An article in a recent issue of the New Scientist Magazine caught my eye it has referred to a paper written by Tilman Spohn of the Institute of Planetary Research in Berlin and was published on 23rd November 2013.
The Article is about the crustal evolution of our planet Earth, whilst this article is recorded as just a thought process, much about this theory when carefully considered rings true.
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The theory proposed is about the Geochemical and Microbial breakdown of early Earths rocky surface, a process that occurred over a vast initial timescale, looking at the rock types that were then and are now generated.
The breakdown of the surface rocks it is argued has been a necessary process leading to the kick starting of today’s familiar plate tectonics, it is postulated that the sedimentary process was needed in order to start the whole
cycle.
The Rock Cycle
Looking in detail at the rock processes involved, including the generation of Volcanic and Hyperbysal Rock types together with the formation processes of the many other rock types. These include the formation of Granite (when were the first Granites formed), the Ophiolite Zones (when did subduction begin) and Hydrothermal Processes (when did complex life start)?
After reading this article myself it seems to make a lot of sense, modern day plate tectonics had to evolve slowly over time and would not have been an instant process.
How for example was the necessary water generated for essential later hydrothermal volcanic processes that were first incorporated into the early Earths crustal rock’s. The sedimentary rock precursors would need to be buried at great depth for the now familiar cyclical crustal processes to work.
The answers to the questions of how crustal evolution was started are vital in the understanding of how today’s plate tectonic system works.
The answers to some of the questions seem strangely to imply that something akin to the now famous Gaia principle was actually in operation. The implication of this is that simple life processes were necessary in the first instance in order to produce the necesary environment that was needed for more complex life to evolve.
Life Starts
Let us look in more detail at the factors that influence the formation of the various rock types encountered today. Firstly there are the sedimentary rocks that are formed as a breakdown product of surface weathering, this must involve water as an essential ingredient in any chemical and biological breakdown of the early crust, RNA in the start of life works best in ice and this was initially formed.
The sedimentary rocks would have firstly formed just on the surface, later to be transported and recycled in today’s familiar tectonic processes, eventually collecting as sediments at the bottom of vast oceans of water.
Granite
For Granite to form for example, a precursor sedimentary rock needs to sink under the crustal rocks, sub-ducted beneath vast amounts of overlying rock over a huge Precambrian timescale. Today it is worth noting, bacteria can be found at great depths in the crust, when I wonder did this process first start and when did the bacteria learn to cope with the challenging environments. This seems to me to be a classic Chicken and Egg situation, which came first, the potential environments with their rich food source or the bacteria that gave rise to these environments.
Similarly, the formation of Granite as a rock type also needs the chemical and organic breakdown of a basic rock precursor; this would then be heated deep within the lower crust. Granite is rich in Sodium, Calcium and Potassium and this lighter material would then rise up within the crust in the form of plutons or Batholiths, these rocks are also rich in Ammonium and this element is also an indicator of a necessary precursor sedimentary origin.
Granite is referred to by geologists as a very dry material, any water having been driven off, it should also been remembered that Aluminium Oxide is also a constituent of granite and that this element is usually produced by the biological weathering processes. Other specialists however argue that it should be considered that acid rain would also produce the same element, volcanic processes are needed however, so how then did this process really start I wonder.
Much of the worlds Granite is of Pre-Cambrian age and any granite would have been emplaced into original crustal rock. Quartz rich veins are associated with Granite and these veins produce the economic minerals that are usually associated with the whole emplacement process. It seems likely that a biological precursor rock cannot be ruled out, which begs the question of when in our planets history did this rock production and the associated plate tectonic process begin?
Ophiolite Zones
Ophiolite zones as a rock type must form from the deep burial of precursor surface igneous rocks like pillow lavas, these together with Serpentine and Chert need to incorporate water and even follow on from earlier subduction. It seems very likely that an early organic process was involved in the oceanic ridge system and that this was necessary in order to kick start today’s crustal rock systems.
Metamorphic Rocks
Lastly we also need to consider when
today’s Metamorphic processes first appear, how did the heat and pressure needed for some of the metamorphic rock types first react with the pre-existing sedimentary and Igneous materials.
These initial rocks must have accumulated in vast quantities in the upper mantle for the system to work, how did this process begin and evolve I wonder, others are asking the same questions.
Barry Taylor
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