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The geological record at the end of the Anthropocene will undoubtedly contain large volumes of plastic. However, some geological processes will completely destroy plastic (e.g. magmatic or chemical processes). I assume that sedimentary rocks will contain plastic pellets at the least and that igneous rocks will not. Which geological processes that create metamorphic rock will be able to completely destroy plastic molecules?

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    $\begingroup$ I would image any metamorphic process that involved the use of heat or heat and water( including steam) would be the most likely as these are most likely to cause plastics to break down. $\endgroup$ – Fred Feb 1 '18 at 2:20
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    $\begingroup$ Interesting question. Makes me want to put some plastic into our experimental petrology machines to see what would happen. $\endgroup$ – Gimelist Feb 2 '18 at 9:28
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    $\begingroup$ On a geological timescale, I wonder how many plastics will be destroyed because they just fall apart from friction processes, UV radiation, and last but not least biological processes. $\endgroup$ – Jan Doggen Feb 5 '18 at 8:53
  • $\begingroup$ Interesting question! I think it would benefit from some more specifics. What are we counting as "completely destroyed" for a molecule? Presumably something like splitting a 1000-carbon chain into two wouldn't qualify, but requiring every single bond to be broken seems excessive. And what kinds of plastics are you interested in? Some are far more resilient than others. $\endgroup$ – Pont Feb 6 '18 at 7:29
  • $\begingroup$ @Pont I'm neither a chemist nor a geologist. But, in terms of this question, I was wondering which processes would have the necessary heat/pressure to burn up or chemically convert the plastic to a molecular form that is found in "nature". The most common type of plastics are of interest. $\endgroup$ – farrenthorpe Feb 6 '18 at 16:51
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Some simplifying assumptions

This is quite a broad and complex question to answer, so I'm going to simplify it shamelessly to make it a little more answerable.

Firstly, there are a huge number of different plastics with a huge variety of physical characteristics. You mention that you're interested in ‘the most common type of plastics’, so I'm going to ignore exotic, high-temperature specialist polymers.

Secondly, you specified that by ‘destroy’, you mean ‘chemically convert the plastic to a molecular form that is found in "nature"’. This could be tricky to guarantee completely: to be really sure you'd have to do a thorough analysis of all the molecular structures in the altered plastic, and make sure that none of them occur in nature. I'm going to reference research on pyrolysis of plastics which classifies end products as gas, wax, oil, and char, and assume that these products contain little or no material which couldn’t be found in nature.

Thirdly, I’m going to ignore metamorphic conditions other than temperature. Temperature will, I’m fairly sure, have the biggest effect on the breakdown of plastics. The chemical environment might make the plastic break down more quickly (by providing reactants), but I don’t think that it will make the plastic break down any more slowly than it would at the same temperature in an inert atmosphere or vacuum. I’ll also ignore any pressure effects.

Grades of metamorphism

Here’s a helpful diagram of metamorphic facies, temperatures, and pressures (source: Wikimedia).

Metamorphic facies

The chart cuts off at 900°C, by which point the rock will generally be melting, putting it into the igneous rather than metamorphic category.

Effects of heating on common plastics

Fortunately for this question, efforts to dispose of waste plastic have led to a lot of research on the effects of heating. Williams and Williams (1997) is a particularly useful reference. They report the effects of heating a sample of mixed plastic waste (low- and high-density polyethylene, polypropylene, polystyrene, polyvinyl chloride, and polyurethane) in an inert nitrogen atmosphere at temperatures ranging from 500°C to 700°C. Here’s what they got out of it (yields are in percentages):

Table of product yields

(I assume that there's a typo somewhere in the 650°C column, since it implies a 110.5% yield.) The yield is 78.91% at 550°C, so maybe we could expect some identifiable plastic residue (though probably no actual plastic) to remain at that point. By 600°C, there's already a 95.51% yield; unfortunately the missing char figures prevent us from quantifying the yield at higher temperatures, but it seems likely that we’d be approaching 100% by 700°C.

There are a few pages of analyses of the products, but they’re mainly focused on their suitability as fuels rather than whether they’re found in nature. Considering the wide variety of compounds found it crude oil, I think it’s plausible that all the plastic pyrolysis products have naturally occurring counterparts, but it would be a significant research project to establish this beyond all doubt.

Putting it together

Referencing the pyrolysis temperatures back to the metamorphic facies, it looks as though the amphibolite facies corresponds reasonably to the range over which common plastics are completely, or near-completely, decomposed by temperature. So, when the aliens land in a few million years and start investigating the Anthropocene, they might well find some of our old rubbish in prehnite-pumpellyite (P-P) facies rocks, but there won't be much to see in the granulites. Here are some examples of metamorphic rocks from those grades.

prehnite-pumpellyite facies
Liberty Gulch Broken Formation, a prehnite-pumpellyite facies metasandstone. Source: Raymond and Berro (2015).

amphibolite facies
Amphibolite facies (staurolite zone) metasediments from the Scottish Dalradian. Source: Dave Waters, Dalradian Metamorphism album.

granulite facies
Strongly deformed high- to medium-pressure pelitic granulites from the Qianlishan Complex. Source: Yin et al. (2014).


References

Loren A. Raymond, David A. Bero (2015). Sandstone-matrix mélanges, architectural subdivision, and geologic history of accretionary complexes: A sedimentological and structural perspective from the Franciscan Complex of Sonoma and Marin counties, California, USA. Geosphere, 11(4), pp. 1077–1110. doi: https://doi.org/10.1130/GES01137.1

E. A. Williams & P. T. Williams (1997). Analysis of products derived from the fast pyrolysis of plastic waste, Journal of Analytical and Applied Pyrolysis. 40-41, pp. 347-363.

Yin, C., Zhao, G., Wei, C., Sun, M., Guo, J., & Zhou, X. (2014). Metamorphism and partial melting of high-pressure pelitic granulites from the Qianlishan Complex: constraints on the tectonic evolution of the Khondalite Belt in the North China Craton. Precambrian Research, 242, pp. 172-186.

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  • $\begingroup$ I really appreciate this answer. Could you perhaps include brief definitions and pictures of amphibolite, prehnite-pumpellyite, and granulite? $\endgroup$ – farrenthorpe Feb 9 '18 at 23:49
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    $\begingroup$ @farrenthorpe Glad you liked it! Pictures added. As to definitions: the most concise definitions of the metamorphic facies are the pressure/temperature conditions shown in the first figure. The appearance and composition of rocks of these facies depends on the nature of the original, unmetamorphosed rock from which they were formed. You can look at Wikipedia's metamorphic facies page to get an idea of which minerals are associated with which rock type / metamorphic facies combinations. $\endgroup$ – Pont Feb 11 '18 at 14:01
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    $\begingroup$ @farrenthorpe pictures will not really help. Pictures mostly help to distinguish the fabric of the rock (schist, gneiss, etc). They are less useful when trying to figure out the metamorphic grade of the rock. $\endgroup$ – Gimelist Feb 12 '18 at 23:25

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