Plate Tectonics & Evidence of Life

[Aaron8547]

Looking for research on the destruction of biological evidence due to the subduction and destruction inherent to plate tectonic activity. One, are there any places on Earth that have not been affected by tectonic activity? And two, what is our chronological limitation in looking back at fossilized remains due to them being destroyed by this activity?

Thanks

 

[Evo]

Looking for research on the destruction of biological evidence due to the subduction and destruction inherent to plate tectonic activity. One, are there any places on Earth that have not been affected by tectonic activity? And two, what is our chronological limitation in looking back at fossilized remains due to them being destroyed by this activity?

Thanks

There are several places, one is Australia.

Jack Hills, Australia, where rocks were found to contain the oldest known minerals on Earth, a 4.4 billion-year-old zircon. Just three of the very oldest zircons have been found, ones that date back to almost 4.4 billion years ago.

http://www.livescience.com/43584-earth-oldest-rock-jack-hills-zircon.html

The Acasta Gneiss from northern Canada is around 4 billion years old. http://paleobiology.si.edu/geotime/main/hadean4.html

 

[tom aaron]

Some of the earliest 'evidence' of life is kind of iffy. It's hard to distinguish it from non biological origins. I was sitting in a TV room with some senior doctors emeritus when the so-called evidence of life on Mars was presented 20 years ago. They all greeted it with various expressions from 'What a bunch of idiots' to 'huh, you got to be kidding'.

Also, it's just not all that studied. This may be because the conclusions are not all that conclusive and the field of study doesn't have a lot of opportunities. This is not my area of study but I have had rock samples that I was curious about, made thin sections and never get much definitive in answers from the few who can even begin to look at them.

Anyways. Plate tectonics hasn't destroyed everything but what's left is not all that great for giving answers.

 

[Torbjorn_L]

One, are there any places on Earth that have not been affected by tectonic activity? And two, what is our chronological limitation in looking back at fossilized remains due to them being destroyed by this activity?

The first question is open, since there isn't any consensus on how long plate tectonics has worked. The last affected places or grains are as stated in the thread. However, it seems that from surviving lone zircon grains you can derive a) age of plate tectonics and b) habitability (ocean and crust 4.4 billion years ago) but possibly also habitation (an early biosphere).

a) Possibly plate tectonics started 4.4 billion years ago.

By measuring magnetites inclusions in Jack Hill zircons (see previous comments; but note that the oldest zircon record is extensive, at least some 5000 zircon grains from before the oldest known rocks), a secure observation of an early geodynamo has just been made. It constrains heat flow so that early plate tectonics is implied.

"The values measured by Tarduno and his team were much greater than 0.6 ?T, indicating the presence of a geodynamo at the core of the planet, as well as suggesting the existence of the plate tectonics needed to release the built-up heat.

"There has been no consensus among scientists on when plate tectonics began," said Tarduno. "Our measurements, however, support some previous geochemical measurements on ancient zircons that suggest an age of 4.4 billion years.""

[ http://www.sciencedaily.com/releases/2015/07/150730162010.htm ]

I think that last bit is tying together the 4.4 billion year old crust observation (from melt characteristics of the rocks that the zircons grew out of, I think) with the process that could have produced it. An early plate tectonics would help explain why there is no surviving rock (as of yet) from early Earth, the early crust was extensively reworked over time.

b) The potential for studying biosphere-litosphere interactions in zircons has just started to come under study:

"This observation, while reflecting 9 granitoids and 289 analyses of zircons from a region where over 400 different plutons have been identified, is consistent with the incorporation of (reduced) organic matter in the former and highlights one possible manner in which life may modify the composition of igneous minerals. The chemical properties of rocks or igneous minerals may extend the search for ancient biological activity to the earliest period of known igneous activity, which dates back to ∼4.4 billion years ago. If organic matter was incorporated into Hadean sediments that were buried and melted, then these biological remnants could imprint a chemical signature within the subsequent melt and the resulting crystal assemblage, including zircon."

[ http://www.ncbi.nlm.nih.gov/pubmed/26153630 ]

 

[Baluncore]

Looking for research on the destruction of biological evidence due to the subduction and destruction inherent to plate tectonic activity.

The ocean floors are made of basalts and are growing at the mid-ocean ridges. The ocean floors are more dense and younger than the continents. The continents are older low density accretions that are not being subducted. Any very old evidence of life must therefore be on, or in, the continents.

It is mountain building on the continents that lifts old evidence to where it can be observed. Evidence of life will be gradually destroyed by erosion after tectonic uplift.
Gentle metamorphism acts to preserve fossil evidence by hardening and sealing a rock. Heavier metamorphism will begin to mobilise the grain boundaries and so begin to destroy evidence of life.

And two, what is our chronological limitation in looking back at fossilized remains due to them being destroyed by this activity?

Plate tectonics does not really destroy the oldest evidence. The oldest evidence is preserved in the continental shields.

 

[davenn]

The ocean floors are made of basalts and are growing at the mid-ocean ridges. The ocean floors are more dense and younger than the continents.

yes, indeed. there is no ocean floor older than around 180 million years. A drop in the bucket on the geological time scale of the Earth