User:Jantho6/sandbox

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Louisiana State University Student Major: Geology

Minor: Business Admin.

Fall 2012
http://en.wikipedia.org/wiki/Wikipedia:USEP/Courses/Plate_Tectonics_(Alex_Webb)

http://en.wikipedia.org/wiki/Wikipedia:USEP/Courses/Plate_Tectonics_%28Alex_Webb%29

Wikipedia Page Proposal
ECLOGITIZATION

I propose to create a page investigating and explaining the process known as eclogitization and its impact on plate tectonics especially in subduction zones.

Outline

1) Introduction to eclogitization and definition

2) Eclogite facies (image of metamorphic facies with regard to temperature and pressure)

3) Where does Eclogitization take place (image of subduction zone)

4) Fluid Filtration of Eclogitization

5) Impacts of Eclogitization

6) Previous Studies detailing Eclogitization

I believe this topic has value because at the moment there is no wiki page detailing any sort of eclogitization and this page will help detail this process. The outline provided above shows how I intend to structure my page with along with images. I hope to first give a clear definition of eclogitization and then provide an orderly way of explaining where and how it occurs and its impacts on plate tectonics. I will create a self made figure showing the zone within the Earth that highlights where eclogitization is possible and the result of the densification of a slab in a subduction zone.

Wikipedia Pages Related to Eclogitization
Eclogite : This page explains the metamorphic rock, eclogite and how it is formed. It also plants the foundation of the type of rock that is involved in wiki-page topic, which is eclogitization. An addition that I would like to see on this page is a more in-depth investigation into the process of eclogitization and what that can show when studying metamorphic rocks that have been exhumed to the surface.

Written Communication: The page explains the eclogite facies very well. Within the the importance of eclogite section eclogitization could be touched on more and described in more detail

Visual Communication: The thin section image of eclogite is an excellent example of the rock under a microscope. To improve the readers inderstanding of where eclogite is formed there could be an image showing a subduction zone and common depths of eclogite formation

Metamorphism : For eclogitization to occur there metamorphism must take place. This page highlights the different metamorphic facies, one being the eclogite facies. This page is well put together but one change that I would like to see would be explaining metamorphic facies earlier in the page.

Written Communication: Metamorphism is a broad topic and the page effectively outlines each type of metamorphism and the facies that can occur. For ordering purposes it would be helpful to flip "Limits of metamorphism" and "Kinds of metamorphism"

Visual Communication: The image of metamrophic facies with regard to temperature and pressure is a simple yet perfect image for this page. The image that shows the metamorphic aureole is somewhat vague and a reader with limited knowledge of geology may not be able to pick out the different rocks in the image. A new image that is more clear may be helpful.

Orogeny : This page explains the term orogeny and where orogeny has occured on Earth. When there is orogeny there is always metamorphism taking place and sometimes pressures and temperatures are high enough to cause eclogitization. An addition that I would like to see on this page is more discussion on the Himalayan orogeny since this is one of the greatest orogenic events in Earth;s history and eclogitization takes place here.

Written Communication: The entire written communication is very orderly and I find it detailed yet simple for the reader.

Visual Communication: The image showing the geologic provinces of the world is a nice image and it very fitting for the page. A picture of the Himalaya Mountains would be an improvable part of the page.



Eclogitization is the process in which the high-pressure, metamorphic rock, Eclogite is formed. Eclogitization is the proper metamorphic term describing the appearance of the eclogite facies. The eclogite facies records pressures between 11 kbar - 20 kbar and a range in temperature of 300°C - 1000°C. Eclogite can be aptly termed a Ultra-High Pressure metamorphic rock (UHP) due to its formation in extreme pressures. To reach the eclogite facies the source melt will cross through different Metamorphic facies, more often than not from the advancement of the blueschist facies.

Geologic Setting and Effect of Eclogitization
Eclogitization occurs due to the subduction of continental or oceanic material into the mantle. Eclogitization is an important metamorphic process because it provides insight into previous plate tectonic processes and records pressures that have occurred in a specific area. At these zones within the earth, high pressures are reached as well as medium to high temperatures and eclogitization commences. Metamorphic re-crystallization during burial can lead to a significant density increase (up to 10 % in the case of eclogitization) or approximately 300–600 km/m3 of crustal rocks and continental lower crust and oceanic crust reach higher density than mantle. This density increase is a main driver in the convection of Earth and is a legtimate answer to questions such as, why is a tectonic unit disconnected from the descending lithosphere and why does the slab undergo exhumation after?

Localities
A difficult aspect of studying eclogitization is that eclogites constitute only a very minor volume of continental basement exposed today at Earth's surface. The few areas that are available to study eclogitization and view eclogites include garnet peridotites in Greenland and in other ophiolite complexes. Examples are also known in Saxony, Bavaria, Carinthia, Norway and Newfoundland. A few eclogites also occur in the northwest highlands of Scotland and the Massif Central of France. Glaucophane-eclogites occur in Italy and the Pennine Alps. Occurrences exist in western North America, including the southwest and the Franciscan Formation of the California Coast Ranges. Transitional Granulite-Eclogite facies granitoid, felsic volcanics, mafic rocks and granulites occur in the Musgrave Block of the Petermann Orogeny, central Australia. Recently, coesite- and glaucophane-bearing eclogites have been found in the northwestern Himalaya.

Field Studies

 * The Western Gneiss Region and the Bergen Arc of Western Norway: Known as one of the largest eclogitized pieces of continental crust that was exhumed during the Caledonian orogeny, studies here have shown that recrystallization of the eclogite facies is also accompanied with a significant reduction in rocks strength. This is shown by a localisnation of shear zones where the host granulites have been transformed to eclogites. The main point of this study was to explore the kinematics of syn-eclogite deformation in the Bergen arc which suggested that eclogitization is ultimately responsible for the separation of tectonic units from the descending lithosphere.  Furthermore, despite density increase, studies show that eclogitization may trigger exhumation due to the reduction in rock strength and requires that eclogitization is not complete.  This is especially true in basic and intermediate lithologies that may become denser than the mantle if eclogitization in case of complete recrystallization which is shown by a localisnation of shear zones where the host granulites have been transformed to eclogites. .  Thus the Bergen Arc provides an excellent example of eclogitization and its impact in a continental subduction region.


 * Mechanical Models: Simulations with viscous (ductile) and plastic (brittle) rheologies have been used to investigate the effect of eclogitization on the dynamics of convergence. A plethora of geologic settings have been considered such as intracontinental deformation, subduction, and continental collision to determine the density and buoyancy impact of eclogitization.  In most models, lithospheric bending or subduction is obtained, material from the lower continental crust and, in the case of the oceanic subduction, from the oceanic crust is entrained to great depths (more than 100 km). In all these cases, the force required for convergence at a constant velocity is significantly reduced in the case with eclogitization, compared to models without eclogitization.  Although models have shown that eclogitization does not impact subduction initiation,  eclogitized oceanic crust contributes to the slab negative buoyancy and could help the subduction of young oceanic lithosphere.