What Is Eclogitization?

Eclogitization is the tectonic process by which the eclogite rock is formed.

Eclogitization is the geological process which leads to the formation of eclogite. Eclogite is an extremely dense rock which is created only through intense pressure. Due to the high density of eclogite, the formation of such rocks in the Earth’s crust is capable of significantly changing the density of the regions in the crust where eclogitization occurs. As the density of the crust changes, it also affects the motion of tectonic plates. The most significant change in motion is noticeable at the boundaries where subduction occurs; these are usually called convergent boundaries and these are the only places where the conditions are just right for eclogitization to occur.

Eclogitization and Slab Pull

Eclogitization has been used to explain the phenomenon of slab pull. According to the observation of geologists, a number of areas of constant continental collision, for example the Himalayas and the Alps, exist on the tectonic plates. The collision of these areas has been ongoing for tens of millions of years and it occurs in the central parts of the landmass. This in turn leads to the creation of Fold Mountains which can rise to significant heights, as has been observed in Asia and in the Alps. This goes against the hypothesis that continental buoyancy should slow down the collision of continents over time, and that as one subduction zone goes inactive, another one should open up in a different area and the oceanic crust should be consumed through the new zone.

The slab pull hypothesis has been presented by geologists in order to explain this constant collision of continents and bring scientific theories in line with observable data from both the Earth’s surface and from the crust. According to the slab pull hypothesis, this constant motion of tectonic plates is initiated by the cooler plates, which are denser than warmer plates. So, when the heavier plates begin to subduct and go under, it continues to subduct until it disconnects. After the disconnection of the descending slab, eclogitization occurs and subduction continues to occur even though the initial slab that had started the subduction process has been detached. This is also used to explain the phenomenon of tectonic slabs detaching from the lithosphere during the descent and the process of exhumation which occurs post-subduction. According to famous geologists, if the process of eclogitization were to cease due to some reason, the constant continental collision will also stop.

Location of Potential Eclogitization

There are two locations where eclogitization can potentially occur. These are (1) in the actual portion of the crust that is being subducted and (2) in the overriding crust at the very base of the root of the crust. These are the only two locations in the fold mountain where the temperature, pressure, and density of the crust are enough to start eclogitization. When re-crystallization occurs during burial, the said area of the crust can gain significant density. If we are to run with a 10% increase in density during the entire process, this will equate to about 19-35 lbs/ft3 in the density increase. The increase in density drives the crust downward, and is therefore one of the primary forces driving the convection system in the mantle of the planet. These locations are the only ones which allow for eclogitization to occur as they have an optimum balance of the resources required for the process to occur.

Countries and Areas Where Eclogitization Occurs

Eclogites are very rare, with only a very small number of them exposed on the surface of the Earth and therefore available for observation. However, they are not completely unknown; some of the most common sights for the study of eclogites are pridote and ophiolite complexes which occur in various parts of the world. The most common places that these occur in are parts of Greenland, Norway, Bavaria in Germany, Newfoundland, Central France, and the northwestern parts of the Scottish Highlands. The rarity of the eclogites is due to the conditions that are required to form them, and due to the locations where such conditions are present in the perfect balance. Another factor is that even though they may be created, they may not be available on the surface of the Earth.

Effects of Fluid on Eclogitization

Although eclogitization requires high temperatures and high pressure to occur, the main driving factor behind the process is fluid. With time, it has been hypothesized that the process will not occur without fluid; this hypothesis has been corroborated through observation of rocks which have been subject to the process only for the extent of the time that fluid was available, leading to their partial eclogitization. Fluids allow the delamination of the base of crustal roots in the Fold Mountains. If fluid is not present in adequate quantities, then the process will not be completed. Partial eclogitization has been observed in rocks uncovered in Norway, China, and the Ural Mountains. In all of these locations, eclogite occurs in the same place as other normal rocks. This shows that eclogitization requires a catalyst in order to be successful. In this case, the catalyst is fluid, for it has been observed that in places where fluid can reach, such as the various fractures in the features, eclogite has been formed. On the other hand, the places that have been deprived of fluid are devoid of eclogite. Also present in these locations are partially eclogitized rocks, such as amphibolites and granulites.

Through these observations, it has been concluded that for eclogitization to occur successfully, fluid must flow into the rocks and the metamorphosis will not be completed in its absence. It has been hypothesized that if water flow were to stop, then without fluid to drive the eclogitization, the rocks will tend to revert to an unstable form known as their “meta-stable” state. As these will not have reached the required density, they will not cause the required undertow, which will in turn result in hindering the continuing subduction and may even lead to complete stoppage of the ongoing subduction.

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