian Institution Washington. http://esci.unco.edu/faculty/

            morrow/TOC%20Course/Sudbury_talk_abs.htm (accessed August 15, 2007).

Gaylard, Mike. 2006. Deep Impact The Vredefort Dome.

http://www.hartrao.ac.za/other/vredefort/vredefort.html (accessed August 15, 2007).

Impact Craters. http://spaceguard.esa.int/NScience/neo/neo-when/impact-

            craters.htmhurri.kean.edu (accessed August 15, 2007).

Impact Crater Images. www.pibburns.com/catastro/impactim.htm (accessed August 15,

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Nelson Stephen A. 2003. Types of Metamorphism. Tulane University Physical Geology

 http://www.tulane.edu/~sanelson/geol212/typesmetamorph.htm (accessed August 16, 2007).

Nelson Stephen A. 2003. Types of Metamorphism & Metamorphic Textures & Structures.

Tulane University Physical Geology.Petrology.http://www.tulane.edu/~sanelson/

eens211/typesmetamorph.htm (accessed August 16, 2007).

Pasadena Bureau Chief. 2000. The Chicxulub Crater Dig

http://www.space.com/scienceastronomy/planetearth/chicxulub_crater_001219-1.html

(accessed August 16, 2007).

Reimold W.U. Meteorites and Meteorite Craters. University of the Witwatersrand.

            http://www.saao.ac.za/~wgssa/as2/reimold.html (accessed August 15, 2007).

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